SkyandTelescope.com March 2014 71The fi nal composite print reveals an impressive gain
of 1.2 magnitudes over the single exposure, from 18.5 to
19.7. This tour de force is doubly impressive, because all
7½ hours of exposures were manually guided, requiring
constant, careful hand-guided corrections at the telescope,
followed by equally intensive work in the darkroom to
develop, mechanically align, and print each negative to
produce the fi nal result.From Film to Bytes
So how do Johnson and Slipher’s laborious eff orts mea-
sure up against today’s digital technology? It wouldn’t
be fair to compare their results with the images that are
produced by modern professional observatories. Even
amateurs using relatively modest equipment routinely
produce higher-resolution images of the bright planets
and much deeper images of deep-sky targets than any-
thing that was possible a half century ago. But the process
of integration printing (now known as stacking) has
become a fundamental principle in nearly all astrophotog-
raphy done today.
In planetary imaging, software programs automati-
cally sort and stack thousands of images to produce com-
posites. These images are so free of noise that aggressive
sharpening techniques can be applied to reveal details
only hinted at in the best images of the fi lm era. Addition-
ally, these programs take stacking to a higher level by
monitoring hundreds of points in a video and then stack-
ing the sharpest portions of each image.Every deep-sky photo-processing program uses a
stacking algorithm to increase the signal-to-noise ratio
— much as Johnson and his colleagues describe in their
1958 paper — while also removing additional unwanted
signals, including satellite and airplane trails. With
electronic autoguiding and automated imaging programs,
professionals and amateurs alike can shoot as deep as
their skies will allow by shooting dozens of hours of
exposures over multiple nights. And you can combine the
exposures nearly automatically on your computer, without
the need for a custom mechanical apparatus or photo-
graphic developing chemicals.
Although we have come a very long way since the age
of emulsion-based astrophotography, the ingenuity of
these pioneers shines through in nearly every planetary
and deep-sky image you see today. ✦Astrophotographer Klaus Brasch wishes to thank Lowell
Observatory librarian and archivist Lauren Amundson for
her invaluable assistance in researching this article.Today, consumer DSLR cameras and amateur telescopes easily
top the overall quality of the best fi lm astrophotographs of the
mid-20th century by relying on the integration principle. The
above image of M33 was recorded by the author using a modifi ed
Canon EOS 6D DSLR with a Celestron EdgeHD 11 Schmidt-
Cassegrain telescope. He stacked fi ve 5-minute exposures to
record considerably fainter stars.KLAUS BRASCHStacking.indd 71 12/23/13 11:35 AM