THE ANN ARBOR TELESCOPE
195
ern sky; therefore the observer decides to
work on a star to the south. He starts a
one-hour exposure. Within ten minutes the
smoke has shifted and is crossing directly in
front of the telescope, making necessary an
increase of exposure time by perhaps 50 per
cent. When at last the exposure is finished,
the observer starts work on a northern star.
Yes, you guessed it! In ten minutes or so the
smoke is crossing the northern field.
A less obvious but very real type of interference is due to the inevitable smog that
hangs over any city. This was almost nonexistent when the 37-inch was new and Ann
Arbor a small town. With the growth of the
city and increased industrial activity, the
smog has become appreciable.
N SPITE OF the many handicaps, the efficiency of our recording of star spectra
has increased considerably over the years.
The gains are due to three main improvements.
1. In 1936 an evaporated coat of aluminum was substituted for the former chemically deposited silver coating of the 37-inch
mirror. Silver is as good as aluminum when
fresh, but it tarnishes rapidly and renewal
twice a year was necessary. An aluminum
coat is still fairly good after two years, but
we usually renew it every year. This change
alone has gained us a factor of about two in
efficiency.
2. In 1949 the surfaces of prisms and
lenses in the spectrograph were given a coating that greatly reduces losses of light transmission due to reflections from the surfaces.
This gained us a factor of about two and a
half.
3. The sensitivity of photographic plates
has been increased by a factor of about two
since twenty years ago.
Taken all together, these gains mean a
factor of about ten in efficiency of recording. But over against this we must set the
deterioration of observing conditions due to
smoke-both the power plant and the general city smog. These are estimated to cut
the efficiency in half. The result is a net
gain of five over the observing in 1911. Still,
this means that the 37-inch is now able to
do what it would have taken an 80-inch to
accomplish fifty years ago.
F OR MANY YEARS the 37-inch was the
Observatory's only major instrument.
When it was new, stellar spectroscopy was
still a burgeoning field of research, and many
interesting discoveries were yet to be made.
even among the brightest stars. It is very
fortunate that the study of spectra was
chosen as the main area of research. Had a
program requiring direct photography been
started, the increase of sky brightness would
have put us out of business more than
twenty years ago. But such a single-purpose
program as stellar spectra is too narrow
for a major observatory today. We have
branched out into several other fields.
In 1927 the Lamont-Hussey Observatory
was established at Bloemfontein, South
Africa, for the discovery and measurement
of southern double stars. Professor Hussey
had planned this program but did not live
to see the observatory become a reality. The
observing with the 27-inch refractor was
carried on for many years by R. A. Rossiter
until his retirement in 1953. After several
years of alternating idleness and occasional
use by visiting astronomers, this distant station has recently been reactivated, and work
on double stars is again going forward.
The 1930's saw the development of the
McMath-Hulbert Observatory for solar research, making the University of Michigan a
leader in that field. After an interruption
due to World War II, theoretical astrophysics was strongly emphasized under the direction of Leo Goldberg (Director from 1946
to 1960). During his administration the
Schmidt camera, 24 inches aperture, was installed at the so-called Portage Lake site (on
Peach Mountain, about a mile from the
WUOM station). This instrument can be
used not only for direct photography, but
for low-dispersion spectroscopy when a
large prism is mounted in front of it. Thus
it can supplement the work of the 37-inch
when necessary or desirable. Finally, the
radio telescopes were installed on Peach
Mountain, further broadening the astronomical work of the University. However, the
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