Evening star., 1928-02-28
u. S. SCIENTISTS TO CUT HOLE
IN GIANT TELESCOPE GLASS
Commercial Firms Refuse to
Undertake Delicate
Task.
Optical Disc Is Largest Ever
Made in This
Country.

! Having successfully completed the
/ task OF casting the largest disk OF
! optical glass ever made in the United
/ States. scientists of the Bureau of
/ Standards will soon undertake 4 job
quite as delicate--that of cutting an
eight-inch hole III the completed disk
/ AS soon as Clifford G. Crump
astronomer OF Ohio Wesleyan Uni
verSity. and his technical advisers have
decided whether the hole shall pass
through or slightly away from the
optical center, A. n. Finn, chief of
the bureaus glass section. and his
assistants will begin the task which
no commercial glass manufacturer in
the United States has been willing 10
assume. The disk was made under
Mir. Finn'S direction for The rejecting
telescope of the Perkins Observatory at
Ohio Wesleyan.
e0-lnCh Diameter.
Thirty-five hundred pounds in
weight and as nearly tlawless as can
hc expected i'll such a large piece of
glass. ii Is 70 inches in diameter and
II inches thick. and is only exceeded
IN size by two other refiectorS the
l0O-lnch mirror III the Mount Wilson
Observatory and the 72-inch disk at
Victoria. B. G., both of which were
cast i'll France and are not as per-
feet as the disc cast at the Standards
Bureau. 1t represents three years of
effort and four failures. development
of an entirely new method of casting
evolved by Mr.. Finn: an outlay of
about $4,000 for materials and fuel and
long months of suspense on the part
work.
Cooling and annealing processes
which kept the disk under electrically
controlled and gradually diminishing
heat from March 7. 1927, until January
y last were entirely effective in re-
lieving it Of internal strains That is
the reason DR.. Finn has no miSgivingS
about the task of boring a hole in the
massive disk.
If the glass had not been properly
annealed. the opening would release
permanent strains" which would break
1t to pieces These strains develop dur
ing the cooling of the glass The sur
face. contracting first and becoming
rigid. eXertS an outward pressure
against the more gradual contraction
OF the interior OF the mass as it co0ls
more slowly. Unless the difference be-
tween the surface and interior tempera-
lures Is held within a narrow range
for a sufficiently long period the
strains become so great that they either
crack the glass Immediately or become
permanent" Existence of permanent
strains in 3,500-pound piece of glass
would make drilling hazardous. for such
masses are likely 10 fly apart with ali
explosion.
Will BE Polished
After the hole is cut. the disk will be
sent TO some optical concern for grind
ing and polishing until the upper sur
face has a parabolic curve, an excep-
tionally difficult operation. The same
curve is used in automobile headlight
refiectors. but in the mirror it will BE-
such small section Of such a large
parabola that it Will be hardly notice
able
Such surface has the property OF
rejecting parallel rays Of light to
focus. In the Perkins Obsers atory the
big mirror is TO be placed near the
base Of the telescope A smaller mirror
above 1t, cutting off only the amount OF
Light that would otherwise be lost in
the opening. Will catch the, rays at an
angle before they reach their pormal
local point. and reflect thenr back