Sbig ST-5C User Manual
Operating Manual
for the
Model ST-5C
Advanced CCD Camera
Santa Barbara Instrument Group
Santa Barbara Instrument Group
1482 East Valley Road • Suite 33
PO Box 50437
Santa Barbara, CA 93150
PHN (805) 969-1851 (FAX) (805) 969-4069
Email: sbig@sbig.com Home page: www.sbig.com
Note: This equipment has been tested and found to comply with the limits for a Class B digital
device pursuant to Part 15 of the FCC Rules. These limits are designed to provide reasonable
protection against harmful interference in a residential installation. This equipment generates,
uses, and can radiate radio frequency energy and if not installed and used in accordance with
the instructions, may cause harmful interference to radio communications. However, there is
no guarantee that interference will not occur in a particular installation. If this equipment does
cause harmful interference to radio or television reception, which can be determined by turning
the equipment off and on, the user is encouraged to try to correct the interference by one or
more of the following measures:
• Reorient or relocate the receiving antenna.
• Increase the separation between the receiver and the equipment.
• Connect the equipment into an outlet on a circuit different from that to which the
receiver is connected.
• Consult the dealer or an experienced radio/TV technician for help.
Changes or modifications not expressly approved by the party responsible for compliance could
void the user's authority to operate the equipment.
Also note that user must use shielded interface cables in order to maintain product within FCC
compliance.
ST-5C Manual
Fifth Printing
May 1998
Table of Contents
1. Introduction to CCD Cameras....................................................................................................3
1.1. How CCDs Work.......................................................................................................................3
1.2. CCDs Applied to Astronomical Imaging..................................................................................4
1.2.1. Cooling..................................................................................................................4
1.2.2. Dark Frames..........................................................................................................5
1.3. The Various CCD Parameters and How they Affect Imaging..................................................5
1.3.1. Pixel Size...............................................................................................................5
1.3.2. Full Well Capacity................................................................................................5
1.3.3. Dark Current.........................................................................................................6
1.3.4. Read Noise............................................................................................................6
1.3.5. Frame Transfer......................................................................................................6
1.3.6. Antiblooming Protection......................................................................................7
1.3.7. A/D Bits and Digitization Rate............................................................................7
1.3.8. Binning..................................................................................................................8
1.3.9. Spectral Response.................................................................................................8
1.4. Camera Hardware Architecture................................................................................................9
2. The First Day with the Camera.................................................................................................11
2.1. Setting up the System..............................................................................................................11
2.1.1 Installing the CCDOPS Software.........................................................................11
2.1.2. Getting Acquainted with CCDOPS Software.....................................................12
2.1.3. Connecting the Camera to the Computer ..........................................................14
2.1.4. Establishing a Communications Link with the Camera.....................................15
2.1.5. Operating your Camera with CCDOPS - a Daytime Orientation.....................15
2.2. The First Night with the Camera ............................................................................................17
2.2.1. Focusing the Camera..........................................................................................18
2.2.2. Finding and Centering the Object ......................................................................19
2.2.3. Taking an Image................................................................................................. 19
2.3. Further Foray’s into CCDOPS................................................................................................ 19
3. Advanced Imaging Techniques................................................................................................21
3.1. Taking a Good Flat Field.........................................................................................................21
3.2. Track and Accumulate ............................................................................................................21
3.3. Color Imaging..........................................................................................................................21
3.4. Autoguiding............................................................................................................................22
3.5. Field Operation........................................................................................................................22
4. Glossary.....................................................................................................................................23
5. Hints and Tips........................................................................................................................... 27
5.1 Question and Answer...............................................................................................................27
5.2. CCDOPS Use Tips...................................................................................................................29
5.3. Hints and Tips for Laptop Users.............................................................................................29
5.4. Telescope Tips......................................................................................................................... 30
A. Appendix A - Specifications....................................................................................................33
A.1 DC Power Jack.........................................................................................................................33
A.1 Communications Port .............................................................................................................33
A.1 Telescope Port .........................................................................................................................34
i
B. Appendix B - Maintenance ......................................................................................................35
B.1. Replacing the Fuse..................................................................................................................35
B.2. Disassembling/Reassembling the Optical Head....................................................................35
B.3. Cleaning the Optical Windows...............................................................................................35
B.4. Replacing the Desiccant..........................................................................................................36
ii
Introduction
Congratulations and thank you for buying the SBIG ST-5C Advanced CCD Camera. This
camera offers incredible performance in a small package for a moderate cost. Using the camera
will expand your astronomical experience by allowing you to easily take images like the ones
you've seen in books and magazines, but never seen when peeking through the eyepiece. CCD
cameras offer convenience, high sensitivity (a typical deep-sky image is several minutes), and
advanced image processing techniques that film just can't match. While CCDs will probably
never replace film in its large format, CCDs offer ease of use and allow a wide range of scientific
measurements. Their use has established a whole new field of Astronomy. Some of the
features you'll discover about your camera include:
• Based on the Texas Instrument TC255 CCD with 320 x 240 pixels that are 10
microns square.
• Double Correlated Sampling readout with 16 Bit A/D for the lowest possible
noise.
• Convenient and fast parallel interface offers full frame download times under 4
seconds.
• Thermoelectric cooling gives sky background limited performance.
• Integral Shutter Wheel that can be replaced with a Color Filter Wheel for Tricolor
imaging.
• Telescope port for use as an Autoguider.
• Advanced CCDOPS software for data acquisition, display and processing.
• Track and Accumulate1for hassle-free long duration exposures.
• 12 VDC operation allows use in field with a car battery.
• Standard T-Thread allows use with a variety of telescope adapters including the
standard 1.25 inch nosepiece and eyepiece projection units.
This manual is organized for two types of use. Some sections have been designed to be read
through from the start while you're learning about the camera and the software whereas other
sections are to be used for reference. Briefly the manual consists of the following sections:
• Section 1 describes CCD cameras and how they work. While it is the first section
of the manual, it is a bit technical, and some users may wish to skip ahead to the
second section then come back to this section once they have had a little hand's
on experience.
• Section 2 tells you how to install the camera and CCDOPS camera software and
takes you step by step through the process of taking your first images. Even if
you have experience with other cameras you should browse through this section
and read any sections that are new to you. Also for further training and detailed
technical information regarding the CCDOPS software for the ST-5C please refer
to the separate CCDOPS manual.
• Section 3 presents some more detailed information about how you use the
camera for some slightly more advanced tasks. The discussion of even more
advanced topics is continued in Appendix D. Once you have become familiar
with the basic operations of the camera you will want to read these sections.
• Sections 4 and 5 provide a Glossary of common CCD imaging terms, a Question
and Answer section on the most common questions you'll have and a section of
1
Track and Accumulate covered by SBIG US Patent 5,365,269.
Page 1
Introduction
useful hints and tips for using the camera. Again, this is a good section to read
once you have had a little time with the camera.
• Finally, the Appendices provide a wealth of technical information about the
camera.
Page 2
Section 1 - Introduction to CCD Cameras
1. Introduction to CCD Cameras
The CCD (charge coupled device) is very good at the most difficult astronomical imaging
problem: imaging small, faint objects. For such scenes, long film exposures are typically
required. The CCD based system has several advantages over film: greater speed, quantitative
accuracy, ability to increase contrast and subtract sky background with a few keystrokes, the
ability to co-add multiple images without tedious dark room operations, wider spectral range,
and instant examination of the images at the telescope for quality. Film has the advantages of a
much larger format, one-step color, and independence of the wall plug (the ST-5C camera can
be battery operated in conjunction with a laptop computer, though). After some use, you will
find that film is best for producing sensational large area color pictures, and the CCD is best for
planets, small faint objects, and general scientific work such as variable star monitoring and
position determination. It is for this reason that we designed the camera to support both efforts,
as an imaging camera and as an auto-guider to aid astrophotography.
1.1. How CCDs Work
The CCD is a solid-state imaging detector that is quite commonly used in video tape cameras
and is starting to find acceptance in still frame cameras. It has been used for Astronomical
Imaging for over twenty years. The CCD is arranged as a rectangular array of imaging
elements called pixels. An image is formed by reading the intensity of these pixels.
The basic function of the CCD detector is to convert an incoming photon of light to an
electron which is stored in the detector array until it is read out, thus producing data which
your computer can display as an image. How this is accomplished is eloquently described in a
paper by James Janesick and Tom Elliott of the Jet Propulsion Laboratory:
"Imagine an array of buckets covering a field. After a rainstorm, the buckets are
sent by conveyor belts to a metering station where the amount of water in each
bucket is measured. Then a computer would take these data and display a
picture of how much rain fell on each part of the field. In a CCD the "raindrops"
are photons, the "buckets" the pixels, the "conveyor belts" the CCD shift registers
and the "metering system" an on-chip amplifier.
Technically speaking the CCD must perform four tasks in generating an image.
These functions are:
1) charge generation
2) charge collection
3) charge transfer
4) charge detection
The first operation relies on a physical process known as the photoelectric effect when photons or particles strike certain materials free electrons are liberated. In
the second step the photoelectrons are collected in the nearest discrete collecting
sites or pixels. The collection sites are defined by an array of electrodes, called
gates, formed on the CCD. The third operation, charge transfer, is accomplished
by manipulating the voltage on the gates in a systematic way so the signal
electrons move down the vertical registers from one pixel to the next in a
conveyor-belt like fashion. At the end of each column is a horizontal register of
pixels. This register collects a line at a time and then transports the charge
packets in a serial manner to an on-chip amplifier. The final operating step,
charge detection, is when individual charge packets are converted to an output
voltage. The voltage for each pixel can be amplified off-chip and digitally
Page 3
Section 1 - Introduction to CCD Cameras
encoded and stored in a computer to be reconstructed and displayed on a
television monitor."
2
Readout Register
Output
Y=1
Amplifier
Y=N
X=1 X=M
Figure 1.1 - CCD Structure
1.2. CCDs Applied to Astronomical Imaging
When CCDs are applied to astronomy, with the relatively long exposure times (compared to the
30 frames per second used in video camera), special considerations need to be applied to the
system design to achieve the best performance. This section discusses the cooling and dark
frame requirements of astronomical imaging.
1.2.1. Cooling
Random noise and dark current combine to place a lower limit on the ability of the CCD to
detect faint light sources. If the CCD is producing more electrons from its own internal
processes than is produced by photons from a distant object, the signal from the object is said to
be "lost in the noise", and will be impossible to display without sophisticated image processing
software. Noise here refers to the "gritty" look of short exposure images.
Internally, the CCD generates thermal noise and readout noise caused by the operation
of the electronics on the chip. The goal is to eliminate unwanted sources of electron production
in the chip and thus make the detector more sensitive to the remaining source of electron
production by incoming photons. As you can imagine, the reduction of unwanted noise is
important for the best performance of the CCD.
Dark current is thermally generated electrons in the device itself. All CCDs have dark
current which can cause each pixel to fill with electrons in only a few seconds at room
temperature even in the absence of light. By cooling the CCD, this source of noise is reduced,
the sensitivity increased, and longer exposures are possible. In fact, for every 8°C of additional
cooling, the dark current in the CCD is reduced to half. In your camera for example, cooling the
CCD from room temperature (25°C) down to 0°C results in an eight-fold reduction in dark
current.
The ST-5C uses a thermoelectric (TE) cooler to cool the CCD. The TE cooler is a solid-
state device that acts like a heat pump. By running electrical current through the TE cooler, heat
is pumped out of the CCD through the TE cooler. The camera also has a temperature sensing
thermistor attached to the CCD to monitor the temperature, and the camera electronics control
2
"History and Advancements of Large Area Array Scientific CCD Imagers", James Janesick, Tom
Elliott. Jet Propulsion Laboratory, California Institute of Technology, CCD Advanced Development
Group.
Page 4
Section 1 - Introduction to CCD Cameras
the temperature at a user determined value for long periods. As a result, exposures up to an
hour long are possible, and saturation of the CCD by the sky background typically limits the
exposure time.
The sky background conditions also increase the noise in images, and in fact, as far as
the CCD is concerned, there is no difference between the noise caused by dark current and that
from sky background. If your sky conditions are causing photoelectrons to be generated at the
rate of 100 e-/pixel/sec for example, increasing the cooling beyond the point where the dark
current is roughly half that amount will not improve the quality of the image. This very reason
is why deep sky filters are so popular with astrophotography. They reduce the sky background
level, increasing the contrast of dim objects, and can be used to advantage with the CCD
camera.
1.2.2. Dark Frames
No matter how much care is taken to reduce all sources of unwanted noise, some will remain.
Fortunately, however, due to the nature of electronic imaging and the use of computers for
storing and manipulating data, this remaining noise can be drastically reduced by the
subtraction of a dark frame from the raw light image. A dark frame is simply an image taken at
the same temperature and for the same duration as the light frame with the source of light to
the CCD blocked so that you get an "image" of the dark. This dark frame will contain an image
of the noise caused by dark current (thermal noise) and other fixed pattern noise When the
dark frame is subtracted from the light frame, this pattern noise is removed from the resulting
image. The CCDOPS software can automatically perform this.
1.3. The Various CCD Parameters and How they Affect Imaging
If you scan the CCD related literature you will see a slew of new terms describing CCDs and
their performance. In this section we will discuss the more common CCD parameters and their
effects in an imaging application.
1.3.1. Pixel Size
Every CCD, independent of the manufacturer, is divided into a relatively large number of small
pixels. In your CCD camera the imaging area of the CCD is 3.2 mm x 2.4 mm and the pixels are
10 microns square (1 micron is one thousandth of a millimeter or roughly 0.00004"). If you
looked at the CCDs available from the various manufacturers you would see that their pixels
typically vary from 7 microns on the small end to 25 microns on the large end. There are
advantages and disadvantages associated with the size of pixels in a CCD.
While having small pixels may seem advantageous in terms of offering "higher
resolution", large pixels gather more light and are thus "more sensitive". You can also adjust
your telescope configuration to accommodate various size pixels, using faster telescopes to
increase the speed of small pixel CCDs or longer focal lengths to increase the resolution of
larger pixel CCDs.
Often times the basic goal is to match the CCD resolution to the telescope resolution and
to the overall seeing. It would be a waste to use a pixel size of 7 microns on a telescope with a
spot size of 25 microns or to configure the CCD/telescope to produce an image scale of 10 arcseconds per pixel when you're looking for fine planetary detail.
1.3.2. Full Well Capacity
The full well capacity of a CCD is the number of electrons each pixel can hold before the pixels
are full. While this may seem like an important consideration in choosing a camera, you need
to think about how the camera is used.
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Section 1 - Introduction to CCD Cameras
The typical CCD astronomer is taking images of faint galaxies and nebulae. While
exposures are long, you very rarely will expose the CCD to more than a small fraction of its full
well capacity on these dim objects. Some stars in the image will expose to the full well capacity,
but not much of the nebulosity. So even though small pixel size CCDs have lower full well
capacities than large pixel CCDs, most applications do not stress this CCD parameter.
1.3.3. Dark Current
Every CCD, independent of manufacturer, will suffer from dark current. One manufacturer's
CCD may have lower dark current than another manufacturer's, but they all have dark current.
The only way to reduce dark current in a CCD is by cooling it, and in general the more cooling,
the better. But there is a limit.
In astronomical imaging, you're looking at objects against the sky background, and that
sky background isn't perfectly dark. City light and sky glow itself cause the sky to actually
have some brightness. As far as the CCD is concerned from a noise standpoint, it can't tell the
difference between electrons generated by dark current and those generated by sky
background. Because of this, cooling the CCD beyond the point where the dark current is less
than the sky background will not result in any further improvement in image quality.
As a matter of interest, your CCD camera produces roughly 1 e-/p/s
(electron/pixel/second) of dark current when cooled to -5°C and a typical dark sky through an
F/10 telescope produces 3 e-/p/s. Sky background scales inversely with the square of the
telescope's F-Ratio and for example is 25 times higher at F/2 than it is at F/10.
1.3.4. Read Noise
The read noise of a CCD is the noise inherent in the CCD's amplifier and charge detection
circuitry. This read noise forms a noise floor below which the CCD will not detect weak signals.
Something you're imaging must rise above the read noise level before you'll be able to see it.
Since the read noise is the noise floor, it is really only important for very short exposures
of dim objects. As the signal from your target or the signal from the sky background builds up,
it will cross the noise floor and the noise in the image will be determined only by the brightness
of the target and the exposure time, not the CCD's read noise. For example, the read noise in
your CCD camera is 30 electrons RMS, and when your signal has built up to 900 electrons (30
electrons RMS squared) the read noise is no longer the dominant noise in the final image. At
F/10 the sky background alone achieves this level in 6 minutes and at F/2 it occurs in 12
seconds!
There are techniques CCD manufacturers can use to reduce the read noise. One of the
techniques is Double Correlated Sampling where some of the noise in the amplifier is
subtracted out by making two measurements per pixel rather than one. Your Advanced CCD
camera uses such a technique.
1.3.5. Frame Transfer
There are two basic types of CCDs available: Frame Transfer CCDs like the one used in your
camera and Full Frame CCDs. A Frame Transfer CCD is divided into 2 separate areas on the
CCD. One area, called the image area, is sensitive to light and that is where the image builds up
when exposed to light. But CCDs can't be "turned off" and they continue to build up signal,
even throughout the readout phase. Remember that during readout, the rows of charge are
shifted up to the readout register. It's sort of like advancing the film in a camera with the
shutter open. Unless the CCD is covered with a shutter during the readout, the continued
buildup of signal throughout the readout phase will cause streaking.
The second area in the Frame Transfer CCD, called the storage area, is shielded from
light by an aluminum layer on the CCD. This storage area is used as an "electronic shutter"
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Section 1 - Introduction to CCD Cameras
whereby data from the image area, after completing the exposure, is rapidly shifted into the
storage area where it is then digitized. A fast shift from the imaging area to the storage area
insures minimal streaking. Once the image is in the storage area, it can read out by the camera
electronics without causing streaking.
The simple answer to streaking you might say is to use a mechanical shutter, and in fact
your camera does have a shutter but the accurate timing of exposures is not limited by the
speed of that shutter but by how rapidly the imaging area can be moved into the storage area.
In this way the mechanical shutter is used to cover the CCD chip for taking dark frames while
short exposure images can be achieved electronically, without the limitations of mechanical
shutters.
1.3.6. Antiblooming Protection
As described above, the individual pixels in the CCD have a limited full well capacity. When a
pixel fills up with charge, the excess charge generated has to go somewhere. Again, there are
two basic types of CCDs available.
Standard CCDs, when reaching the saturation point, will spill the charge into
neighboring pixels, typically up and down the column in a line that is called blooming. If, for
example, you had a pixel that was exposed to 10 times its full well capacity, it would bloom
until a column of ten pixels was saturated, causing streaks in the image. The second type of
CCD offers Antiblooming protection.
In an Antiblooming protected CCD, when the charge in the pixel gets above some
threshold, typically one-half the full well capacity, the majority of the excess charge gets bled off
into a drain on the CCD. For example, a CCD with a 100X Antiblooming protection will drain
off 99% of the excess charge, allowing a pixel to overexpose to 100-fold before blooming occurs.
There is a price to pay, however, with Antiblooming protection and that's why
manufacturers produce both protected and unprotected CCDs. First off, the process of
Antiblooming protection causes a nonlinearity in the response of a CCD. If you were trying to
make accurate Photometric measurements, you would want the integrated star brightness kept
below the knee where the Antiblooming kicks in. The second detriment to some Antiblooming
protected CCDs is that at the integrated circuit level, the Antiblooming structures can reduce
the sensitive area of the individual pixels, causing a slight reduction in overall sensitivity.
The Texas Instruments (TI) TC255 CCD used in your camera offers variable
Antiblooming protection and, according to TI, the structures required to implement the
Antiblooming protection do not cause any reduction in sensitivity. The benefit of the variable
Antiblooming protection in the TC255 is that you can select the amount of Antiblooming you
want, using just a small amount for fields where no bright stars would cause blooming and a
large amount for objects like the Orion Nebula where a bright star in the field of view would
otherwise bloom. With the TC255, using the minimum amount of Antiblooming protection also
has a beneficial effect in that it reduces the dark current in the CCD.
1.3.7. A/D Bits and Digitization Rate
If you browse through the literature on specifications of the various CCD cameras, you see
some of them are 8 bits, some are 12 bits and some are 16 bits. While in general an A/D
(Analog to Digital) converter with greater precision is desired, there is a point where the extra
precision doesn't get you any increased performance. In most CCD cameras it's actually the
CCD that limits the performance, not the A/D converter.
As a starting point, you can take the CCD's full well capacity and divide by the CCD's
read noise to come up with a figure for the CCD's dynamic range. In this way the dynamic
range is the ratio of the brightest object you could image without saturating to the dimmest
object you could detect. You could see that a 16 bit A/D with a dynamic range of 65,000 is
overkill for a CCD with a dynamic range of 4000, for example. Let's look at your camera. The
Page 7
Section 1 - Introduction to CCD Cameras
CCD has a full well capacity of 50,000 electrons and a read noise of 30 electrons RMS giving a
dynamic range of roughly 1700. A 12 bit A/D offers a dynamic range of 4096 and would cover
that CCD fairly well.
One thing you do want to do with the A/D is make sure that the A/D's noise (typically
1 count) is lower than the CCD's noise so that you are truly CCD limited. Setting the A/D to
have one-eighth the noise of the CCD allows averaging several images to improve noise.
Applying this criteria to the TC255, you would want an A/D with a dynamic range of 10,000 or
so (a 14 bit A/D has 16,000 dynamic range).
Finally, the CCDs can be used in a mode where the pixels are combined in a process
called binning which is described in detail below. Binning reduces the CCD's spatial resolution
like increasing grain size in film, but increases the CCD's dynamic range. With your camera
you can bin the CCD 2 by 2 resulting in another factor of 4 in dynamic range which gets us up
to 40,000 or so, hence the need for a 16 bit A/D. While the camera only achieves 16,000 counts
unbinned, allowing it to achieve 65,000 counts binned allows taking advantage of the extra
dynamic range inherent in binning.
The final consideration regarding the A/D is how fast the data is digitized and
downloaded from the CCD to the computer. The A/D is not the only contributor to that time.
The actual transmission of the data to the computer is a significant portion of it. In your camera
an entire image can be digitized by the A/D converter and sent to the PC in 3.5 seconds. This is
done using the PC's standard Parallel port without requiring the addition of expensive (and
difficult to configure) SCSI adapter cards.
1.3.8. Binning
Binning is a process where multiple pixels in the CCD are combined to form a single larger
pixel. This reduces the CCD's resolution but increases the sensitivity. Different CCDs from
various manufacturers support different types of binning.
Some CCDs support on-chip binning, where all the pixels in the group are combined in
the CCD itself. This has the advantages of lower noise (a single read noise is generated for the
group of pixels) and higher speed digitization since fewer pixels are involved. In imaging
applications you tend to bin the images in both directions to preserve a "square" aspect ratio.
The one final advantage of binning is that it increases the overall image throughput, reducing
the digitize and download times due to the reduced number of pixels involved.
The camera software allows you to select a High resolution 10 micron square mode
where the pixels are unbinned and a Low resolution mode resulting in 20 micron square pixels.
The latter is useful for fast acquisition of faint objects.
1.3.9. Spectral Response
Like film, CCDs have a varying response to differing wavelengths. The basic fabrication
techniques used in manufacturing the CCDs greatly affect their spectral response. At the
extreme Red end of the spectrum, the CCDs loose their sensitivity because the photons simply
do not have enough energy to generate electrons in the CCD wells. At the Blue end of the
spectrum, the photons do not penetrate deep enough into the CCDs to get into the wells and are
stopped by the top layers of the CCD. Between the Red and the Blue, interference effects in the
top layers of the CCD can also cause peaks and valleys in the response.
This affects you in several ways. The most obvious is the overall effect that causes you
to take longer exposures with CCDs for various colors. For example, when taking color images,
your Blue exposure is typically several times longer than the Red exposure to give an image
with similar quality or Signal/Noise. One last interesting note about CCD's spectral response
is that they have much more response in the near infrared than film.
Page 8
Section 1 - Introduction to CCD Cameras
OR
The TC255 CCD used in your camera is made using Texas Instrument's Virtual Phase
technology that gives excellent Blue response compared to other CCDs. This is achieved by
reducing the number of photon-absorbing clocking gates in the CCD.
Figure 1.2 - CCD Quantum Efficiency
1.4. Camera Hardware Architecture
This section describes the ST-5C Advanced CCD Camera from a systems standpoint. It
describes the elements that comprise a CCD camera and the functions they provide. Please
refer to the figure below as you read through this section.
CCD
Shutter/
Filter Wheel
Wall
Xfmr
(US &
Japan)
Cigarette
Adapter
Microcontroller
Gate
Array
CPU
Clock
Drivers
Postamp/
A/D
Converter
Host Computer
Optical
Head
Preamp
TE Cooler
Figure 1.3 - CCD System Block Diagram
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