"Color Astrophotography, Getting the Colors Right"

from the December/January 1988/1989 article on how to color balance long exposure astrophotographs

by Robert C. Price

Getting the colors right is a commercial slogan you may have heard on television. How do you get the color right when you are using color film for long exposure astrophotography? Getting the color right is not an easy task as illustrated by the beautiful green (grossly inaccurate color for the Orion nebula) photograph of the Orion nebula on posters and on the cover of Scientific America; and the awful blue-green color of the Ring nebula, M57, in the April 1987 issue of Astronomy magazine (page 111). Both these nebulae are very bright and only take a few minutes exposure to reveal their true color, The Orion nebula is a beautiful pink-magenta and the Ring nebula is red and yellow. If such short exposure photographs have color balance difficulties what about longer exposures where color balance is grossly shifted by reciprocity failure? All color film is manufactured without any attempt to optimize its use for astrophotography (that is, to minimize its reciprocity failure). Color films exhibit reciprocity failure of varying degrees depending on the film type, exposure length, and temperature. The long exposure color balance problem exhibited by color film is characterized by different color sensitive emulsion layers exhibiting different reciprocity failure rates. If all color sensitive layers had the same reciprocity failure, color films would maintain the same color balance regardless of exposure time. Color film is balanced for short exposure under sunlight illumination. Long exposures such as those used by astrophotographers cause the colors to become unbalanced. Balancing the color content of long-exposure color negative can be done when printing the negative by using the same gray card method used in printing standard daylight color negatives. In the gray card color balance method for daylight photography, a color/gray card is photographed under sunlight conditions. The negative is printed using a filter pack in the enlarger which results in a print that duplicates the color/gray card. Other negatives taken under the same conditions (the same film, exposure, and illumination) will result in correctly balanced color prints when printed with this same filter pack. This same approach can be used for long exposure color photographs. To use this method to color balance long exposure color astrophotos requires a long exposure color negative of a color/gray card. Since it would be difficult to place a gray card among the subjects photographed, we must make a gray card that accurately indicates the shift in color caused by long exposure photography. Just as a gray card photographed on a sunny cloud-free day can be used in the printing process to obtain the correct color filter pack for all negatives taken under those similar conditions; a gray card taken under low sunlight type illumination be used in the printing process for long exposure astrophotographs. To obtain a gray card that accurately reflects long exposures, the gray card negative must be a product of the same exposures that are used in astrophotography. Ten minutes to one hour exposures of a gray card will produce a negative that will reflect the shift in color balance produced by long exposures similar long exposure astrophotographs. One obvious method to produce such a gray card is to use sufficient ND filters to require 10-60 minutes of exposure when photographing a gray card in sunlight. The author has not been successful in attempting this because SLR cameras are not designed to be very light tight when the mirror is raised and the shutter curtain makes the exposure. THe camera is sufficiently light tight for exposures of fraction of seconds, but not tens of minutes. A second method is to use the full moon as a low level sunlight source and photograph a gray card by moonlight. The full moon provides the advantage of a low level sunlit balanced source, complete with blue sky, just as the sun provides when a sunlit gray card is photographed. Since the moon is reflected sunlight from a mostly colorless surface, the long exposure gray card will indicate what filter pack should be used to produce a color print that corrects the color shift caused by the long exposure. Table 1 provides gray card exposure times, F-stop settings for various films under cloudless full moon conditions. Actual color shifts from daylight exposures are given in table 2 and are for a specific temperature. When using this gray card method to color balance long exposure astrophotographs, several limitations must be taken into account. The gray card conditions must match all the astrophoto conditions. these conditions are:

1. Film should be from the same batch and be aged similar to gray card negative.
2. Exposure must be close.
3. Temperature is a definite factor that will alter the color balance of long exposures. Gray card negative temperature must be close to actual long exposure astrophotograph temperature.

One factor to consider when choosing a film, is the amount of color balance shift from the daylight exposure. If the color shift is severe, there may be additional complications in correcting the astrophotograph in order to obtain proper color balance. Color film is usually balanced for daylight exposures. The color layers will have the same latitude, plus or minus so many F-stops, centered on a specific exposure density. As the color balance shifts, so does the exposure latitude of each emulsion layer. Thus a photographic color layer may saturate or not be able to tolerate any underexposure. This will upset the films ability to be accurately color balanced for long exposures. Typical daylight to long exposure color shifts are illustrated in table 2.

Below: Table 1 Gray card exposure times

Film Gray card exposure* Konica SR-V 100 40 minutes @ F/5.6 Konica SR 400 20 minutes @ F/5.6 Konica SR 1600 20 minutes @ F/11 Konica SR-V 3200 10 minutes @ F/16 * Density required to equal recommended daylight exposure

Below: Table 2 Daylight and Gray Card filter packs

Film Daylight Gray Konica SR-V 100 55Y 50M 105Y lOOM Konica SR 400 55Y 40M Konica SR 1600* 25Y 30M 15Y 55M Konica SR 1600 25Y 30M 40Y 60M Konica SR-V 3200 40Y 55M 55Y 70M * hypersensitized