Red Light Sky Photography

Red Light Sky Photography

When a massive star forms, not only does it shine with the light of 10,000 suns, but it also lights up the surrounding gas. These glowing clouds of gas are called gaseous nebulae. Stars shine at all wavelengths. But these nebulae are more particular as they radiate most of their energy at only a few special places in the spectrum. With cheap and simple equipment, the astrophotographer can capture these faint glowing clouds of gas by viewing them in their special wavelengths. Many of these nebulae cover several degrees of sky – thus they’ll look impressive in the field-of-view of an ordinary 35-mm camera. Besides a camera, all that is needed to photograph these objects is a clock driven mount, the proper filter and some special film.

The best wavelength for photographing these nebulae is that of the Hydrogen-alpha (H-alpha) emission line at 656 nanometers wavelength. Light at 656 nm is deep red – deeper than ordinary film will record. (Ordinary film stops working before the eye stops seeing to prevent the lips and noses from coming out too light on snapshots.) Special film is required for deep sky nebular photography, and the most useful is Kodak 103aE. It is good for two reasons: 1) The basic emulsion – type 103 – has been treated to make it sensitive to some red wavelengths – which is what the E stands for, and 2) 103aE film has been treated to supress reciprocity failure – which is what the a means. Also suitable is 103aF film, though it is not as good because it is sensitive too far into the red.

Hubble. Hubble Space Telescope-Image of Supernova 1994D (SN1994D) in galaxy NGC 4526 (SN 1994D is the bright spot on the lower left). Source of the photograph : NASA/ESA

How can a film be too sensitive? In nebular photography, you want to capture objects which emit a very peculiar spectrum with light mainly in one narrow line. But must of the sources of contaminating background light emit over a wide part of the spectrum. If we can select a narrow “window” in the spectrum, using a filter and film properly chosen to make that window as narrow as possible, we can record more of the Nebula without recording background light which will fog and destroy the image of the nebula. So we must choose a filter (called a short cut filter) which cuts off all wavelengths shorter than the line we are to record, and we must choose a film that isn’t sensitive to wavelengths longer then we want. 103ae was designed to cut off at wavelengths just a bit longer than H-alpha. On the other hand, 103aF was designed to record very deep red light, and in so doing, it records unwanted background light when you photograph the nebulae.

The unwanted background light comes from many sources. Even on the very darkest night the sky has some brightness, called nightglow. The light from billions of stars in our galaxy can also wash out the nebulae, most of which lie against the background of the Milky Way. And unfortunately, city light pollution often provides the greatest source of unwanted illumination. A carefully-chosen film-filter combination can help – but not eliminate – the effects of city lights. The moon can be a nuisance too, and deep sky astrophotography should be done when the moon is down or close to new.

In choosing your filter, you’ll especially want to avoid transmitting the bright oxygen airglow emission lines at 630 and 636nm (part of the nightglow). A filter that is as opaque as possible at 636nm – but as transparent as possible at 656nm – is best. Two kinds of filters are available: glass and gelatin. The best inexpensive filters are made of colored glass; the Corning #2403 and Schott #RG-645. They begin to transmit light at wavelengths around 640nm, and continue being transparent well into the infrared. In combination with an E film, which cuts off soon after the H-alpha line at 656nm, it is possible to get a peak of sensivity of the film and filter together that is only 20 to 30nm wide and centered on H-alpha. An F film, with its sensitivity extending farther into the red, gives sensitivity to a wider range of wavelengths.

Starting your experiments in H-alpha photography, a gelatin filter has two advantages: cheapness and availability. A two-inch square Kodak Wratten filter will be available in your photostore, whereas glass filters must be ordered from industrial suppliers. You best bet in a gelatin filter is the Kodak Wratten #92. Other deep red filters such as a Wratten #29 will work, but not as well because they pass more of the sky background. Gelatin filters are soft; they scratch easily and can be damaged by moisture. With reasonable care, however, they’ll give adequate service.

The proper film and filter can record the H-alpha line without recording much unwanted glow from the night sky.

Wratten filters may be ordered through your local photo dealer. Kodak publication B-3 (“Kodak filters for Scientifc and Technical purposes”) may be ordered from Dept. 454, Eastman Kodak Company, Rochester, N.Y., 14650, or obtained through your local photo dealer.

Corning filter information is available from Robert C. Saxton, Optical Products Dept. , Corning Glass Works, Corning, N.Y. 14830. The #2403 filter must be ordered through Corning dealers (names are supplied by Corning), as Cs 2-58. A two inch square filter (polished) costs about $10. Corning’s catalog can be purchased for $1.

Schott filters can be ordered from Schott Optical Glass, Inc., Duryea, Pa 18642. The RG-645 (polished) filter, two inches square and 3mm thick, costs $12.15. (Schott’s minimum order is $30). No catalog is available, but inquiries may be directed to Jim Speicher, Product Manager.

Baird-Atomic, Inc. 125 Middlesex Turnpike, Bedford, Massachusetts 01730, is one of many suppliers of interference filters. Their standard filter 11-3-4 (6563-A) costs $77 in the two-square-inch size. Better filters are available at higher prices, and their catalog is available upon request.

(This text was published in Astronomy Magazine, July 1976, and has an historic value).