Santa Barbara Instrument Group (SBIG) - Deep space spectrograph (optimized for ST-402ME and f/10 SCT scopes)

SBIG’s new Deep Space Spectrograph (DSS-7) is a spectrograph optimized for the types of spectral
observations that an amateur has always been interested in, from stellar classification to
nebular analysis to galactic red shifts.  It is a more general purpose instrument than our Self
Guided Spectrograph (SGS), which is optimized for stellar work, and is much less expensive.  It
is optimized for the ST-7XME or the low cost ST-402, and will work well with ST-8/9/10/2000
cameras and ST-237s.  It will not work with the STL series due to their deeper backfocus required
by the built in filter wheel.  This memo describes the DSS-7 in detail, and present examples of
observations that can be made by the amateur. 

Spectroscopy Fundamentals: a spectrograph is a device 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.  The DSS-7 spectrograph 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
CCDs used in SBIGs cameras have a larger range of sensitivity than the eye.  Most stars put out a
continuum of wavelengths with a number of absorption lines superimposed on it.  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).  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).  Galaxies have a spectrum that is an
aggregate of many stars, and have a similar spectrum.  Most galaxies only have a few obvious
features – the cores tend to show a sodium absorption 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 6563
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.

Optical Design: 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.