Sky and Telescope - June 2016 - 38

Early Pro-Am Split

E. E. BARNARD, A PHOTOGRAPHIC ATLAS OF SELECTED REGIONS OF THE MILKY WAY

California. It was here, in 1892, that he garnered worldwide recognition with his discovery of Jupiter's ﬁfth and
innermost moon, Amalthea.
Barnard began experimenting in wide-ﬁeld celestial
photography in 1889, six years before Isaac Roberts's
unﬂattering display of his work. In the interim, he accumulated a series of extraordinary skyscapes with the Willard camera in guided exposures up to ﬁve hours long.
The camera's achromatic doublet lens (reﬁgured by the
renowned Pittsburgh instrument maker John Brashear)
was mounted in a simple wooden box and strapped to a
6½-inch guiding refractor (pictured at right). It produced
photographic images of the Milky Way that, in Barnard's
words, showed "for the ﬁrst time, the vast and wonderful
cloud forms, with all their remarkable structure of lanes,
holes and black gaps and sprays of stars." On cold nights,
he suited up in an "Esquimaux coat made of reindeer
skin, and heavy rubber overshoes." When asked by one

shivering guest how to keep warm, he deadpanned, "We
don't." Barnard was easily riled - by poor sky conditions, bureaucratic bungling, or as most recently lodged
by Isaac Roberts, wrongful criticism of his work.

Framing and Surface Brightness
In his written response to Roberts, which was read before
the Royal Astronomical Society on November 8, 1895,
Barnard complained, "It is unjust to use an enlargement such as Dr. Roberts used, because it necessarily
puts these pictures at a disadvantage. My picture [of
the Christmas Tree Cluster region] was simply spoiled
by this, while Dr. Roberts' retained its original qualities, not being enlarged." The diaphanous cloud that
stands out distinctly in Barnard's original, 10°-tall image
appears only as a subtle ﬁeld-brightening across the 2°
plates from Roberts's 20-inch reﬂector. When imaged in
a wide ﬁeld that surrounded it with darker sky for comparison, Barnard insisted, the nebulosity left no doubt
that it was actual diﬀ use matter and not the light of
unresolved stars. "All that is wanted to show it fairly well
with his reﬂector is a somewhat longer exposure and a
larger plate to give more sky around it for contrast."
Barnard went on to emphasize an aspect of photography that Roberts had failed to grasp. True, a 20-inch
telescope has more light-gathering power than a 6-inch
camera lens, and it will record fainter stars. But unlike a
point star, a diﬀ use nebula is spread out. What matters
is its surface brightness: its light per unit area of sky, and
per square millimeter on a photographic emulsion. A
large-aperture telescope renders an interstellar cloud no
brighter, in terms of surface brightness, than a smallaperture scope with the same f/ratio. Also important are
sky transparency, exposure time, plate sensitivity - and
a ﬁeld wide enough to frame a very dim large object in a
darker surround.
Barnard included three additional wide-ﬁeld prints of
the 15 Monocerotis-Cone Nebula region, demonstrating,
in his opinion, "that Dr. Roberts' reasoning is decidedly
wrong. This diﬀ used light is in nowise conﬁ ned to the
star areas. It will be also readily seen that it spreads over
a large region where there are essentially no stars at all
- even where Dr. Roberts' reﬂector can show no stars.
That this is real diﬀ used nebulosity there is no reason
whatever to doubt."
But Roberts refused to yield: a large-aperture telescope picks up fainter stars and therefore, he asserted,
was able to reveal fainter nebulosity - which, as his
photographs showed, was not there.

A Sinking Ship
GOING DEEP IN MONOCEROS Barnard's plate of the Milky Way in northern
Monoceros, made with a 6-inch studio portrait lens, showed nebulosity all
around the star cluster NGC 2264 (center), the Christmas Tree Cluster including
4.6-magnitude 15 Monocerotis. This section of the plate is 7°° tall.

June 2016 sky & telescope

Barely had the dust settled than a second battle erupted,
this time over the presence of extended nebulosity around
the Pleiades. Barnard had taken wide, four- and ten-hour
exposures of the Pleiades and their neighborhood. These

Sky and Telescope - June 2016

Table of Contents for the Digital Edition of Sky and Telescope - June 2016