Curiously this powerful methodology is scarcely used by amateurs, most likely because real spectroscopy is highly technical and quantitative. But you need not have a Ph.D. in physics to learn about stars, just a way to disperse the wavelengths. Ordinary prisms and thin film gratings (actually inexpensive plastic replicas of true gratings) are fine for demonstration purposes, but to spread starlight by eye requires a grating optimized for this purpose, and at least one such product is available commercially to amateurs.

The Rainbow Optics’ Star Spectroscope transmission diffraction grating (review in October 1995 Sky and Telescope) looks like a filter and simply screws into the eyepiece. Compared to a plastic film, these rulings on glass have been optimized for angular dispersion and blazed to send most of the light to one side so the spectrum is brighter. On a medium-sized telescope, and using the provided cylindrical lens to broaden the spectrum, absorption lines can be readily seen on bright stars, their strength distinguishing among the classical spectral types: O, B, A, F, G, K and M.

In a historically interesting way, this grating can be used to photograph spectra with enough resolution to classify spectral types—the forefront of astrophysics in the early 20th century. Here’s how I took the photographs at right. The grating was slipped into filter holder in front of a telephoto lens (focused at infinity) on a 35-mm camera loaded with fast print film. The camera was fixed on a tripod and pointed at a bright star for exposures of up to 4 minutes. The trick is to orient the grating's dispersed light and camera frame perpendicular to the star’s motion across the sky. Trailing broadens the spectrum so that lines and bands are visible—effectively using the Earth’s rotation as the slit in a conventional spectrograph. The film was processed and printed as for any other nighttime photos. Spectra of Sirius and the Great Orion Nebula showing absorption and emission lines (respectively) are presented at right.

Granted, not much if any modern, cutting-edge astrophysics can be done with this setup. But turned to an exceptionally bright nova or Milky Way supernova, changing spectral lines would reflect the life and death of the star. Since neither is likely to occur anytime soon, collecting more light and using a more sensitive detector is the way forward in amateur astronomical spectroscopy. 

Two great resources for those interested in this activity are: the Forum for Amateur Astrospectroscopy web site, and Practical Amateur Spectroscopy edited by Stephen F. Tonkin (Springer, 2002).

Moondark is written by Doug Miller, published on the web, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 22 June 2003. Text and images copyright © 2003 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.