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SBIG - Deep Space Spectrograph (optimized for ST-402ME and f/10 SCT scopes)

Model#: DSS-7

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Manufactured by: Santa Barbara Instrument Group (SBIG)


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.

Additional Information

Name SBIG - Deep Space Spectrograph (optimized for ST-402ME and f/10 SCT scopes)
Manufacturer Santa Barbara Instrument Group (SBIG)
Model DSS-7