Sky and Telescope - November 2018 - 32

Imaging History

responsible for establishing the film-hypersensitizing processes
(already initiated at the UK Schmidt) to improve the sensitivity of the photographic plates. Hypersensitization reduces
reciprocity failure of photographic materials. Reciprocity
failure is a well-known characteristic of photographic emulsions, which can be understood as the effective speed of the
emulsion falling dramatically during long exposures. When
exposure times are in minutes (rather than the fractions of
a second for snapshots), raw film does not record double the
photons on the negative if it's exposed for twice the time. In
the late '70s, this failure was found to be reduced by baking
the plates in an oven for a few hours in nitrogen gas and then
soaking them in hydrogen gas. Once hypersensitized, the
plates acquire a brief shelf life and have to be exposed as soon
as possible (within just a few hours!) and developed quickly
to obtain optimum results.
During the mid-'70s, professional observatories collected
most of their photons using photographic plates, though some
electronic detectors were used for recording spectra. These
plates, manufactured by Kodak, were glass sheets coated with
a thin emulsion layer that was sensitive to a broad wavelength
range, making them suitable for astronomy. They were very
rigid and stable, though expensive, and didn't curl or buckle
during long exposures like commercial film acetates. Their
large size allowed them to cover 6.6° × 6.6° on a 356-mm (14inch) square plate when exposed in the UKST. The AAT used
smaller plates covering one square degree of sky.
Once exposed and developed, the plate became a negative image where the stars appear as black dots, galaxies and
nebulae are gray smudges, and the sky background is a uniform
gray. Astronomers preferred to study original plates, as creating
a positive image does not add scientific value to an exposure,
and any additional process tends to degrade
the original quality. Additionally, human
vision can pick out faint structures more easily on a negative image than on a positive.
All this work occurred in the black-andwhite domain, but sometimes filters were
employed to isolate a particular region of the
visible spectrum. In general, however, the
main purpose of photography with a large
telescope was to study faint objects. Exposing
plates directly at the focal plane of a telescope
created nice monochrome images with very
little information regarding an object's color.
From his earliest days working with
the AAT, however, Malin was interested in
u TECHNICOLOR GLORY Before the 1980s, color
photographs of deep-sky objects were a rare treat to
come across. It wasn't until David Malin perfected the
technique of combining B, V, and R spectroscopic
plates as blue, green, and red images that dazzling
color pictures of objects became possible, like this
portrait of the Zeta Orionis region in Orion captured
with the UK Schmidt Telescope.

32

N OV E M B E R 2 018 * SK Y & TELESCOPE

obtaining color images. But telescope time for these experiments was difficult to find. He first tried to jury-rig a Hasselblad roll-film holder at AAT's prime focus in May 1976. Two
months later, he stuck large sheets of color negative film onto
blank glass plates to make color exposures.
Unfortunately, color film had some characteristics that
made it inefficient for astronomical use - the lack of contrast
and non-uniform spectral response being the most obvious.
Also, color films intended for daylight snapshots suffered from
reciprocity failure and did not work well with long exposures.
The tests with color film were interesting, but the low
contrast and weird color balance convinced Malin that this
was not the correct approach. The solution, it turned out, had
already been proposed more than a century earlier.
In 1861, the Scottish scientist James Clerk Maxwell had
presented the fundamentals of color synthesis and photography in a famous lecture at London's Royal Institution. In
trying to understand the process of color vision, he showed
that color pictures could be made by registering three positive, monochrome images on a screen that were projecting
through the same color filters used to take the images. The
most realistic images were those taken and projected through
red, green, and blue filters. Maxwell had established the basis
of both color photography and color vision, and is considered
to be the father of color photography. Now Malin had to
apply it to astrophotography.

Tricolor Photography
Malin realized that the most promising option to extract
color from the objects in the universe was to use a modification of Maxwell's additive three-color process in the astronomical arena. This required three plates of the object (and



Sky and Telescope - November 2018

Table of Contents for the Digital Edition of Sky and Telescope - November 2018

Contents
Sky and Telescope - November 2018 - Cover1
Sky and Telescope - November 2018 - Cover2
Sky and Telescope - November 2018 - 1
Sky and Telescope - November 2018 - Contents
Sky and Telescope - November 2018 - 3
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