G3 and G4 CCD
Operating Manual
Version 2.3
Modified on December 4th, 2012
All information furnished by Moravian Instruments is believed to be accurate.
Moravian Instruments reserves the right to change any information contained
herein without notice.
G3 and G4 CCD cameras are not authorized for and should not be used within
Life Support Systems without the specific written consent of the Moravian
Instruments. Product warranty is limited to repair or replacement of defective
components and does not cover injury or property or other consequential
damages.
Copyright © 2000-2012, Moravian Instruments
Moravian Instruments
Masarykova 1148
763 02 Zlín
Czech Republic
tel./fax: +420 577 107 171
www: http://www.gxccd.com/
e-mail: ccd@gxccd.com
Table of Contents
Introduction....................................................................................................5
Camera Technical Specifications...................................................................7
CCD Chip...............................................................................................10
Model G3-6300.................................................................................10
Model G3-1000.................................................................................11
Model G3-11000...............................................................................11
Model G3-11000C............................................................................11
Model G4-9000.................................................................................12
Model G4-16000...............................................................................12
Camera Electronics.................................................................................12
Model G3-6300.................................................................................13
Model G3-1000.................................................................................13
Model G3-11000...............................................................................13
Model G4-9000.................................................................................14
Model G4-16000...............................................................................14
CCD Chip Cooling.................................................................................15
Power Supply..........................................................................................15
Mechanical Specifications......................................................................16
Package Contents....................................................................................17
Optional components..............................................................................18
Filter wheel for five 2 inch threaded filter cells................................19
LRGB filter set..................................................................................19
Clear (C) filter...................................................................................19
UBVRI filter set................................................................................20
Separate filters...................................................................................20
Telescope adapters for G3 cameras..................................................20
Telescope adapters for G4 cameras..................................................22
Getting Started..............................................................................................24
Camera System Driver Installation.........................................................24
Windows 7 System Driver Installation.............................................25
Windows XP and Windows Vista System Driver Installation.........25
Windows 2000 System Driver Installation.......................................26
SIPS Software Installation......................................................................27
SIPS configuration files....................................................................27
G3 and G4 CCD Camera Driver for SIPS..............................................28
Using of multiple configuration files for different cameras.............29
Cropping of the CCD area................................................................30
Camera Connection................................................................................31
Camera LED state indicator..............................................................32
Working with Multiple Cameras............................................................32
Camera Operation.........................................................................................34
Camera and the Telescope......................................................................34
Temperature Control...............................................................................36
First Images............................................................................................38
Brightness and Contrast – Image Stretching..........................................40
Calibration..............................................................................................41
Color Images with monochrome camera and filters...............................43
Color images with color camera.............................................................46
Balancing colors.....................................................................................48
Some General Rules for Successful Imaging...............................................50
Camera Maintenance....................................................................................53
Desiccant exchange................................................................................53
Changing Filters......................................................................................54
Changing the Whole Filter Wheel..........................................................56
Changing the Telescope Adapter............................................................56
Power Supply Fuse.................................................................................57
Introduction
Thank you for choosing the large-format CCD camera. G3 and G4 series of
CCD cameras were developed for imaging under extremely low-light
conditions in astronomy, microscopy and similar areas. Design of this series
inherits from G2 cameras, with which they share precise electronics providing
uniform frames without artifacts and extremely low read noise limited only by
CCD detector itself. Also the robust construction, rich software support and
easy manipulation are the same. However, G3 and G4 CCD camera head is
large enough to contain detector up to 24×36 mm for G3 series and 36×36 mm
for G4 series.
G3 cameras can contain also filter wheel with 5 positions for 2 inch (50 mm)
diameter filters. Internal filter wheel is not available for G4 cameras, because
a filter wheel capable to carry 50×50 mm square filters is too big.
External Filter wheels with 5 positions for 50×50 mm square filters or
7 positions for 2 inch (50 mm) diameter filters are available for G4 cameras.
Also G3 cameras can be equipped with external filter wheel, but it is not
possible to combine internal and external wheels on single camera. G3 camera
mus be made without internal filter wheel to be compatible with external filter
wheels.
Please note the G3 and G4 CCD cameras are designed to work in cooperation
with a host Personal Computer (PC). As opposite to digital still cameras, which
are operated independently on the computer, the scientific slow-scan, cooled
cameras usually require computer for operation control, image download,
processing and storage etc. To operate the camera, you need a computer which:
1. Is compatible with a PC standard.
2. Runs a modern 32 or 64-bit Windows operating system.
Drivers for 32-bit and 64-bit Linux systems are also provided, but
camera control and image processing software, supplied with the
camera, requires Windows operating system.
3. Provides at last one free USB port.
G3 and G4 cameras are designed to operate with USB 2.0 high-speed
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(480 Mbps) hosts. Although they are fully backward compatible with
USB 1.1 full-speed (12 Mbps) hosts, image download time can be
somewhat longer if USB 1.1 connection is used.
A simple and cheap device called USB hub can expand number of
available USB port. Typical USB hub occupies one computer USB
port and offers four free ports. Make sure the USB hub is USB 2.0
high-speed compatible.
But keep on mind that if more USB devices connected to one hub need
to communicate with a host PC, USB hub shares its single up link line
to the host PC. Although G3 and G4 cameras can operate through a
USB hub, it can negatively affect the camera performance, like
download time etc. It is recommended to connect other USB devices
through USB hub (e.g. the mouse) and to provide the camera a direct
USB connection to the host PC.
The G3 and G4 cameras need an external power supply to operate. It is not
possible to run the camera from the power lines provided by the USB cable,
which is common for webcams or very simple imagers. G3 and G4 CCD
cameras integrate highly efficient CCD chip cooling, shutter and possibly filter
wheel, so their power requirements significantly exceed USB line power
capabilities. On the other side separate power source eliminates problems with
voltage drop on long USB cables or with drawing of laptop batteries etc.
Also note the camera must be connected to some optical system (e.g. the
telescope) to capture images. The camera is designed for long exposures,
necessary to acquire the light from faint objects. If you plan to use the camera
with the telescope, make sure the whole telescope/mount setup is capable to
track the target object smoothly during the exposure.
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Camera Technical Specifications
G3 and G4 series of CCD cameras are manufactured with two kinds of CCD
detectors:
● G3 and G4 cameras with Kodak KAF Full Frame (FF) CCD
architecture. Almost all Full Frame CCD detector area is exposed to
light. This is why these detectors provide very high quantum
efficiency. FF CCD detectors, intended for research applications, are
not equipped with so-called Anti Blooming Gate (ABG – a gate,
which prohibits blooming of the charge to neighboring pixels when
image is over-exposed) to ensure linear response to light through the
whole dynamic range. FF CCD detectors used for astrophotography
are equipped with ABG to eliminate disrupting blooming streaks
within field of view.
Cameras with Full Frame, non-ABG detectors are suitable for
scientific applications, where linear response is necessary for
photometric applications in astronomy, microscopy etc. High quantum
efficiency could be used also for narrow-band imaging, where
overexposure is a rare exception, and for imaging of small objects
without a bright star in the field of view.
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Illustration 1: “Full Frame” CCD schematic diagram
● G3 cameras with Kodak KAI Interline Transfer (IT) architecture
(G4 cameras are not produced with IT detectors). There is a shielded
column of pixels just beside each column of active pixels on these
detectors. The shielded columns are called Vertical registers. One
pulse moves charge from exposed pixels to shielded pixels on the end
of each exposure. The the charge is moved from vertical registers to
horizontal register and digitized in the same way like in the case of
Full Frame detectors. This mechanism is also known as “electronic
shuttering”, because it allows very short exposures and also
digitization of the image without mechanically shielding of the
detector from incoming light.
Also G3 cameras with IT CCDs are equipped with mechanical shutter,
because electronic shutter does not allow dark-frame exposures,
necessary for proper image calibration etc.
The price for electronic shutter if lower quantum efficiency
(sensitivity) of IT detectors compared to FF ones. Also all IT detectors
are equipped with ABG, so they can acquire images of very bright
objects without charge blooming to neighboring pixels.
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Illustration 2: “Interline Transfer” CCD schematic diagram
G3 camera models:
Model G3-6300 G3-1000 G3-11000 G3-11000C
CCD chip KAF-6303E KAF-1001E KAI-11002 KAI-11002
Resolution
3072×2048 1024×1024 4032×2688 4032×2688
Pixel size
9×9 µm 24×24 µm 9×9 µm 9×9 µm
CCD area
27.7×18.4 mm 24.6×24.6 mm 36.3×24.2 mm 36.3×24.2 mm
ABG No No Yes Yes
Color mask No No No Yes
G4 camera models:
Model G4-9000 G4-16000
CCD chip KAF-09000 KAF-16803
Resolution
3056×3056 4096×4096
Pixel size
12×12 µm 9×9 µm
CCD area
36.8×36.8 mm 36.9×36.9 mm
ABG Yes Yes
Color mask No No
Cameras with “C” suffix contains CCD detector covered with so-called Bayer
mask. Color filters of three basic colors (red, green, blue) cover all pixels, so
every pixels detects only light of particular color.
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These cameras are able to acquire color image in single exposure, without the
necessity to change color filters. On the other side color mask brings lower
sensitivity and limits the capability to perform exposures using narrow-band
filters etc.
Because each pixel is covered by one of three basic color filters, it is necessary
to compute (interpolate) remaining two colors for each pixel, which of course
limits resolution of color image. Imaging using color detectors is described in
the “Color images” chapter.
CCD Chip
Quantum efficiency (sensitivity) of CCD detectors used in G3 and G4 cameras
depends on the particular camera model.
Inherent dark current of these detectors is quite low compared to other CCD
detectors, suitable for scientific applications, which results into very good
signal/noise ratio.
Model G3-6300
G3-06300 uses 6 MPx Kodak KAF-6303E Class 1 or 2 CCD chip.
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Illustration 3: Quantum efficiency of Kodak CCD detectors used in G3 and G4 cameras
Resolution
3072×2048 pixels
Pixel size
9×9 µm
Imaging area
27,7×18,4 mm
Full well capacity Approx. 100 000 e-
Output node capacity Approx. 220 000 e-
Dark current 1 e-/s/pixel at 0°C
Dark signal doubling 6.3 °C
Model G3-1000
G3-01000 model uses 1 MPx Kodak KAF-1001E Class 1 or 2 CCD chip.
Resolution
1024×1024 pixels
Pixel size
24×24 µm
Imaging area
24.6×24.6 mm
Full well capacity Approx. 220 000 e-
Output node capacity Approx. 650 000 e-
Dark current 17 e-/s/pixel at 0°C
Dark signal doubling 5.5 °C
Model G3-11000
G3-11000 uses 11 MPx CCD Kodak KAI-11002 Class 1 or 2.
Resolution
4032×2688 pixels
Pixel size
9×9 µm
Imaging area
36,3×24,2 mm
Full well capacity Approx. 60 000 e-
Dark current 12 e-/s/pixel at 0°C
Dark signal doubling 7 °C
Model G3-11000C
G3-11000C uses 11 MPx CCD Kodak KAI-11002 Class 1 or 2 with color
(Bayer) mask.
11
Model G4-9000
G4-9000 uses 9 MPx CCD Kodak KAF-09000.
Resolution
3056×3056 pixels
Pixel size
12×12 µm
Imaging area
36,8×36,8 mm
Full well capacity Approx. 110 000 e-
Dark current 0,5 e-/s/pixel at 0°C
Dark signal doubling 7 °C
Model G4-16000
G4-16000 uses 16 MPx CCD Kodak KAF-16803.
Resolution 4096×4096 pixels
Pixel size
9×9 µm
Imaging area 36,9×36,9 mm
Full well capacity Approx. 100 000 e-
Dark current 0,3 e-/s/pixel at 0°C
Dark signal doubling 6,3 °C
Camera Electronics
16-bit A/D converter with correlated double sampling ensures high dynamic
range and CCD chip-limited readout noise. Fast USB interface ensures image
download time within seconds. Maximum length of single USB cable is 5 m.
This length can be extended to 10 m by using single USB hub or active USB
extender cable. Up to 5 hubs or active extenders can be used in one connection.
Third party USB extenders allow extending of one USB connection up to
100 m.
ADC resolution 16 bits
Sampling method Correlated double sampling
Read modes Standard
Low-noise
Horizontal binning 1 to 4 pixels
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Vertical binning 1 to 4 pixels
Sub-frame readout Arbitrary sub-frame
TDI readout Only for KAF-equipped cameras
Computer interface USB 2.0 high-speed
USB 1.1 full-speed compatible
Binning can be combined independently on both axes.
Image download time and system read noise depends on the CCD chip used in
particular camera model.
Model G3-6300
Gain 1.5 e-/ADU (1×l binning)
2.3 e-/ADU (other binnings)
System read noise 12 e- (LN read)
15 e- (Standard read)
Full frame download 9.8 s (LN read)
8.1 s (Standard read)
Model G3-1000
Gain 3 e-/ADU (1×l binning)
5 e-/ADU (other binnings)
System read noise 12 e- (LN read)
15 e- (Standard read)
Full frame download 2.0 s (LN read)
1.7 s (Standard read)
Model G3-11000
Gain 0.8 e-/ADU (1×l binning)
1.6 e-/ADU (other binnings)
System read noise 11,5 e- (LN read)
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13 e- (Standard read)
Full frame download 16.9 s (LN read)
14.1 s (Standard read)
Model G4-9000
Gain 1.5 e-/ADU (1×l binning)
1.7 e-/ADU (other binnings)
System read noise 10 e- (LN read)
12 e- (Standard read)
Full frame download 16.3 s (LN read)
13.1 s (Standard read)
Model G4-16000
Gain 1.6 e-/ADU (all binnings)
System read noise 9 e- (LN read)
11 e- (Standard read)
Full frame download 26 s (LN read)
22 s (Standard read)
System read noise depends on the particular CCD detector. For instance KAF6303 can be read with 11 e- RMS.
Download times are valid for USB 2.0 host and may vary depending on host
PC.
G4-16000 cameras use the same gain for unbinned and binned reading when
used with system driver version 2.3 and higher. When older driver is used,
stated gain 1.6 e-/ADU is used for 1×l binning only, while all other binning use
2.7 e-/ADU. Gain setting was unified to utilize the full A/D converter dynamic
range, because particular detector output node cannot pass higher output signal
levels either way.
The gain in binning modes of G4-9000 cameras was adjusted from 2.5 e-/ADU
to 1.7 e-/ADU beginning with system driver version 2.3 from the same reasons.
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CCD Chip Cooling
Regulated two-stage thermo-electric cooling is capable to cool the CCD chip up
to 45 °C below ambient temperature. The Peltier hot side is cooled by a fans.
The CCD chip temperature is regulated with ±0.1 °C precision. High
temperature drop and precision regulation ensure very low dark current for long
exposures and allow image proper calibration.
The camera head contains two temperature sensors – the first sensor measures
directly the temperature of the CCD chip. The second one measures the
temperature of the air cooling the Peltier hot side.
The cooling performance depends on the environmental conditions and also on
the power supply. If the power supply voltage drops below 12 V, the maximum
temperature drop is lower.
CCD chip cooling Thermoelectric (Peltier modules)
TEC modules Two stages
Maximal ∆T
45 °C below ambient
Regulated ∆T
40 °C below ambient (75% cooling)
Regulation precision
±0.1 °C
Hot side cooling Forced air cooling (two fans)
Optional heat exchanger for liquid coolant
Maximum temperature difference between CCD and ambient air may exceed
45 °C when the cooling runs at 100% power. However, temperature cannot be
regulated in such case, camera has no room for lowering the CCD temperature
when the ambient temperature rises. The 40 °C temperature drop can be
achieved with cooling running at approx. 85% power, which provides enough
room for regulation.
Power Supply
The 12 V DC power supply enables camera operation from arbitrary power
source including batteries, wall adapters etc. Universal 100-240 V AC/5060 Hz, 60 W “brick” adapter is supplied with the camera. Although the camera
power consumption does not exceed 55 W, the 60 W power supply ensures
noise-free operation.
15
Camera head supply 12 V DC
Camera head power consumption 14 W without cooling
52 W maximum cooling
Adapter input voltage 100-240 V AC/50-60 Hz
Adapter output voltage 12 V DC/5 A
Adapter maximum power 60 W
1. Power consumption is measured on the AC side of the supplied 12 V
AC/DC power supply. Camera consumes less energy from 12 V power
supply than state here.
2. The camera contains its own power supplies inside, so it can be
powered by unregulated 12 V DC power source – the input voltage
can be anywhere between 10 and 14 V. However, some parameters
(like cooling efficiency) can degrade if the supply drops below 12 V.
3. G3 and G4 camera measures its input voltage and provides it to the
control software. Input voltage is displayed in the Cooling tab of the
CCD Camera control tool in the SIPS. This feature is important
especially if you power the camera from batteries.
Warning:
The power connector on the camera head uses center-plus pin. Although all
modern power supplies use this configuration, always make sure the polarity is
correct if you use own power source.
Mechanical Specifications
Compact and robust camera head measures only 154×154×74 mm (approx.
6×6×3 inches). The head is CNC-machined from high-quality aluminum and
black anodized. The head itself contains USB-B (device) connector and
12 V DC power plug, no other parts (CPU box, USB interface, etc.), except a
“brick” power supply, are necessary. Integrated mechanical shutter allows
streak-free image readout, as well as automatic dark frame exposures, which
are necessary for unattended, robotic setups. Integrated filter wheel contains
5 positions for standard 2-inch filter cells with M48×0.75 thread. There are
three M3 threaded holes around each filter position, which allow fixing of
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filters without cells (only a glass) up to 51 mm diameter.
Internal mechanical shutter Yes, blade shutter
Shortest exposure time 0.25 s
Longest exposure time Limited by chip saturation only
Internal filter wheel
(G3 cameras only)
5 positions 2" threaded filter cells or glass
filters up to 51 mm diameter
Head dimensions
154×154×77 mm (G3 with wheel)
154×154×65 mm (G3 without wheel, G4)
Back focal distance 29 mm (G3 with wheel)
17 mm (G3 without wheel, G4)
Camera head weight 1.7 kg (G3 with wheel)
1.5 (G3 without wheel, G4)
Package Contents
G3 and G4 CCD cameras are supplied in the foam-filled, hard carrying case
containing:
● Camera body with a user-chosen telescope adapter. The standard 2"
barrel adapter is included by default for G3 cameras. G4 cameras are
equipped with M68×1 threaded adapter. If ordered, the filter wheel is
already mounted inside the camera head and filters are threaded into
place (G3 cameras only).
If the camera is ordered with external filter wheel, the external filter
wheel is attached to the camera head.
● A 100-240 V AC/50-60 Hz input, 12 V DC/5 A output power supply
adapter with 1.5 m long output cable. The adapter includes AC cable.
17
Illustration 4: 12 V DC/5 A power supply adapter for
G3 and G4 CCD Camera
● 5 m long USB A-B cable for connecting camera to host PC.
Warning:
5 m (approx. 15 feet) is the maximum allowed length of USB cable.
Do not try to connect 5 m USB A-B cable with USB A-A extension
cable to get a longer connection. When the distance between camera
and host PC is longer, USB hub or an active USB extender cable can
be used as a repeater to provide up to 10 m long connection. Thirdparty USB extenders allow USB connection tens or even hundreds
meters long.
● An USB Flash Driver or CD-ROM with camera system drivers,
drivers for third party software packages, SIPS software package with
electronic documentation and PDF version of this manual.
● A printed copy of this manual.
Optional components
Number of optional parts are available for G3 and G4 CCD cameras. These
parts can be ordered separately. Refer to our web site for the pricing, please.
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Filter wheel for five 2 inch threaded filter cells
Because the filter wheel is not necessary for some applications (e.g. in
microscopy), a version of G3 camera without filter wheel is also offered. Make
sure you choose the variant with filter wheel included, even if you plan to use
own filters and you order camera without LRGB filters.
G3 cameras without filter wheel do not allow later installation of the internal
filter wheel. They can be only attached to external filter wheel. It is of course
possible to supply a camera intended for installation of the internal filter
without the wheel, but it is necessary to state such requirement in the camera
order.
It is not possible to install internal filter wheel into G4 camera head. G4
cameras can be used with external filter wheel only.
Another filter wheel can be also ordered separately. Replacing the whole filter
wheel is easier than replacing individual filters. It is possible to thread for
instance LRGB filter set into one wheel, BVRI set into second wheel and
narrow-band filters (Hα, OIII, ...) into the third one.
LRGB filter set
High quality 2" LRGB filter set optimized for CCD imaging. This set contains
high-pass Red, Green and Blue filters plus Luminance filter covering the
combined RGB spectral range, blocking IR and UV portion of spectrum for
maximum color accuracy.
Clear (C) filter
Optional clear filter (optical glass with AR coatings) of the same thickness like
RGB filters can be used in addition to (or instead of) Luminance filter to use
maximum chip QE for luminance images. Optical glass is used instead of
simple empty hole in filter wheel to ensure proper focus and to eliminate
refocusing when changing from filtered to unfiltered exposures.
The Clear filer can be also combined with RGB so it is possible to install
CRGB filter set and to leave one empty filter wheel position.
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UBVRI filter set
Scientific-grade UBVRI filter set with AR coatings in 2" cells. This set
contains scientific Blue, Visual, Red and Infra-red filters for photometric
applications.
Separate filters
Number of 2" filters for narrow-band imaging and other special applications
(UHC, Hα, Hβ, OIII, SII, etc.) are available. Visit our web site for current filter
offering.
Telescope adapters for G3 cameras
The G3 cameras are supplied with standard 2" barrel adapter by default, but the
user can choose any other adapter he/she prefers. Another adapters can be
ordered separately.
It is possible to choose among various telescope/lens adapters:
2" barrel adapter Adapter for 2" focusers.
T-thread short M42×0.75 mm inner thread,
7.5 mm long.
T-thread long M42×0.75 mm inner thread,
preserves 55 mm back focal
distance as defined by
Tamron.
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Pentax (Praktica)
lens adapter
M42×1 mm inner thread,
preserves 46.5 mm back focal
distance.
M48×0.75 thread
adapter
Adapter with inner thread
M48×0.75
Zeiss adapter
M44×1 mm outer thread,
includes fixing nut.
PSB-1100 adapter Threaded adapter for TeleVue
PSB-1100 coma corrector
Canon EOS lens
adapter
Standard Canon EOS bayonet
adapter
Nikon F lens adapter Standard Nikon F bayonet
adapter
T-thread (M42×0.75) adapter or M42×1 adapter cause vignetting when used
with G3-11000. Also common coma correctors (often equipped with T-thread)
cause vignetting with this large chip. E.g. TeleVue Paracorr PSB-1100 with
appropriate adapter must be used with KAI-11000 CCD.
If the mounting standard defines also back focal distance (distance from adapter
front plane to detector), the particular adapter is constructed to preserve defined
21
distance (for instance T-thread defines back focal distance to 55 mm, but
certain distance is defined also for Pentax (Praktica) thread, for Canon EOS and
Nikon bayonets etc.).
Adapters are attached to the camera body using four M3 (3 mm metric) screws.
These threaded holes are placed on the corners of 44 mm square. Custom
adapters can be made upon request. A nut with the same thread is screwed
Telescope adapters for G4 cameras
The G4 cameras are supplied with standard M68×1 threaded adapter. This
adapter consists of two parts. The first part, attached to the camera head, offers
4.5 mm long outer thread M68×1. The second part is a 12 mm thick nut with
inner M68×1 thread, screwed to the outer thread. So the user can choose to use
outer thread (without a nut) or 7.5 mm of inner M68×1 thread (when the nut is
screwed in). The back focal distance is of course longer in the second case.
Illustration 5: M68×1 adapter for G4
camera with a nut
Only other option for G4 camera is Canon EOS lens adapter, all other standards
(2” barrel, T-thread, …) cause vignetting in the case of G4 cameras, because
the detectors used measure 52 mm diagonally.
M68×1 adapter
Adapter with both inner and
outer M68×1 thread
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Canon EOS lens
adapter
Standard Canon EOS bayonet
adapter
Adapters are attached to the camera body using four M3 (3 mm metric) screws.
Threaded holes on the camera body are placed on the corners of 52 mm square.
Custom adapters can be made upon request.
G2 and G3 cameras use smaller 44 mm threaded hole square on the camera
head for mounting adapters, so adapters for G2/G3 series and G4 series are not
compatible.
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Getting Started
Although the G3 and G4 cameras are intended for operation at night (or for
very low-light conditions at day), it is always better (and highly recommended)
to install software and to make sure everything is working OK during day,
before the first night under the stars.
The G3 and G4 CCD cameras can be in principle operated under various CCD
control software packages (refer to our web site for available drivers), this
manual demonstrates camera operation under the SIPS (Scientific Image
Processing System) – camera control and image processing software suite
supplied with the camera.
Camera System Driver Installation
Every USB device requires so-called “system driver”, incorporated directly into
the operating system kernel. Some devices (for instance USB Flash Disk
dongles) conform to some predefined class (USB mass-storage device class in
this case), so they can use the driver already present in the operating system.
But this is not the case of the G3 and G4 CCD camera – it requires its own
system driver to be installed.
Although 64 bit operating system can run 32 bit application without any
problems, it is basically impossible to combine e.g. 64 bit process with 32 bit
dynamic link library. The same is valid for operating system kernel - 64 bit
kernel cannot use 32 bit system driver. This is why all G3 and G4 camera
drivers are supplied in two versions, one for 32 bit systems (marked x86
according to Intel 386, 486 CPUs) another for 64 bit systems, marked x64
(according to CPUs supporting 64 bit instructions marked x86-64 or only x64).
The simplest way to install camera system driver is to utilize Plug-and-play
nature of USB devices and Windows operating system. Simply plug the camera
to power supply and connect it to the host PC using the included USB A-B
cable. Windows detects new USB device and opens hardware installation
wizard. The system driver installation is slightly different on different Windows
systems.
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Windows 7 System Driver Installation
Windows 7 does not offer users the possibility to install system drivers using
Plug-and-Play, like in the case of older Windows 2000, Windows XP and
Windows Vista. It is necessary to pre-install all drivers, else the operating
system only informs user that it cannot find appropriate driver for newly
connected device.
We can only estimate reasons for this limitation of system functionality,
probably it has something common with the inability of many hardware
vendors to provide drivers complying to Plug-and-play standards (notification
requiring installation of the software first and plugging of the device later was
present on many devices).
Although the Plug-and-Play mechanism is hidden in Windows 7, it is possible
to use it. Newly connected device appears in the “Device Manager” as
“Unknown Device” (such device is usually marked by a question mark on
yellow background icon). It is enough to click on such device by right mouse
button to invoke pop-up menu and choose “Update Driver...” menu item.
Operating system then opens driver installation wizard, basically identical to
the one in Windows XP and Windows Vista.
Let us note that 64 bit versions of Windows 7 and Windows Vista require
digitally signed drivers. Drivers without digital signature cannot be installed on
these systems.
All G3 camera drivers supplied by Moravian Instruments are digitally signed
from the beginning of the year 2010.
Windows XP and Windows Vista System Driver Installation
The operating system notifies the new USB device was plugged in the “Found
new hardware bubble”. The system then opens the “Found New Hardware”
Wizard.
1. The wizard offers searching for suitable driver on Windows Update
site. Reject this offer (choose “No, not this time”) and click “Next”
button.
2. Choose the “Install the software automatically” in the next step.
Insert the CD-ROM into the drive and the wizard will continue by the
25
next step.
It is not necessary to install files from CD-ROM. It is possible to copy
the folder containing driver files e.g. to shared network volume, USB
Flash Disk etc. Then it is necessary to choose the “Install from a list or
specific location” and to define the path to driver files.
3. The wizard starts to copy files. But Windows XP checks for driver file
digital signature. If it cannot find the signature, it notifies the user by a
message box. Click “Continue Anyway”, the digital signature is only
an administrative step and does not influence the proper functionality.
4. The wizard then finishes the installation and the camera is ready to
work.
Windows 2000 System Driver Installation
The operating system notifies the new USB device was plugged in the “Found
new hardware” message box. The system then opens the “Found New
Hardware Wizard”. Click the “Next” button to start the installation.
1. Choose the “Search for a suitable driver for my device” and click
“Next” button.
2. Drivers are supplied on the CD-ROM, choose “CD-ROM drives” and
continue by clicking “Next” button.
It is not necessary to install files from CD-ROM. It is possible to copy
the folder containing driver files e.g. to shared network volume, USB
Flash Disk etc. Then it is necessary to choose “Specify a location” and
define the path to driver files.
3. The Wizard notifies it found proper driver and installs it. The
installation is finished and the camera is ready to work.
Please note the Windows system keeps the information about installed devices
separately for each USB port. If you later connect G3 camera to different USB
port (different USB connector on the PC or through the USB hub), Windows
reports “found new hardware” again and asks you to install the software.
Repeating the installation again brings no problem, just choose “Install
automatically” option and Windows will reuse already installed drivers.
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SIPS Software Installation
The Scientific Image Processing System (SIPS) software package is designed
to operate without the necessity to be installed in any particular folder. The
package can be even run directly from USB Flash Drive or CD-ROM.
SIPS package is distributed in the two forms:
1. On the USB Flash Drive or CD-ROM supplied with the camera. There
is a folder named “SIPS” on the storage device, which contains the
unpacked version of the software. It is enough to just copy the folder
to any place on the hard drive the user chooses.
2. In the form of zipped archive “SIPS.zip”, which can be downloaded
from the web site. Similarly, it is enough to un-zip the archive file to
any folder the user chooses.
Un-installing of the SIPS is also quite easy – just delete the SIPS folder.
No matter how is the package installed, the software is run by launching the
“SIPS.exe” main program file.
SIPS configuration files
The software package distinguishes two types of configuration:
● Global configuration, common for all users.
● User-specific configuration.
Global configuration defines which hardware is used and which drivers
controls it. The configuration is stored in the simple text file “SIPS.ini”, which
must be placed in the same folder as the “SIPS.exe” main executable. The file
may look for example like this:
[Camera]
g1ccd = g1ccd.dll
g2ccd2 = g2ccd2.dll
g3ccd = g3ccd.dll
[GPS]
GarminUSB = gps18.dll
NMEA = nmeagps.dll
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[Telescope]
NexStar = nexstar.dll
LX200 = lx200.dll
Individual sections define which driver would be loaded and asked to
enumerate all connected devices of particular type (CCD cameras, GPS
receivers, telescope mounts).
SIPS package already contains this file containing all included drivers. This file
is not modified programmatically, it is necessary to edit it manually if new
device driver, not included into basic package, is installed.
User-specific configuration is stored in the file named also “SIPS.ini”, but this
file is placed in the “\Documents and Settings\%user_name%\Application
Data\SIPS\” folder. Number of setting is stored in this text file, beginning from
the position and open state of individual tool windows, to the preferred
astrometry catalog and parameters for searching stars in images.
G3 and G4 CCD Camera Driver for SIPS
SIPS is designed to work with any CCD camera, providing the driver for the
particular camera is installed. The driver for G3 CCD camera is include into the
basic SIPS package and is not necessary to install it separately.
Every CCD camera driver for SIPS (including the G3 and G4 CCD drivers) is
required to provide information about available filters (if the particular camera
has the integrated filter wheel, of course). But the user can order camera with
various filters, or he or she can change individual filters or the whole filter
wheel etc. There is no way how to determine the actual filters in the filter wheel
automatically. This is why the G3 and G4 CCD camera driver for SIPS reads
the “g3ccd.ini” file to determine actual configuration of filters, which will be
then reported to SIPS.
Let us note that G3 and G4 CCD cameras are software compatible so they use
the same driver g3ccd.dll as well as driver initialization file.
The “g3ccd.ini” file is placed in the same directory where the camera driver
and the SIPS itself is installed. This file is ordinary text file following the .INI
files conventions. Here is the example of the “g3ccd.ini”:
[filters]
Luminance, Gray
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Red, LRed
Green, LGreen
Blue, LBlue
Clear, 0
The file should contain just one section “[filters]” (other sections, if present, are
ignored). Every line in this section defines one filter position. The first string
defines the filter name, which will be displayed in various parts of the SIPS
GUI or will be part of file name of acquired images (if the user chooses
including of filter name to file name). The second parameter, delimited by
comma, represents a color by which the string is displayed. The color can be
expressed by a name (White, Red, LRed, etc.) or directly by number
representing the particular color (0 represents black).
The above mentioned information will be displayed e.g. in the filter-choosing
combo-box this way:
If there are more filters in the camera than the configuration file describes,
another filters will be added with undefined name. And if the configuration file
describes more filters than the number of filter in the camera, last descriptions
will be omitted.
Using of multiple configuration files for different cameras
It is sometimes necessary to work with multiple cameras, sharing single driver,
on single computer (whole G2 revision 3+, G3 and G4 series of CCD cameras
are software compatible, so they use the same driver g3ccd.dll). If multiple
cameras have different filter wheels with different filters, it is rather
complicated to adopt g3ccd.ini configuration file to currently connected
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Illustration 6: Filters
offered by the CCD
Camera tool
camera. If there are multiple cameras connected at once, adopting of
configuration file is not possible.
This is why SIPS camera drivers (and also camera drivers for other programs)
introduced enhanced naming convention of driver configuration file. Every Gx
series camera has unique identification number, stated on the camera shell (this
number is also displayed in the list of all connected cameras in the SIPS “CCD
Camera” tool). Camera driver tries to open configuration file, which name is
extended with the camera ID number. If for instance camera ID is 1234, driver
first tries to open configuration file named “g3ccd.1234.ini”. Only is such file
does not exists, general configuration file “g3ccd.ini” is used. So it is possible
to create separate configuration files describing filters in every connected
camera.
Cropping of the CCD area
G3 and G4 camera drivers are able to crop the image matrix even before the
image is passed to SIPS. Although it is possible to define sub-frames directly in
SIPS camera control tool, limiting camera resolution this way is not very
convenient when multiple frames of different types (light, dark, flat) are
acquired. If for instance the user wants to use only center area of a large CCD
because the optics used cannot utilize such large CCD detector, it is possible to
read only a sub-frame (sub-frame 512, 512, 3072, 3072 converts 16Mpx G416000 camera into 9MPx camera). But different sub-frame is used e.g. when
focusing the camera and it is necessary to properly restore above mentioned
subframe before each dark, light of flat field is acquired. And 1 pixel difference
between light and dark frame harms the possibility to properly calibrate images.
This is why the 'g3ccd.dll' driver allows definition of sub frame in the
appropriate .ini file in the [crop] section:
[crop]
x = 512
y = 512
w = 3072
h = 3072
Such camera will report resolution 3072×3072 pixels to SIPS and all other subframes, defined in the SIPS camera control tools, will be related to the above
defined subframe.
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Camera Connection
Camera connection is pretty easy. Plug the power supply into the camera and
connect the camera to the computer USB port using the supplied USB cable.
Note the computer recognizes the camera only if it is also powered. Camera
without power act the same way as the unplugged one from the computer point
of view.
When the camera is powered and connected to the computer (with appropriate
drivers installed), it starts to initialize filter wheel. The internal filter wheel
starts to rotate and the camera control unit searches for the filter wheel home
position. This operation takes a few seconds, during which the camera does not
respond to computer commands. Camera indicates this state by flashing the
orange LED. See the “Camera LED state indicator” chapter for details.
The camera is fully powered by the external power supply, it does not use USB
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Illustration. 7: Power connector (right) and USB connector (left) on the bottom side
of the camera head
cable power lines. This means it does not draw laptop batteries and long USB
cables with thin power lines (which can cause voltage drops and power-related
problems for USB-powered devices) does not affect the G3 camera operation.
Camera LED state indicator
There is a two-color LED on the camera body, close to the USB connector. The
LED is functional only upon camera startup not to influence observations.
The LED starts blinking orange when the camera starts to initialize filter wheel.
Orange blinks are not always the same – they depend on the filter position
when the camera is powered up.
If the case the camera control unit cannot find the filter wheel origin, the
camera notifies the user by 2 s long red flash immediately after filter
initialization failed (orange blinking terminates). Please note the while filter
wheel initialization is skipped by the firmware when the camera is supplied
without the filter wheel. So if you notice orange blinks followed by 2 s red
blink, the filter wheel failed to initialize. Although the camera continues
operation like the model without filter wheel, it is not recommended to start
work with such camera – it is not clear which filter is behind the CCD or the
wheel can be in the inter-filter position. Return the camera to manufacturer for
maintenance in such case.
Camera firmware finishes initialization by signaling the USB speed, on which
it is currently operating.
● USB 2.0 High Speed (480 Mbps) is signalized by 4 short green blinks.
● USB 1.1 Full Speed (12 Mbps) is signalized by 4 short red blinks.
Working with Multiple Cameras
It is possible to connect multiple CCD cameras to single computer, be it
directly to USB ports available on the computer I/O panel or through the USB
hub. The operating system assigns unique name to every connected USB
device. The name is rather complex string derived from the device driver
GUID, USB hub identifiers, USB port number on the particular hub etc. Simply
put, these identifiers are intended for distinguishing USB devices within
operating system, not to be used by computer users.
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Illustration 8: Camera Id number is displayed in brackets after
camera name in SIPS
But the user always needs to distinguish individual cameras – for instance one
camera should be used for pointing, another for imaging. This is why every
camera has assigned unique identifier (ID number). This number is printed on
the sticker on camera body and it is also displayed in the list of all available
cameras in the CCD Camera tool in SIPS. This enables the user to select the
particular camera he or she needs.
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Camera Operation
Camera operation depends on the software used. Scientific cameras usually
cannot be operated independently on the host computer and G3 and G4 CCD
also needs a host PC (with properly installed software) to work. Camera itself
has no displays, buttons or other controls. On the other side, every function can
be controlled programmatically, so the camera is suitable for unattended
operation in robotic setups.
Plug the camera into computer and power supply and run the SIPS program.
Open the “CCD Camera” tool (choose the “Tools” menu and click the “CCD
Camera...” item or click the tool button). The camera name (e.g. “G311000”) should be displayed in the title bar of the tool window.
If you run the SIPS before the camera was plugged and powered, SIPS does not
know about it and it is necessary to scan for available cameras. Select the
“Camera” tab and press the “Scan Cameras” button. Connected camera should
appear in the displayed tree. Select it (click its name by mouse – its name
should be highlighted) and press “Select Camera” button.
If the camera does not appear in the tree of available cameras, check the
following items:
1. Check the USB cable – make sure both connectors are properly
inserted to PC (or USB hub) and to camera head.
2. Check the camera power – the power adapter should be plugged to AC
source (the green LED on the adapter should shine) and the power
output cable connector must be properly inserted to camera head
connector.
3. Check if the camera system driver is properly installed. Refer to the
“Camera System Driver Installation” chapter for information about
system driver installation.
Camera and the Telescope
The camera needs some optical system to capture real images. It depends on the
telescope adapter to which telescopes (or lenses) the camera can be connected.
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Standard 2" barrel adapter allows camera connection to vast majority of
astronomical telescopes.
2" barrel adapter can cause vignetting (partial shielding of detector corners),
especially if a fast optical systems is used with the G3-11000 camera with
24×36 mm CCD detector. Also T-thread (M42×0.75) or Praktica (M42×1)
adapters cause even bigger vignetting. G3 cameras can use threaded adapter
with 2.156-24UN thread, designed for TeleVue Paracorr PSB-1100 coma
correctors (thread diameter is approx. 55 mm). Standard adapters for Canon
EOS or Nikon do not cause vignetting even with large chip.
Illustration 9: G3 CCD camera connected to telescope focuser
Photographic lens or some small refractor is the best optical system to start
experimenting with the camera. If you are using some bigger telescope at home
for the first experiments, make sure the telescope can be focused to relatively
nearby objects in the room.
It is better to start experimenting at night, because it is very easy to saturate the
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camera at daylight. The shortest exposure of G3 or G4 cameras is around
0.25 s, which can be too long at daylight conditions.
The following chapters provide only a brief description of camera operation
under SIPS (Scientific Image Processing System) program, supplied with the
camera. Refer to the SIPS User's Guide (click “Help” and “Contents” from the
SIPS main menu) for thorough description of all SIPS features.
Temperature Control
Active chip cooling is one of the basic features of scientific CCD cameras
(SIPS User's Guide explains why cooling is important to reduce thermal noise).
If you plug the camera to power supply, you may notice the fans on the back
side of the camera head start operation. These fans take away the heat from the
hot side of the Peltier modules, which cool the CCD chip. Fans are running
continuously, independently on the Peltier cooler (they are also used to cool
down the camera power supplies etc.).
Peltier cooler can be controlled from the “Cooling” tab of the SIPS CCD
Camera tool.
Although the Cooling tab displays number of values and graphs, only two
values can be modified by the user. The “Set Temperature” count-box defines
required CCD chip temperature and the “Max. dT” count-box defines the
maximum speed, with which the temperature can change. If the required
temperature is greater or equal to the current CCD chip temperature, the Peltier
cooler is off. The “Cooling utilization” indicator displays 0% and the camera
consumes minimum energy.
To cool down the CCD chip, set the required temperature to target value.
Camera does not switch the Peltier cooler to 100% immediately, but starts
changing of the target temperature according to defined maximal speed. The
target temperature is displayed in cyan color on the graph. The current chip
temperature is displayed in red. Also notice the blue line, which displays the
cooling utilization – it starts to grow from 0% to higher values.
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Also notice the yellow line in the graph – it displays camera internal
temperature. This temperature also somewhat grows as the cooling utilization
grows. The hot air from the Peltier hot side warms up the camera interior
slightly.
How fast can be the chip cooled? Can be the chip damaged, if it is cooled too
fast? Unfortunately the maximum speed of temperature change is not defined
for Kodak CCD chips (at last the author does not know about it). But in general
slow temperature changes cause less stress to electronic components than rapid
changes. The SIPS temperature change speed default value is 3 °C per minute.
It is usually no problem to switch the camera earlier and to provide time for
slow cooling. However, if it is necessary to cool the camera rapidly, alter the
“Max. dT” value.
It is also easier to achieve higher temperature differences if the temperature is
changed only slowly. Switching the Peltier cooler from zero to 100%
immediately provides a lot of heat and, especially in the case of air-cooled
Peltier, the overall camera temperature can raise more than necessary. The
result is the chip temperature is higher in absolute numbers, because the hotside temperature is also higher. It takes long time before the hot side slowly
37
Illustration 10: “Cooling” tab of the CCD Camera Control tool
settles.
What is the best temperature for the CCD chip? The answer is simple – the
lower the better. But the minimum temperature is limited by the camera
construction. The G3 and G4 cameras are equipped with two-stage cooler,
which can cool the chip more than 45 °C below ambient temperature with air
cooling. But it is not recommended to use maximum possible cooling. If the
environment conditions change, the camera may be unable to regulate the
temperature if the environment air temperature rises. Set the target temperature,
which requires approx. 85% of the cooling utilization. This provides enough
room for e.g. environment temperature changes etc.
The power supply voltage is also displayed in the “Cooling” tab. Especially
when the camera is powered from 12 V battery, this information can be used to
estimate when the battery should be replaced and recharged. Note that working
with less intensive cooling can significantly prolong the battery life.
First Images
Actual exposure is performed from the “Exposure” tab of the CCD Camera
tool.
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Illustration 11: Exposure tab of the CCD Camera Control tool
It is necessary to define few parameters before the first shot. First, it is
necessary to define the image type – choose “Light” from “Exposure” combo
box. Then choose the exposure time. If you experiment with exposures in the
dark room with a camera connected to some small refractor, start with
1 second. Do not forget to review the image handling options on the right side
of the “Exposure” tab. Let the “Open new Light image window” and
“Overwrite image in selected window” check-boxes checked, uncheck other
options for now (we do no plan to save our first images).
Then click the “Start Exposure” button. Camera will open the shutter, perform
1 s exposure, close the shutter and download the image. Image is then opened
in new image window. If this is the first shot, it will probably be far from sharp
focused image. Alter the focuser and try again.
Notice that options determining the new image handling on the right side of this
tab changes with every change of the exposure type. SIPS remembers these
options for every exposure type separately. So it is possible e.g. to define
separate folders for dark frames and for flat fields.
Always check whether new image processing options are defined properly
39
before you start any exposure.
If you choose “Dark” from “Exposure” combo box (remember the image
handling options on the right side changes – make sure they are properly
defined), image will be captured without opening the shutter. The captured
image will represent the thermal noise, generated by the CCD chip itself,
combined with the CCD chip and camera electronics read noise. Such images
are subtracted from normal images during image calibration to reduce the dark
current effects.
Brightness and Contrast – Image Stretching
The G3 and G4 CCD camera dynamic range spans 65 536 levels. But only
imaging of perfectly illuminated and perfectly exposed scenes can result in
images with pixels spanning this range. Usually only a fraction of this range is
used, e.g. the black background can have values around 500 counts and the
brightest part of the image can have around 10 000 counts. If we assign the
black to white range to the full possible range (0 to 65 535), the image with 500
to 10 000 counts will be displayed only in dark gray tones. This is why image
brightness scale should be “stretched” before they are displayed.
Open the “Histogram and Stretch” tool .
Illustration 12: Histogram and Stretch tool
The exact meaning of the histogram chart is explained in the SIPS software
documentation. Now only try to play with “Low” and “High” count-boxes or
better with the related horizontal sliders. Observe how the image view is
changed when you alter these values.
The best positions of Low and High control are as follows: the Low count
40
should be on the count value representing black on the image. Any pixel with
value lower than this count will be displayed black. The High count should be
on the count value representing white on the image. Any pixel with value
higher than this count will be displayed white.
Similar adjustments are usually called brightness and contrast adjustments.
● Brightness is changed by moving both Low and High values together
up and down. Try to move both values using the second slider below
the histogram chart.
● Contrast is changed if the relative distance between Low and High
values changes. Try to narrow or widen the distance between Low and
High values.
But astronomers often need precise control of Low and High values so the
terms brightness and contract are not used within SIPS.
Calibration
If you preform short exposure of bright object, the signal to noise ratio of the
image is very high. Image artifacts related to CCD chip (like hot/cold pixels or
thermal noise) almost do not affect the image. But all unwanted effects of
unevenly illuminated field, CCD thermal noise etc. significantly degrade image
quality when imaging dim deep-sky objects for many minutes.
This is why every CCD image should be calibrated. Image calibration basically
consists of two steps:
1. Dark frame subtraction
2. Applying flat field
Image calibration is supported by the “Calibration” tool in SIPS .
The raw image downloaded from the camera contains not only the information
desired (the image of the target field), but also CCD chip thermal noise and
artifacts caused by unevenly illuminated field (vignettation), shadows of dust
particles on camera cover glass and filters etc.
41
Illustration 13: The raw image
downloaded from the camera
The Dark frame is taken with the same exposition time at the same CCD chip
temperature. Because hot pixels are less stable than normal pixels, it is always
better to take more dark frames (at last 5) and to create resulting dark frame as
their average or better median.
Illustration 14: The dark frame
corresponding to the above raw image
Illustration 15: The raw image with
subtracted dark frame
Subtraction of the dark frame eliminated majority of thermal noise, but
unevenly illuminated field is still obvious. Image center background is much
brighter than the border parts.
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Illustration 16: Flat field represents the
telescope/camera response to uniformly
illuminated field
Illustration 17: Fully calibrated image
with dark frame subtracted and applied
flat field
CCD image calibration is described in detail in the SIPS User's Guide. Refer to
the “Introduction to CCD Imaging” and “Calibrate Tool” chapters for
calibration description in theory and in practice.
Color Images with monochrome camera and filters
Color images are definitely more appealing than black and white ones. It is also
easier to gather more information from color images – for instance it is possible
to distinguish which part of the nebula is emission (red) and which is reflection
(blue). But astronomical cameras are only rarely equipped with color CCD
chips from number of reasons. The color and monochrome chips are discussed
in the SIPS User's Guide – refer to the “Introduction to CCD Imaging” chapter.
Although the G3 camera is equipped with monochrome CCD chip, it is
definitely capable to capture color images, at last when the internal filter wheel
contains RGB filters. Instead of shooting single color image, three images –
each for Red, Green and Blue colors, must be obtained and combined. This
process is not suitable for fast moving/changing objects, but astronomical
objects usually do not change so fast.
Taking three images and combining them is undoubtedly more complex
procedure than shooting simple color image. But using of monochrome chip
brings so important advantages for astronomical usage, that bothering with
multiple images is definitely worth the effort:
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● Color CCD chips have one fixed set of filters without the possibility to
exchange them or to completely remove them. Monochrome chip is
capable to take images with narrow-band filters like Hα, OIII, etc.
● Color chips have less Quantum Efficiency (QE) then monochrome
ones. Limiting QE from around 80% to around 30% by color filters
only wastes light in number of applications.
● Interpolation of pixel luminance from surrounding pixels, necessarily
performed when processing images from color chips, introduces
significant error and prohibits precise measurement of position
(astrometry) and brightness (photometry).
● Color CCD chips do not allow reading of binned images.
● Color CCD chips do not allow so-called Time Delay Integration (or
Drift-Scan Integration).
Another huge advantage of monochrome chip is the possibility to combine
color images from three color images and one luminance image. Luminance
image is captured without filter, using maximum chip sensitivity. This
technique is often called LRGB imaging.
Inserting the color filter into the light path significantly reduces the amount of
light captured by the chip. On the other side the human eye is much less
sensitive to changes of color than to changes of brightness. This is why the
CCD chip can be binned when capturing color images to 2×2 or 3×3 to
significantly increase its sensitivity. Luminance image is taken without binning
so the image resolution is not degraded.
Let us note that imaging through separate color filters is close to impossible in
some cases. For instance taking images of some fast evolving scenes, like
planet occultation by Moon, imaging of fast moving comet etc. There is no time
to take separate exposures through filters, because the scene changes between
individual exposures. Then it is not possible to combine red, green and blue
images into one image. In such cases using a single-shot color camera is
necessary.
The color images can be combined in the (L)RGB Add Tool in SIPS. This
tool is thoroughly described in the SIPS User's Guide.
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Illustration 18: “(L)RGB Image Add“ tool in SIPS...
Illustration 19: ...and a resulting image
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If we take images for individual colors and also luminance image, possibly with
different binning and exposure times, the calibration starts to be relatively
complex. We need dark frame for every exposure time and binning. We need
flat field for every filter and binning. We need dark frames for every flat field.
This is the price for beautiful images of deep-sky wonders.
Color images with color camera
Single-shot color cameras use special CCD detectors with red, green and blue
color filters applied directly on individual pixels. G3 cameras can be equipped
with such detectors (the name of the camera is then followed by the letter “C”
to indicate color CCD).
Every pixel receives light of particular color only (red, green or blue). But color
image consists of pixels with all three colors specified. So it is necessary to
calculate other color from the values of neighboring pixels.
Covering pixels with such color mask and subsequent calculations of remaining
colors was invented by Mr. Bayer, engineer working at Kodak company. This
is why this color mask is called Bayer mask and the process of calculation of
missing color is called Debayer processing.
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Illustration 20: Schematic diagram of color CCD detector
There are several algorithms for calculating missing color components of
individual pixels – from simply using of color from neighboring pixels (this
method provides quite coarse images) to more accurate methods like bilinear or
bicubic interpolation. There are even more sophisticated algorithms like pixel
grouping etc.
No G3 camera performs the Debayer processing itself. The raw image is always
passed to the host PC and processed by control software. It is also possible not
to perform Debayer filtering and save images in the raw form for processing by
some other software packages.
SIPS software implements bilinear Debayer interpolation. It is possible to
perform Debayer processing immediately when the image is downloaded from
the camera (color image is then immediately displayed and/or saved and no raw
monochrome image is shown) or to perform this processing anytime later.
Debayer processing can be performed from “Image Transform” tool (to open
this tool click button in the tool-bar or choose “Image Transform” from the
“Tools” menu). Check box “Debayer new images” allows immediate Debayer
processing of images downloaded from the camera. The button performs
Debayer processing of currently selected image.
The Bayer mask displayed on the schematic image above begins with blue
pixel. But there are no rules specifying the color of the first pixel – in principle
there can be also green pixel from the blue-green line on the upper-left corner
as well as green pixel from the green-red line or red pixel.
There is no way how to determine the Bayer mask organization from the image.
This is why the “Image Transform” tool provides two check-boxes called
“Bayer X odd” and “Bayer Y odd”. Combination of these check-boxes allows
specification of Bayer mask organization on the particular CCD.
State of “Bayer X odd” and “Bayer Y odd” check-boxes are always updated
when you connect camera with color CCD according to the information
provided by the driver. Is is necessary to update them manually only if the raw
color image is loaded from the disk file and needs to be processed without
connected camera.
Wrong definition of these two flags results in wrong color calculation. Proper
settings can be easily determined by the try-and-error method. But Debayer
processing discards the original raw image so it is always necessary to backup
47
the original raw image.
Also please note the settings of the “Bayer X odd” and “Bayer Y odd” check
boxes must be altered when any geometric transformations are applied to the
raw image (e.g. mirroring, rotation, etc.). Some transformations (e.g. soft
binning or resampling) cannot be performed on raw image at all. It is always
better to Debayer images first and process them later.
Also note that stacking of raw color images results in loss of color information.
Stacking algorithms align images regardless if the particular pixel is red, green
or blue. SIPS allows also sub-pixel stacking, which can mix pixels of different
colors. Images must be Debayer processed first and then stacked.
Balancing colors
CCD chip sensitivity to red, green and blue light is different. This means the
exposure of uniformly illuminated white surface does not produce the same
signal in pixels covered with different color filters. Usually blue pixels gather
less light (they have less quantum efficiency) then green and red pixels. This
results into more or less yellowish images (yellow is a combination of red and
green colors).
The effect described above is compensated by so-called “white balancing”.
White balancing is performed by brightening of less intensive colors (or
darkening of more intensive colors) to achieve color-neutral appearance of
white and/or gray colors. Usually is one color considered reference (e.g. green)
and other colors (red and blue) is lightened or darkened to level with the green.
Automatic white balancing can be relatively easy on normal images, where all
colors are represented approximately uniformly. But this is almost impossible
on images of deep-space objects. For instance consider the image of emission
nebula, dominated by deep-red hydrogen alpha lines – any attempts to lighten
green and blue color to create color-neutral image result to totally wrong color
representation. Astronomical images are usually color balanced manually.
As already described in the “Brightness and Contrast – Image Stretching”
chapter, image can be visually brightened by altering its stretch limits. SIPS
“Histogram and Stretch” tool displays and also enables altering of stretching
curve limits and shape for red, green and blue color individually.
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Illustration 21: Histogram and Stretch tool shows histograms of individual colors
Some General Rules for
Successful Imaging
Advanced CCD cameras caused a revolution in amateur astronomy. Amateurs
started to capture images of deep-sky objects similar or surpassing the ones
captured on film by multi-meter telescopes on professional observatories.
While the CCD technology allows capturing of beautiful images, doing so is
definitely not easy and straightforward as it may seems. It is necessary to gain
experience, to learn imaging and image processing techniques, to spend many
nights mastering the technology.
Although CCD camera can convert majority of incoming light into information,
it is not a miracle device. Keep on mind that laws of physics are sill valid.
● CCD camera does nothing more than converting image created on the
chip by telescope (or objective lens) into information. A quality
telescope and quality “photographic-class” mount is absolute must for
successful imaging. If the mount cannot keep the telescope on track or
the telescope cannot create perfectly focused image, result is always
distorted and blurred.
● Ideally the exposures should be automatically guided using guiding
CCD camera or at last webcam or similar device. Tracking errors
caused by drive periodic error, mount polar misalignment or other
mechanical issues (often unnoticeable by eyes) cause streaking of star
images. Note the exposure time for each color often reaches tens of
minutes or even hours if the really high quality images are taken.
The G1 series of CCD cameras are especially designed with guiding
on mind. G1 CCD cameras are equipped with “autoguider” connector,
which allows direct connection between the G1 camera head and
telescope mount. 16-bit digitization and using of sensitive Sony ICX
CCD detectors provide higher sensitivity and dynamic range
compared to typical video or web cameras. The SIPS software
package supports both imaging and guiding cameras and implements
sophisticated guiding algorithms.
● Focus image properly. Almost unnoticeable focuser shift affects star
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diameter. Focusing, especially on fast telescopes, is critical for sharp
images. Electrical focuser is a huge advantage, because it allows
focusing without shaking the telescope by hand and with precision
surpassing the manual focusing.
Keep on mind that the star images are affected not only by focusing,
but also by seeing. Star images will be considerably bigger in the night
of poor seeing, no matter how carefully you focus.
● Master image calibration (dark frame subtraction and flat fielding) and
carefully calibrate all images. Various artifacts (thermal noise, hot
pixels, gradients, telescope/lens vignettation, dust particles on filters
etc.) degrade the image and properly calibrated image always looks
better. Take care to obtain dark frames and flat fields for all filters
used, for all resolutions/binnings etc.
● If the image is processed to be as aesthetic as possible, other
processing than basic calibration can significantly improve its
appearance. Nonlinear stretching (called “curves” in some imageediting packages), special filters (hot/cold pixels removal, noise
reduction etc.) and other processing (e.g. deconvolution) enhances the
image.
Warning:
Never perform these enhancing filters on images intended for data
reduction processing. It is always good idea to store original image
and to enhance only its copy. Scientific information can be
significantly degraded by various noise filters, deconvolution etc. If
for instance the image of some galaxy contains newly discovered
supernova, photometric reduction of the original image can be
scientifically very important.
● A common saying “there is a science in every astronomical picture” is
especially true for CCD images. Examine your images carefully, blink
them with older images of the same object or field. There is always a
chance some new variable star, new asteroid, new nova or supernova
appear in the image.
● Be patient. Although many advertisements proclaim “capture images
like these your first night out”, they probably mean your first
successful night out. Nights can become cloudy or foggy, the full
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Moon can shine too much, the seeing can be extremely bad… Number
of things can come bad, but the bad luck never lasts forever. Start with
bright objects (globular clusters, planetary nebulae) and learn the
technique. Then proceed to more difficult dimmer objects.
If you are new to CCD imaging and terms like “dark frame”, “read noise” and
“image binning” sound strange to you, refer to the “Introduction to CCD
Imaging” chapter of the SIPS software documentation. This chapter explains
basic principles of CCD operation and their usage in astronomy, discusses color
imaging, CCD chip dark current and camera read noise, chip resolution and
pixel scales in relation to telescope focal length, explains basic image
calibration etc.
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Camera Maintenance
The G3 CCD camera is a precision optical and mechanical instrument, so it
should be handled with care. Camera should be protected from moisture and
dust. Always cover the telescope adapter when the camera is removed from the
telescope or put the whole camera into protective plastic bag.
Desiccant exchange
The G3 and G4 camera cooling is designed to be resistant to humidity inside
the CCD chamber. When the temperature decreases, the copper cold finger
crosses freezing point earlier than the CCD chip itself, so the water vapor inside
the CCD chamber freezes on the cold finger surface first. Although this
mechanism works very reliably in majority of cases, it has some limitations,
especially when the humidity level inside the CCD chamber is high or the chip
is cooled to very low temperatures.
This is why a small cylindrical chamber, filled with silica-gel desiccant, is
placed inside the camera head. This cylindrical chamber is attached to the
insulated cooled CCD chamber itself.
Warning:
High level of moisture in the CCD chip chamber can cause camera malfunction
or even damage to the CCD chip. Even if the frost does not create on the
detector when the CCD is cooled below freezing point, the moisture can be still
present. It is necessary to keep the CCD chamber interior dry by the regular
exchange of the silica-gel. The frequency of necessary silica-gel exchanges
depends on the camera usage. If the camera is used regularly, it is necessary to
dry the CCD chamber every few months.
It is possible bake the wet silica-gel in the oven (not the microvawe one!) to dry
it again. Dry the silica-gel for one hour at 150 to 160 °C. Exceeding the 170 °C
can damage the silica-gel and its ability to absorb moisture will be limited.
The silica-gel used in G3 and G4 cameras changes its color according to
amount of water absorbed – it is bright orange when it is dry and turns to
transparent without any color hue when it becomes wet.
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The silica-gel container is accessible from the back side of the camera head.
The slotted desiccant chamber cap can be unscrewed e.g. by a coin. Pour out
wet silica-gel and fill the chamber with a dry one.
The desiccant container can be left open without the fear from contamination of
CCD chamber interior by dust. There is a very faint stainless steel grid between
the CCD chamber and the desiccant container, so dust particles cannot enter the
chamber itself. It is even recommended to keep the desiccant container cap off
for a couple of hours when the camera is in the room with low humidity. This
helps drying the CCD chamber interior and prolongs the silica-gel exchange
interval.
The desiccant chamber used in G3 cameras can be filled with a hot silica-gel
without a danger of damaging of the container.
Changing Filters
It is necessary to open the camera head to change filters or the whole filter
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Illustration 22: Silica-gel container is under the screwed cap with slot, left from the
cooling fans
wheel of the G3 camera. Opening the head is quite simple – it is just necessary
to unscrew she eight bolts, which holds the camera head together.
Warning:
The blade shutter rotates 180° between individual snapshots. Camera cover
could be opened only when the shutter is closed. If for instance the camera is
unplugged from power adapter while exposing and the shutter remains open, it
can be damaged while removing the camera cover.
After removing the screws carefully turn the camera head by the telescope
adapter upward. Gently pull the front part of the case. Notice there are two
cables, connecting the filter wheel motor and the filter position optical bar,
plugged into the electronics board. It is not necessary to unplug these cables to
change filters, but if you unplug them, take care to connect them again in the
proper orientation!
Every filter position in the wheel is defined by the index hole. The hole
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Illustration 23: Filters can be changed after opening the camera case front shell
defining the first position is preceded by another hole. Filter numbering is
illustrated by the following picture:
Changing the Whole Filter Wheel
The whole filter wheel can be changed at once. It is necessary to remove the
front part of the camera case the same way as in the case of changing filters.
The filter wheel can be removed when you unscrew the bolt on the center of the
front part of camera case. Take care not to damage the horseshoe-shaped
optical bar when replacing the filter wheel.
Changing the Telescope Adapter
The camera head contains bolt square. The telescope adapter is attached by four
bolts. If you want to change the adapter, simply unscrew these bolts and replace
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Illustration 24: Filter positions in the G3 filter wheel
the adapter with the new one.
Power Supply Fuse
The power supply inside the camera is protected against connecting of invertedpolarity power plug or against connecting of too-high DC voltage (above 15 V)
by a fuse. If such event happens and the cooling fans on the back side of the
camera do not work when the camera is connected to proper power supply,
return the camera to the service center for repair.
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