Space Photo

Space Photo

Your glass of gelatin filter should be mounted in front of the camera lens. Tape and Plasti-tak will work, but it’s a good idea to get a gelation filter mount for your camera. It is important that the filter not rattle about, for it the filter moves during an exposure, double images may result.

A third type of filter – the interference filter – exist; however, those with a range of 5 or 10nm around two inches square may cost $100 or more. Also, several technical difficulties are associated with interference filters. Only light passing perpendicularly through an interference filter is properly filtered; light at all other angles is filtered at a shorter wavelength. This will result in a loss of sensitivity for nebulae at the edge of your camera’s field of view. This effect shows on some of the pictures in this text. Some H-alpha interference filters also leak light at shorter wavelengths – enough that they may even look blue! In this case, an extra red filter (which may be difficult to mount properly) is necessary. If you have a Schmidt camera, an interference filter will not work very well because of the camera’s fast optics. Considering the cost and other problems involved, don’t star with and interference filter. After you’ve gained experience with glass and gelatin filters, you may feel the added expense is justified.

Rosette Nebula. The nebulosities NGc-2237 (The Rosette) and NGC-2264. Image by Oliver Stein

Gaseous nebulae are very faint, so long exposures are needed. The best exposure depends on the brightness of the nebula and the sky, the f-ratio of the camera, the type of filter and the sensitivity of the film. All these can vary; in particular, film sensitivity varies substantially from one production run to another, so it isn’t possible to recommend exact exposure. You’ll have to start with a trial exposure and then adjust the time as necessary. Too short a time will not reveal the full extent of the nebulosity, while too long a time will result in sky fogging the exposure completely.

On a dark night away from city lights, with an f/2 or f/2.8 camera lens and any of the recommended filters, your trial exposure should be about an hour. Develop the film after each night’s work so you know what alterations to make in the next night’s shooting. To help judge what these corrections should be, it is essential to keep a record of the length of each exposure, the filter used, and how bright the sky appeared. In city suburbs, your exposures may fog in only 5 or 10 minutes – but you’ll still find you’ll record objects you couldn’t without the filter.

Of course, there is a tradeoff to be made between exposure times and f-ratios. Low f-rations allow shorter exposures but suffer more from lens aberrations that deform the star images. For most nebulae, even the shortest exposures necessary are long enough that some method of guiding is needed. You’ll need a well-aligned equatorial mount with an accurate drive for guiding hour long exposures. (The problem of accurate guiding was discussed in Photography in Astronomy in January 1975 and June 1976.)

For locating nebulae suitable for H-alpha photography, one of the best resources available is the Skalnate Pleso Atlas of the Heavens and its companion catalog. These are publications of the Czechoslovakian Academy of Sciences, but are available in the United States from the Sky Publishing Corporation. In the Atlas, emission regions are indicated in green (though unfortunately so are reflection nebulae which emit blue light rather than red). Some of the large, very faint nebulae, such as those associated with the stars Zeta Ophiuchi and Lambda Orionis, are not properly shown in the Atlas. However, it does show most of the objects appearing in the list accompanying this text. Norton’s Atlas is less useful for finding nebulae; the maps use Herschel designations which need to be converted to NGC numbers to be of use, and Norton’s is not as complete as the Skalnate Pleso Atlas.

Although there is a wealth of nebulae to be photographed, astrophotographers seem to concentrate on the same old ones such as the North America nebula and the Orion. But many other areas deserve as much attention. The nebulosities in Monoceros are good subjects for 35mm work. Although you’ve seen many pictures showing the nebulosity in Sagittarius, you probably haven’t seen any showing the nebulae in Scorpius and around Zeta Ophiuchi. Pictures of Cassiopeia would be rewarding if they showed the nebulosities to the north and east of the constellation.

Although we’ve been concerned here with wide-angle 35mm work, those with telephoto lenses or Schmidt cameras will find there are many smaller objects that deserve attention. If you like the Cygnus loop, there is a similar object in Gemini: IC-443. Instead of the North America nebula, why not try the nearby IC-1396? The Lambda Orionis nebula would just about fill the field of view of a small Schmidt camera: IC-405 and IC-410 in Auriga would also look good. An interesting subject for Schmidt work is the Dividing Cell nebula (NGC-6888) in Cygnus.

H-alpha photography is comparatively simple and a very successful technique for amateur astrophotographers. The results here, though not perfect, should be encouraging to others.

Extended list of Deep Sky Nebulae. H-Alpha Objects For Your Camera

Barnard’s loop, Lambda Orionis, NGC-2264, NGC-2237 (Rosette nebula), M-42 (Orion nebula), IC-434 (Horsehead nebula), IC-2177, NGC-1499 (California nebula), IC-405, IC-410, IC-443, HGC-7822, IC-1805, IC-1848, IC-1570, Gamma Cygni, NGC-7000 (North America nebula), Cignus loop (Veil nebula), IC-1396, , NGC-6820, NGC-6888 (Dividing cell nebula), M-8 (Lagoon nebula), M-16 (Eagle nebula), M-17 (Omega nebula), IC-4628, M-20 (Trifid nebula), Zeta Ophiuchi, Tau Scorpii, Pi Scorpii, Sigma Scorpii, IC-4608.

(By John Davis, William Tobin and Joel Eaton, Astronomy Magazine, August 1976)