The Star Spectroscope is a diffraction grating manufactured by Rainbow Optics. (link) Over the years, many amateur astronomers have gotten good results with the Star Spectroscope. Our RSpec software works fine with any spectra, including those from a Star Spectroscope grating.
The Rainbow Optics grating has 200 lines per millimeter. The more lines/mm, the more spread out (“dispersed”) the spectrum is. Many experts in the amateur astronomical community feel that that a 100 line/mm grating (like the Star Analyser, link) is a better grating for users, especially newcomers. (And we’re not just saying that because we sell the Star Analyser!)
The Star Spectroscope was developed quite some time ago, in the mid-1990′s. It is not optimized for modern spectroscopy, which may use video and where software is used to calibrate spectra. Because of the Star Spectroscope’s high dispersion, in many equipment configurations, the zero-order star and its spectrum may not both fit on your CCD chip. The absence of the zero-order limits your calibrating options.
Another reason that a 100 line/mm grating is preferred is that it is more difficult to image dimmer objects with a 200 line/mm grating. That’s because a 200 line/mm grating spreads out the spectrum on the chip twice as much.
At first, you might think that getting dimmer images from a 200 line/mm grating would be a small price to pay in exchange for the potential doubling of resolution. But, due to other optical limitations, a 200 line/mm grating seldom outperforms a 100 line/mm grating when processing spectra of astronomical objects. We’re all familiar with the fact that at a certain point, adding additional magnification to a visual telescope can actually cause an image to deteriorate. Similar optical limitations occur when going from 100 to 200 line/mm gratings. For example, the larger deviation angle (6.4°) of a 200 line/mm grating is so far off-axis that it increases the chromatic aberration and eventually limits its resolution
For a worksheet that shows the resolution limitations that a 200 line/mm grating encounters, see Ken Harrison’s TransSpec.xls worksheet in the RSpec Yahoo forum’s Files section. The worksheet’s creater (author Ken Harrison: link) is active on our Yahoo forum (link), so feel free to post questions about it there. Our forum members are experienced experts (and include the designer of the Star Analyser). They love to answer questions.
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