"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