SBIG’s New Low Cost ST-i Spectrograph
SBIG has developed a new spectrograph specifically for our ST-i camera. This unit is intended to enable an amateur to characterize his/her skies, flat field sources, filter passbands, and other light sources. At the moment the unit has no means to allow a specific star or nebular region to be put on the slit. We have some ideas about how to add this capability on our drawing boards, but it will be some time well in the future before this happens, if at all. At the moment its main utility is to allow an amateur to customize his flat field sources to better match his sky conditions. The reason this is important can be found in my write-up on Flat Fields – The Ugly Truth on the SBIG web site. This unit also provides a good way for an amateur to compare his light pollution situation to users at other sites. The cold reality of the world is that good paying jobs are associated with cities and light pollution, and light pollution is fact of life for most of us!
The optical design of the spectrograph is shown in Figure One. I have chosen to depart from modern designs and use a prism as the dispersing element instead of a grating. The reasons for this are two-fold. Most important, the optical efficiency is high across the spectrum, roughly twice that of a grating on average. This is valuable since the night sky is not that bright! Secondly, the spectral range without confusion from multi-order light is from 430 to 1050 nm, encompassing the full sensitivity range of SBIG CCD products. The skyglow in the near infrared (700 to 1050 nm) is significant to the CCD camera, even though it is invisible to your eye. This near infrared response is important in detecting faint stars and distant galaxies, but also is not well baffled by many commercial telescope optical systems, so it needs to be understood.
Figure One: ST-I Spectrograph Optical Design
The design is simple: light enters the spectrograph through a 25 micron entrance slit and is collimated by an achromatic lens. It the passes through the Schott SF11 glass prism, where blue light is bent through a greater angle than red wavelengths. A second achromat focuses the light onto the CCD, with an additional plano-convex lens to shorten the focal length and increase the photographic speed of the system to F/3.66. The speed is important when trying to capture the sky background. The plane of the CCD is actually tilted a little bit relative to the angle of incidence of the light, as shown, to reduce the contribution of chromatic aberration to the optical blur. The spectrum of a neon gas discharge tube captured with this system is shown in Figure Two. Note that the spectral lines are curved, top to bottom. This curvature is a natural result of the off axis rays from the slit being bent by a slightly greater amount by the prism. In the production version the slit will be curved the other direction to compensate, and the lines will be straight. Straightening out the lines will allow greater vertical binning of the CCD for faint targets. Here the curvature is accentuated by the 4:1 vertical binning employed.
Figure Two: Spectrum of Neon Discharge Tube
Figure Three illustrates the sky background from my back yard, which has about 5th magnitude skies. To capture this, the spectrograph is simply pointed straight up. As a result, background starlight is also included in this spectrum and is visible as the semi-uniform continuum baseline. The resolution around the natural airglow line at 557.7 nm (marked) is adequate to separate it from the pervasive mercury line at 540.6 nm, and the sodium line at 568.8 nm (both from streetlights).
Figure Three: Airglow from my Backyard (Magnitude 5 Skies)
A VB.NET program for a PC is included with the spectrograph that allows the user to control the ST-I and acquire spectra, to create a wavelength calibration for the spectrograph, output text files of the data for processing with Excel or another program, and re-bin the data into a format with uniform sized wavelength bins for simpler comparison with grating data or from other sources. The intent is to make this functionality easy to use, but to provide a window into the interesting spectral properties of light sources around us. Figure Four illustrates a screen shot from that program. A graph of the spectrum is shown, as well as a graph of the spectrum binned into spectral intervals of equal width to correct for the non-linearity of the prism design.
We hope this simple to use spectrograph will become an essential part of the amateur astronomer’s toolkit. It enables the amateur to visualize the spectrum of the light around him, and to easily measure the transmission of filters, the reflectivity of diffuse surfaces, and even atmospheric transmission with a bit more time and setup. I will describe this technique in a future paper.
Figure Four: ST-I Spectroscopy Program
|Name||SBIG - ST-i Spectograph|
|Manufacturer||Santa Barbara Instrument Group (SBIG)|
|Manufacturer Part No||ST-i Spectrograph|