The San Francisco call., 1909-12-02

| cOlOSSd1 tuU6 GlGClcu III a 50
cluded spot of the old mission
vineyard and about which visitors
at the college have made many in
quiries, is a solar telescope. known
among the astronomical craft as hori-
zontal photoheliograph. It's object is
to take sun pictures. and it performs
its office well. A senior student of the
class of philosophy repairs thither for a
few moments every day. and, awaiting
the golden opportunity. takes a snap
at the sun
An ordinary camera tried on the sun
is dismal failure; the object is too
distant the light too dazzling. even the
rapidest focal plane shutter too slow.
Recourse is had to other means. At
the beginning of celestial photography
small telescope was devoted to the
sun. the object lens was corrected for
Photographic rays. the eye piece was
knocked out of the principal focus and
its place taken by an enlarger altering
the rays into camera box, where
thin plate with suitable slit. drawn
across by powerful spring, made an
exposure of from .00O1 to .OOO5 of a
second. This was the case at Kew and
at Greenwich.
+ Later on and ever since with a few
exceptions the sun has been approached
with pieces of ordnance that the art
of mechanics does not we'll like TO con-
template. The tube which constitutes
the dark chamber of even a small pho
tographic lens is already so unwieldy
width that accurate motions can not
easily be secured. let alone the ex-
penses of the, big dome.
The science of optics quickly sug-
gested a remedy. One of two ways
was open. In either case have your
tube as long as the focus of the lens
demands. If you choose the first way
set your tube firmly to level on an ac-
curate north and south line on sup
ports free from annoying tremors; at
the north end of the tube insert the
photo lens with a light tight cover
all around; on tremorless pier in
front station a coelostat or siderostat.
with its polar axis in the meridian and
at the height of the local latitude.
Everything is now ready to throw a
sheaf of powerful rays horizontally to
the south end of the tube. which is
snugIy ensconsed in the photographer s
dark room or near by, where every
precaution has been taken that the
tremors of the slitted exposing slide
he Cd oif hx~
a powerful spring or by push or pull
of the operators hand, or by sheer
gravity.
The other and less practical way is
to have your tube straight up in the
air like a smokestack. build a strong
iron tower around it with a platform
on top to which the object end of the
tube is stoutly fastened. On the plat
form place the coelostat as described
above. At a suitable distance south
of it. have a second rejecting surface,
inclined at an angle of 45 degrees RAYS
from the eoelostat's reflector falling
at an angle of IS degrees upon the sec
ond reflector will be thrown vertically
downward and form a bright ina ge Of
the sun on the focal plane of the photo
lens.
Should the CcleSlidl
care to operate with due regard to all
the minute precautions that insure
final success he may dig deep into the
ground and have his darkroom. say. 20
feet below the surface, where the tenn
perature is more constant and where
the obstruction of earthly light can be
avoided more easily Another advan-
tage is had also at the top end of the
tube, where, on account of the consid-
erable height, the operation of sun pic
ture taking is not likely TO be marred
by ground reflections and radiations.
This second method of erecting a solar
telescope has been adopted, we believe
for the first time, on Mount Wilson. by
that very careful scientist, DR.. George
Z. Hale.
over here at Santa Clara we fol-
lowed UP the first way as more expedi-
tious- and safe enough. Our pier is 10
feet long, rising pyranlidically sky
ward. Near the edge at top the coelo-
stat is securely fastened. Thence the
telescope tube spreads its huge length
straight southwark into the operating
room. being supported all along by a
colonnade of single stout poles imbed-
ded in cement and surmounted by cir
cular iron clasps going a third of the
way around the tube, which is nearly
81 feet long and two feet In diameter,
while the photo lens is only six inches
clear aperture
Experts in solar work have found at
their own expense that large tubes are
essential for sharpness of definition
and clearness of detail in the picture;
for such tubes only can afford large,
tranquil, homogeneous column of air
for>the pencil rays to pass through; TO
insure which one further precaution is
taken. namely. having the tube rooted
over so that one side shall not be
warmer than the other. The direction
has been adhered TO.
The machinery on the pier control
ling the movement of the reflector Is
not. as we were told. after the style of
the FoucaIt heliostat but rather re-
sembles the English form of coelostat
by Cooke & Sons of London. The re-
fiector is very different from the usual
ones. 1t has been found that specula
with silver on AT the back soon lose
their efficiency by the silvery surface
i,t.oa king IN iuto minute fra on,eu fart