Suggestions for Better Spectra

• If the star passes overhead the distorting effects of turbulence is greatly reduced because its light passes through less atmosphere, resulting in brighter and sharper spectra. Another advantage is that the light enters the atmosphere at right angles, reducing atmospheric refraction which can distort star images into tiny rainbow streaks. The constellation of Cygnus makes an ideal target because it passes through zenith in the fall season for mid-northern lattitudes.

• If you are using a large telescope 8 inches in diameter or above, a 5 inch aperture stop can dramatically reduce the effects of turbulence. An aperture stop is a big lens cap with a small off center hole cut out.

• If you use a Dobsonian and your telescope's eyepiece has poor color correction or large off-axis star abberations, you may have to perform frequent re-centering to keep the drifting star image in the sweet spot near center of the field of view (along the optical axis).

• If your telecope has above average tracking capabilities, try placing a slit in the focal plane. This should greatly reduce stray light, while sharpening the resulting spectra, since atmospheric turbulence becomes less of a nuisance. Note that Nagler eyepieces don't allow access to the focal plane. Photographically generated slits can be obtained from Learning Technologies.

• If you can obtain a long focal lenght cylindrical lens or beam expander, you can give the spectra a bit more width in the direction perpendicular to the dispersion axis. This makes spectral features easier to identify because instead of dots, the emission lines become small lines and stand out more clearly (like the digitized spectra diagrams above).

• You can also obtain a commercial amateur astronomer spectroscope from Rainbow Optics which uses a blazed replica transmission grating with lower dispersion than holographic film. Edmund Scientific offers a stellar spectroscope based on multiple prisms to increase dispersion.

Suggestions to Reduce Light Pollution

1. If you observe from the city, light pollution can greatly reduce the contrast of the spectrum relative to the background sky. The glare becomes much worse near the horizon so restrict amateur spectroscopy to high sky elevations.

2. If your site suffers from very bad light pollution, keep the star centered in the field of view and slowly step away from the eyepiece while keeping the grating near your eye until you find that the diffracted spectrum falls outside the field of view. The spectrum now appears against a much darker background greatly improving the contrast.

3. If the surface of the ground in the area where you observe is too bright from indirect light pollution from the sky, you can increase the contrast by placing the grating over the hole in a somewhat larger piece of opaque carboard or plastic. The carboard should be sufficiently large to sheild your eye from ambient light.

4. To reduce stray light from light pollution you can surround your head with a black 'photographers' cloak.

5. Instead of a grating, a prism could be used to break up the spectrum, although its efficiency is near 100 percent, the prism's dispersion or deflection is much smaller. It is much more difficult to star hop with the prism in place. For strong light pollution only a high dispersion holographic diffraction grating is capable of 'bending' the spetrum out of the field of view (see suggestion 2) while simultaneously providing the high diffraction efficiency necessary for good contrast.

REFERENCES

1. IDA : International Dark sky Association
2. Star spectroscope (about $200 US)
   (Oct.95 Sky and Telescope)
   Rainbow Optics,
   1593 E Street,
   Hayward, CA 94541
   
3. 5 prism stellar spectroscope, fits standard eyepiece (item 50023)
   Edmund Scientific