70 March 2014 sky & telescopeImaging Historyto refer to as canals throughout his long career. Slipher
never realized that he was in fact recording subtle contrast
and color diff erences between adjacent albedo features:
only a few of his photographs were combined into color
prints during his lifetime. One year after Slipher died in
1964, NASA’s Mariner 4 spacecraft photographed parts of
Mars and conclusively showed that the canals were in fact
wind-driven streaks of sand and dust. Slipher might have
reached the same conclusion if he had had access to the
ultrafi ne-grained fi lm that became available later.
Fortunately for us, Slipher passed along his integration
printing technique to his eventual successors at Lowell
Observatory, including Charles F. “Chick” Capen. Capen
shared the information with members of the Association
of Lunar and Planetary Observers (ALPO).Deep-Sky Improvements
Although integration printing led to advances in plan-
etary photography, the technique was slower to enter the
realm of deep-sky astrophotography. During his seven-
year tenure at Lowell Observatory (1952–1959), Harold L.
Johnson devoted almost all his eff orts to perfecting photo-
electric photometry, ultimately leading to the Johnson-
Morgan UBV (ultraviolet, blue, and visible) system of
stellar observing that is still in use today.
Almost forgotten amid his many other achievements is
a 1958 paper coauthored by Johnson with R. de F. Neville
and Braulio Iriarte in the Lowell Observatory Bulletin (Vol.
IV, No. 93), modestly titled “A method of increasing thephotographic limiting magnitude of an astronomical
telescope.” Directly infl uenced by Earl Slipher’s planetary
work, the team used the 13-inch photographic refractor
(the same used to discover Pluto nearly three decades
earlier) to take several identically developed 45-minute
exposures of the galaxy M33 on the widely used East-
man 103a-0 plates. Prints were then made from a single
plate and compared with a composite image generated
by combining 10 plates with the same superpositioning
apparatus developed by Slipher.Using the integration printing technique, Slipher documented Mars for more than fi ve decades. Over that period he recorded changes in
albedo features (see page 50), captured atmospheric belts for the fi rst time (left), as well as numerous dust storms (right).While working at Lowell Observatory, Harold L. Johnson pub-
lished a short paper on integration stacking of deep-sky images.
Near right: This is a single negative of M33 taken through a
13-inch refractor. Far right: By integrating ten 45-minute nega-
tives of M33, he increased the limiting magnitude of the image.
Note the increased visibility of stars and the spiral arms.Stacking.indd 70 12/26/13 7:12 PM