The evening times. [volume] (Grand Forks, N.D.) 1906-1914, 1913-07-26
THE TELESCOPE
OF THE FUTURE
Saving in Cost and Space VIII
Cause Large Refractor to
Give way to Reflector
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tscientiac Amerlcan.l


%(Scientifc Amerlcan.)
1t may not be out of place to recall
the essential difference. between re-
tractor and reflector. The former is
the one usually called to mind when
we think of a telescope. 1t has a lens
at the upper end Of a tube through
which the light passes. At the lower
n Is the eyepiece through which we
look. On the other hand the reflector
mas no lens, but a large concave mir
FOR which takes the place of the lens.
Ihis mirror is placed at the lower end
of the tube, the latter being open at
the upper end. The light passes
down through. the tube to the mirror,
brought to a focus where the eye
piece or Photographic plate is placed.
one of the principal points in favor
Of the reflector is the saving in cost.
The Clarks received $4,000 for the
36-inch ems of the Lick Observatory,
m a mounting similar to that would
cost abou an equal amount. Thus
this large %refractor may be considered
to represent an investment Of approx
imately 800. 900.
Now compare this cost with that Of
a reflector of the same aperture. 1t is
possible to obtain a three-foot mirror
for about $4,000. A suitable mounting
can be had for about $4,000, or ever
less II a very simple design is chosen.
A well-equipped reflector would there
fore cost In the neighborhood Of $780,
900. or approximately one-1fth that Of
a %refractor of the same siz.
For telescopes Of this a%perature,
which must be permanently mounted
n properly protected from the
weather, an additional item must be
considered. namely, the cost of build
ing and dome. The reflector car be
made much shorter than the refractor.
SO that a dome with a diameter Of 40
feet Is ample for a three-foot instru-
ment Of this type. while the Great
Lick ~refractor requires a dome having
a diameter Of Is feet. Since the cost
of domes varies approximately as the
cube of the diameter. the r~efractor
dome would cost e six times as
much as a SIMILAR structure for an in
strument Of the other type.
1t may therefore be said that, in
general, the properly housed r~efractor
of large size will cost at least five. and
possibly six. times as much as a re-
Hector Of the same. aperture. For
even higher.
There is another point. however.
which must be considered under the
heading of efficiency. Telescopes are
ever increasing in size. In order to.
have a lens or mirror perform we'll it
must be supported in such a way that
the fexure due to its own weight does
not i%njuriously deform the optical sur
face. A lens must be supported from
an increase in size will add suficient
extra weight to produce appreciable
reformation and. thereby injure the
performance of the telescope. Certain
tests made, with the forty-inch lens of
the gYerkes Observatory lead to the
conclusion that 1t would not be adj
visable to make a refractor Of a larger
size unless a new way Of supporting
great lenses is found. With a mirror.
glass of sufficient thickness to be fair
ly rigid and then have a system of
supports so designed as to support the
entire back Of the mirror and thus
completely prevent f1exure. ,i
Furthermore. the larger a lens the
Kicker the glass. This e in
creased loss Of light due to absorption.
this is really a serious matter
will be realized when 1t is known that
the great Lick and Yerkes lenses
transmit only about so per cent of the
Photographic rays falling on them.
This loss Is due primarily to absorp-
ton. Doubling the size Of such a lent
would about double Its thickness and
1t would therefore transmit only about
rs per cent Of the actinic rays. This
mean that while the lens area has
been increased four times the amount
Of light reaching the image would
only be doubled. Now the silver film
Of q mirror in good condition reflects
about go per cent Of the Photographic
rays. Therefore by doubling the aper-
ture OF a mirror. the light reflected Is
increased in the same ratio as the
or four-fold. Accordingly for
large instruments. the reflector is the
more efficient In Its l~ight-grasp.
A ~refractor is not. well adapted to
most kinds Of direct Photographic
work. It can. however, be used for
both visual and spectrographic obser-
yations. On the other hand. the re-
Hector Is we'll adapted to visual work
and 10 at it, best IN photographic and
direct photosrpahic applications. II
ts therefore the more universal instFu-
ment