22 AUSTRALIAN SKY & TELESCOPE JULY 2016
garnered worldwide recognition with his discovery of
Jupiter’s fifth and innermost moon, Amalthea.
Barnardbeganexperimentingwithwide-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
skyscapeswiththeWillardcamerainguided
exposuresuptofivehourslong.Thecamera’s
achromaticdoubletlens(refiguredbytherenowned
Pittsburgh instrument maker John Brashear) was
mountedinasimplewoodenboxandstrappedto
a 165-mm guiding refractor (pictured at right). It
produced photographic images of the Milky Way that,
inBarnard’swords,showed“forthefirsttime,thevast
andwonderfulcloudforms,withalltheirremarkable
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
tokeepwarm,hedeadpanned,“Wedon’t.”Barnard
waseasilyriled—bypoorskyconditions,bureaucraticGOING DEEP IN
MONOCEROS
Barnard’s plate
of the Milky
Wayinnorthern
Monoceros, made
witha15-cm
studio portrait
lens, showed
nebulosity all
around the star
cluster NGC
2264 (centre), the
Christmas Tree
Cluster including
4.6-magnitude 15
Monocerotis. This
section of the
plate is 7°tall.
E. E. BARNARDA PHOTOGRAPHIC ATLAS OF SELECTED REGIONS OF THE MILKY WAYbungling, or, as most recently lodged by Isaac Roberts,
wrongfulcriticismofhiswork.Framing and surface brightness
In his written response to Roberts, which was read
before the Royal Astronomical Society on November
8,1895,Barnardcomplained,“Itisunjusttousean
enlargementsuchasDr.Robertsused,becauseit
necessarily puts these pictures at a disadvantage. My
picture [of the Christmas Tree Cluster region] was
simplyspoiledbythis,whileDr.Roberts’retainedits
original qualities, not being enlarged.” The diaphanous
cloud that stands out distinctly in Barnard’s original,
10 °-tallimageappearsonlyasasubtlefield-brightening
across the 2° plates from Roberts’ 50-cm 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 diffuse
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 emphasise an aspect of
photography that Roberts had failed to grasp. True, a
50-cm telescope has more light-gathering power than
a 15-cm camera lens, and it will record fainter stars.
But unlike a point star, a diffuse nebula is spread
out. What matters is its surface brightness: its light
per unit area of sky, and per square millimetre on a
photographic emulsion. A large-aperture telescope
renders an interstellar cloud no brighter, in terms of
surface brightness, than a small-aperture 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 diffused light is in
nowise confined 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 diffused
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
Barely had the dust settled than a second battle
erupted, this time over the presence of extendedAmateur vs Professional