Sbig DSS-7 User Manual

OPERATING INSTRUCTIONS

FOR THE

SANTA BARBARA INSTRUMENT GROUP

DEEP SPACE SPECTROGRAPH

(DSS-7)

AND

DSS-7 SPECTRAL CALIBRATION

PROGRAM (SCP)

Alan Holmes

4/27/2005

Overview: SBIG’s DSS-7 spectrograph will provide a powerful spectral measurement
capability to the amateur. Spectra of nebular, stellar, and galactic objects can now be
obtained with modest amateur telescopes as small as 4 inch aperture (but bigger is better).
While the SBIG Self Guided Spectrograph is optimized for high resolution stellar
measurements, the DSS-7 is more for spectral measurements of extended objects like
nebula and galaxies. The SGS has the ability to guide on a star while acquiring its
spectra, but the DSS-7 does not. This is because we have made the assumption that with
an extended object you can drift a little bit and still have the emitting region on the
entrance slit, at least more so than with a star. However, the DSS-7 is certainly capable
of stellar measurements. This manual describes connection of the spectrograph to an
SBIG camera, data collection at the telescope, and analysis of the resulting images to
obtain a spectrum of the source.

Spectroscopy Fundamentals: a spectrograph is an instrument that can produce a graph of
the intensity of light as a function of color, or wavelength. A spectrometer is a device
that measures only one selectable color, and a monochromator is a device that transmits
only one color. Wavelength is measured in Angstroms. An angstrom is one ten billionth
of a meter. You will also quite often see wavelengths written in nanometers, which is
one billionth of a meter. 6563 angstroms (A) is 656.3 nanometers (nm). The DSS-7
instrument is designed to separate and focus wavelengths from 4000 to 8000 angstroms
across the width of an ST-7 CCD. The human eye is sensitive from about 4500 (deep
blue) to 7000 (deep red) angstroms, with its peak sensitivity at 5550 angstroms. The
silicon CCD used in SBIGs cameras has a larger range of sensitivity than the eye. Most
stars put out a continuum of wavelengths with a number of absorption lines superimposed
upon the continuum. Most emission nebula like the Orion Nebula produce a spectrum
this is composed of a few bright emission lines, such as H-alpha (a hydrogen line at 6563
angstroms), H-beta (a hydrogen line at 4861 angstroms), and O-III (a triply ionized
oxygen line at 5007 angstroms). Galaxies have a spectrum that is an aggregate of many
stars, and have a similar spectrum to stars. Most galaxies only have a few obvious
features – the cores tend to show a sodium absorbtion line due to the older stars there.
Seyfert galaxies and other active galaxies show an excess of H-alpha, which is great since
it makes a red shift much easier to determine. Quasars, nova and supernova in general
exhibit strong 6563A emission. In the case of quasars it can be red shifted quite a bit,
hundreds of angstroms, so it may actually appear at a different wavelength. For a nova,
the line will only be shifted slightly since the star is in our own galaxy, but it may be
greatly broadened. The individual hydrogen atoms are moving very fast due to the
tremendous temperatures involved, producing Doppler broadening that smears out the
line.
Stars can be classified spectrally into the well know OBAFGKM groups. The
very hot stars have few features in their spectrum, perhaps only a few hydrogen lines.
The spectrum of Vega shown later illustrates this. The cool stars tend to be old, with
many metallic lines producing a very complex and structured spectrum. There are also
several types of peculiar stars, which show strong emission lines or other structure. The
DSS-7 can reveal these features.

Specifications: The DSS-7 will work with the SBIG ST-7/8/9/10/2000 cameras (with no
filter wheel attached), and the ST-402/1603/3200 line of cameras. It will not work with
the STL series of cameras since it cannot reach focus with them. It will also work with
ST-5C and ST-237 cameras, but with reduced spectral range due to the smaller CCD. It
might work with the ST-6 cameras, but we don’t offer an attachment that we know
works. The supplied software, SPECTRA, will work with images captured by the ST­7/8/9/10/2000 line of cameras, and also with the ST-402/1603/3200 cameras, but not with
older cameras. The cameras that have larger CCDs than the ST-7 or ST-402 offer no
significant advantage in terms of spectral coverage – the design does not cover the larger
CCDs. The dispersion of the DSS-7 is 600 nanometers per mm, or about 5.4 angstroms
per pixel for the ST-7 (9 micron pixels). The resolution, which is less than the dispersion
due to blur and the finite slit width, is about 3 pixels, or 16 angstroms. The spectral range
captured by an ST-7 or ST-402 is about 4130 angstroms. The grating blaze is optimized
for 5000 angstroms.

Optical Design and Operation: the optical design of the DSS-7 is illustrated in Figure
One. Light enters the spectrograph through an entrance slit and is folded and then
collimated (made parallel) by the collimation lens. The light then impinges upon a
diffraction grating, which causes different colors to be reflected at different angles. You
can see a similar effect in the light reflected from a CD or DVD. The light diffracted
from the grating is then collected by a focusing lens, and imaged onto the CCD. Light of
a discrete wavelength through the slit will be imaged into a vertical line. If the light does
not fill the slit (such as is the case with a star) the discrete wavelength will produce a
starlike point on the CCD, with different wavelengths spread out along a line. This is
illustrated by the next few figures.

Figure One: Optical Layout of DSS-7 Spectrograph

Entrance Slit
Location

CCD Location

Grating

Figure Two shows the DSS-7 entrance slit. The narrow (50 micron) slit in the center is
flanked by a wider slit above (100 micron), and an even wider slit below (200 micron).
400 micron slits lie at the extreme top and bottom of the pattern. 50 microns is about

0.002 inch, smaller than a human hair, so the slits are quite narrow.

Figure Two: DSS-7 Entrance Slit

Figure Three shows the spectrum collected when this slit is illuminated by hydrogen light
– the two major wavelengths, 6563 and 4861 anstroms, produce two images of the slit
displaced horizontally. Figure Four shows a spectrum collected while examing P Cygni,
a peculiar star with permanent emission lines. The broadband radiation from the star
produces a horizontal line, while the emission lines show up as bright points, and the
airglow lines (some natural, some light pollution) show up as copies of the slit pattern.
For this image the airglow lines have been exaggerated to illustrate them better – P Cygni
is bright enough that exposures are short and airglow is not so prominent.

Figure Three: Hydrogen Spectra

Figure Four: Spectra of P Cygni and SkyGlow: Bright Points are 4861 and 6563

Angstroms

An obvious question at this point is “how does one put the star onto a 50 micron slit at
the focus of a telescope with an 2000 mm (80 inch) focal length”? Figure Five illustrates
the mechanics of the DSS-7.

Figure Five: DSS-7 Mechanical Configuration

The orientation shown here matches that of the optical schematic. The light enters from
the left. Both the slit and diffraction grating are motorized. The slit is motorized such
that it can be flipped in or out under computer control. The grating is motorized such that

it can be driven between the first order (produces a spectrum) and the zeroeth order
(produces an image) position. In the zeroeth order position the grating acts like a mirror,
producing an image of the slit, if it is in place, on the CCD. To put a star on the slit, the
user commands the slit out of the way, and the grating to the zeroeth order, and sets the
camera to FOCUS mode. He then uses his telescope controls to move the star into a
software generated box marking the slit position, and then clicks the CAPTURE
SPECTRUM button. The computer immediately inserts the slit, flips the grating to first
order, and starts the exposure. Figure Six illustrates the image quality in the zeroeth
order position of the grating. The image is good. Much light has been diffracted out of
this order by the grating, though, so objects are about 5 to 10X dimmer than without the
DSS-7.

Figure Six: Full Zeroeth Order Image on an ST-7XME (Nova Scorpius-2 in Center)

The DSS-7 is designed to accept an F/10 cone of light, a value typical of popular
commercial Schmidt-Cassegrain telescopes. In the imaging mode, it acts like a 2:1 focal
reducer, increasing the field of view of the CCD. It also is effectively a 2:1 focal reducer
in spectrograph mode, increasing the sensitivity to extended objects like nebulas or
galaxies. It will accept the center portion of the cone of light from a faster telescope, but
light is lost around the edges of the collimator lens. This 2:1 focal reduction also reduces
the slit width on the detector by a factor of 2, so the 50 micron slit has a projected width
of 25 microns, or around 3 pixels.

The small DC motors in the DSS-7 are powered by a 9 volt battery. The motors are
controlled by signals from the CCD camera’s relay port through a phone jack connector.
There is no provision for guiding. The length of exposure one can take will be limited by
your telescope’s ability to track unguided unless you have another camera set up to work
as a guider. For stellar work, it is not easy to keep the star on the narrowest slit. For