Maurice Gavin is among a small group of amateurs who constantly pushes the limits of their equipment, producing amazing images of interesting stars, nebulae and comets. These are not pretty pictures per se, but spectra—light spread into its rainbow of frequencies and wavelengths. Emission peaks and absorption bands in spectra are interpreted by real astronomers to reveal most of what we know—temperature, chemical composition, relative motion and even distance—about almost everything in the sky. Apparently, amateurs like Gavin and his colleagues can do astrophysics too.

Intrigued and encouraged by what I found, I dug out a Rainbow Optics Star Spectroscope diffraction grating (the photo/CCD model) and mounted it in front of my Canon Digital Rebel 300D camera. I was about to try what is technically known as “slit-less, wide field, objective grating, low dispersion spectroscopy.” Using fast film and a similar set-up, I had previously captured spectra of several bright stars. Now, with this digital camera fixed on a tripod aimed roughly at Sirius, would the camera record its spectrum, widened into a colorful band trailed due to the Earth’s rotation? 

So far, the results are encouraging. It took one entire, bitterly cold evening to get the hang of pointing my spectrograph. The camera needs to be aimed above or below the target star since the spectrum is primarily dispersed to one side. At the same time, the camera and grating needs to be mounted such that the Earth’s rotation trails the star and its spectrum perpendicularly to light’s dispersion, that is, the long axis of the image should be oriented north-south. This is not nearly as easy as it sounds to accomplish with the head of a typical tripod, particularly for stars to the east or west. It is especially frustrating in the cold, with gloves and in the dark. 

On the next night, I improved the focusing enough to just start to bring out bands in some stars. I used a neighbor’s porch light and its sharp emission lines to refine and recheck the focus. The zoom lens set to 55 mm and a 30-second exposure seemed to work. Boosting the ISO-speed sensitivity to 1600 helped record fainter stars as well. In addition to Sirius, I shot Orion’s Betelgeuse, Rigel and the Belt, Aldebaran, Mars and the Pleiades. An early morning session netted the first-magnitude stars of spring and early summer: Regulus, Spica, Arcturus, Vega and, still low in the east, Antares. 

That list includes stellar representatives of the common spectral types: B and A, to K and M. Procyon (spectral class F), and Capella (G) are needed to round out this list. Things to try on the next clear evening include: a longer focal length lens for increased dispersion, determining what spectral resolution is possible, and using the camera’s RAW file format for better dynamic range. Tools for extracting spectra from the digital images are built-in to popular image processing software such as Richard Berry and Jim Burnell’s AIP4Win 2.0 and Christian Buil’s IRIS.

So amateurs can easily do simple astrophysics with modest gear and a reasonable investment of time. Said another way: equipment is important, but the effort is more so. You naturally want the newest and best, but just dig out what you’ve got and go observing. Whether it's catching tiny stellar rainbows, completing your Messier list or knocking off a few faint NGC objects, the sky is sure to reward your efforts. 

Moondark is written by Doug Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 26 February 2006. Text and images copyright © 2006 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.