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SkyandTelescope.com March 2014 69

with dedicated software. Likewise, deep-sky enthusiasts

take many CCD or DSLR images of a single object and

combine them using any number of special software

applications. The goal in all this is to produce compos-

ite images with less noise, higher contrast, and better

resolution than a single exposure can provide. Yet as

accustomed as we are to stacking images using digital

technology and software, it’s perhaps surprising to learn

that image stacking isn’t a recent innovation — it has

been around for close to a century.

Two pioneers in composite image stacking (known

then as integration printing) were famed Lowell Obser-

vatory astronomers Earl C. Slipher (1883–1964) and

Harold L. Johnson (1921–1980). Together they developed

improved astronomical photographic techniques that later

became standard throughout the age of fi lm, and even

made the transition into the digital age.

Planetary Pioneer

Earl C. Slipher, or “E. C.” as he was known, was an extra-

ordinary planetary photographer. He was the brother of

renowned astronomer Vesto M. Slipher (1875–1969), who

was best known for his discovery of galactic redshifts and

the subsequent realization that many of these “nebulae”

are moving away from us (September 2009 issue, page 30).

Earl Slipher began systematic photography of Mars

with Lowell’s famed 24-inch Clark refractor in 1907, and

he continued photographing it and other planets into the

early 1960s. Many of his fi nest images were published in

two classic books: Mars, the Photographic Story (1962) and

A Photographic Study of the Brighter Planets (1964).

A life-long adherent of Percival Lowell’s belief in

Martian “canals,” Slipher was determined to demonstrate

their reality photographically. This proved a near-impossi-

ble task, and not just because of their dubious nature: the

extremely slow and grainy emulsions available to him at

the time made the work daunting. In an eff ort to circum-

vent both the technical limitations inherent in black-

and-white photography and the relatively low contrast of

emulsion-based images, he experimented with various

color fi lters, telescopes, focal lengths, exposures, and

types of emulsions.

Although most of Slipher’s experiments with photo-

graphy did little to improve the quality of his early plan-

etary pictures, his methodical experimentation in 1918 led

to the standardization of photographic plates for astro-

nomical photometry, which were used until the advent of

digital photography.

Sometime around 1930, Slipher became aware of a

new photographic technique known as integration print-

ing. The method consisted of capturing several photos

of a stationary subject and then meticulously aligning

and exposing these plates on a single print. Due to the

random distribution of the emulsion grain on each indi-

vidual negative, combining precisely registered (aligned)

negatives into one photographic print resulted in a photo

with much less fi lm grain than a single negative could

produce, increasing the signal-to-noise ratio in the image.

Producing an integrated print required dividing the

total printing exposure needed for a single negative

among the number of negatives used. Unfortunately, pho-

tographic paper doesn’t respond to light in a linear fash-

ion, so each successive negative would have to be exposed

for a shorter duration to contribute equally to the fi nal

image. For example, one study recommended combining

three negatives by exposing the fi rst for 59% of the total

time, the next for 23%, and the fi nal negative for 18%.

Slipher quickly adapted this technique to planetary

photography. He took many sequential exposures of a

planet on a single photographic plate in rapid succession,

and then he combined the images into a composite print

using a specially constructed apparatus that precisely

adjusted the paper’s position to register each negative.

The resulting images exhibited higher contrast and mark-

edly reduced graininess compared with single exposures.

Using this complex technique, Slipher was able to

produce some of the most detailed images at the time of

Mars, Jupiter, and Saturn during favorable apparitions.

His images of Mars show hints of what he continued

During the early 20th century, the technique of “integration

printing” became the standard method used for photographing

the planets. A single exposure produced a grainy image with the

fi lm available at the time (top). But by combining multiple expo-

sures together in a single print, the random grain distribution on

each negative would be reduced while details recorded in every

negative, such as the atmospheric bands on Saturn (bottom),

would be enhanced.

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