The sun. [volume] (New York [N.Y.]) 1833-1916, 1910-10-09
WANING TELESCOPE MIRRORS
4 Disk IOO Inches In Diameter Vow
BelnE Prepared.

WANING TRLESCOPE MIRRORS.
4 Disk IOO inches In Diameter Now
Being Prepared.
The factory which most successfully
casts the rough disks ~ glass for the
mirrors u great telescopes i the ancient
and famous one At 50 GohaIn France
which was esta bli shed IN loa5 The writer
had the rare privilege 81 visiting this old
walled town last year writes G. " Ritchey
IN HdrprrS Mogdrirc and ~ studying
there the details Of this dificult work
This firm has successfully made two
nearly perfect disks a. inches in dia meter
and inches thick weighing 900 pounds
each. together win a large number ~
smaller disks. They are now engaged in
making disk ioo inches ia diameter and
II Inches thick weighing 1000 pounds
for the mirror 81 the Hooker telescope
/ Disks M 20 ~ inches in diameter are
poured from the large fire clay melting
pots which are used for melting and pour
d ing plate glass The iNp inch glass - 50
large that u cannot No poured from the
usual pots, hence = special furnace with
melting tank capabIe - holding N tsp1
pounds Of molten glass has been con
! stTucted for ~ lnnumsrnhle precau-
tiuuS are necessary IN order 10 prevent
the presence ~ az bubbles and, of -tr1E
. or pouring marks m the resulting disks
After a disk - SIcce-sfIly cast and
before ~ ~ removed from Nhe mould ~ IL
placed IN tho ann~ahnR oven Disks up
10 ~ I feet IN diameter are Anneals .d
10y being cooled down slowly and unit
fornly Tor several days or several weeks.
depending upon their gIy" special
method - ann-aling the result ~ long
experimenting - used Tor such great
disks a. the a. inch and the l0p inch the
anneAiing ~ the latter requires ahutt
a hundred davS
Avery important f-atur. a preparing Tor
tbs. grinding and polishing 81 the glasses
~ the optical room ur hulidIug Thus
must lie 80 constructed that y can b- kept
~CrupulOunly 'T.. from dust tut that th..
temperature San ls Ind-r prrfeCt control
Another important item ~ III" grinding
and polishing machine CoutluV.S Nj
kltCnry Ih- ma"i.'i-- which ! us' at
p,'-ruf ary IA" result ~ - MY years
rkI"r.-uc'' and gradual improvement
BV 'h'''r aId thu ElH-- dlsk- ir' chang. Vt
from rough "s-tings Wr'h rHgErd "dg'.-
and irregular surfaces 20 brilliantly
polished optical mirrors their surfaces
correct "a form 20 ~ IA much los than
one OU- hundred thuu-andth part 81 an
inch After tht- 'uH final nur--.r' .lg aud
~~tuu,hIug - tn surfaces a." dpu'
manly Nv hand and IA" mt"hII" plav-
a l~.- import an ptr'
III, OPT ri WORK = a., dot iuf" fulr
marked stages A. follows Rnugu grind.
ius IN" grinding polishing ugIrug and
'e-tIuk and puillv -Ilv"".HE
Tn" grinding - done hv means 81 larEr
plates Ur tools ~ ~aat IRON Winch ~"
Gre, turned and ground =" ~" rrorpr
,IrVatuIe Ia" abrItiuE material ~ q
t-.tN'..n tho tools and tn glass NAV
euh"r sand emory or Csrh"ruudUm
grams IN ned 18th IN a''Irilucr uf wat"l
tarborvndum grnd~ mos rapidly r..
. "a III
The glass les m q nor .ontsl posItruu
on rtH slowly rotating tuTn'ahl" - tb''
,msck,n.. while IN" grinding plates a."
moved about upon IS surface IN elliptic. ]
uath- or strokes Uy IN" acton 81 IN.
machine Rough grinding compAra-
'vclv smyl" aud consists merely 10 CIV
ing tho glass - apuroN U1'" form thIs
mus t-. dome v'rv dolIi "ri'"Iy huWrN o..
20 Prevent heating and consequent "rsok
.n9 81 nv" glass
Tn. urincip pal glass ~ . reflect nE 'ol~-
-nng.. ha' on m" hacs and coucnys uu
'h. fac" Tn form ~ tho back reed %"
only approximately truo. bIf m" form
q 'he cuuc1v" face must l." uyii,ailv
perfect IN" back ~ - o,ur>" bound
with \ fat tool or Plat" and IN" fn,,' with
4 convex ono
Pine grinding ~ much moro difficult
sine" - thus stage the Accurate surface
y revolution r the cunciv" FAC" must
ba secured Fyery pr"clutIon - nov
conditions and Uniform speed - rotation
Il tho glass b.nNlfh the grinding plates
Fine grinding gives = slrfac" which -
almost automatically true. and which
10 "NguIsitely Nna md sruop'h rsadv fpr
polishing fhis 20 Accomplished Nv Using
with N" IRON grinding plat"s \ -ric~~~sion
~ finer and finer grades ~ CtrborinduT
pDWdSr Wtih water These U0Wdsrs ~
any desIrNd d"gr"s - fnen"-a are EASILY
prepared Nv 'ne'hpd = Washing or
..Nt tEaa
Tho ro0lS ~ pla'a- Used Or polishing
are very different from tho grinding
plates. they are much lighter. heing con
striIc-ed ~ . basis ~ aluminium ~ vopd
coated on IN" fac" wirh small rluar~s ~
moderately ~ rpsIn or pitch which m
turn ~ coated WI'h n thin l-v"r ~ h.'.'a yyas
Tho polishing powder us"d - eft rcd
optical roug" mixed with an amIndarr"
- clean water
Tho polishing tools are moved ahnui
IN ths glass by IA" sam" rn7rhius vi. u
Vh"h Tho grinding IS don" n thus rtaS.
NJ vol a. ~ rho fin" grinding ~" utmost
care and clean lInosss are needed - progent
scratoha- n he I'm" grinding has harn
~"ll dom" "Igh' Or Ten hours work ~
usually su1icI-nT ~ the case ~ y large
glass ~ hing IA" srrftc" - perfect
brilliant polish
As soon ~ the polish begins ~ npr"rr
tho exact form - the conc1y" surface
can b." determined hy optical tests ~
the polishing proceeds This testing ~
don" several tunes = day. and The stroke
- tho polishing tools - rrDdih'.t - a. -
prpduC" moro and more nearly ~" .Nar'
form - surface d"slr"d This OPERATION
- cIli"i figuring 10. surface
u Sill b" -'-'n hit polishing figuring
and eating are Cnrrted on simultaneously
and thar ~" production . No nptiraliy
yarf""' surf4T - accomplished Wt'h
polishing tpuis lust"ld - with grinding
tp-pls Even IN" finest grinding would b."
tpu crude for tho purpose and h.sI,Ios
tho surface mus b" \ polished one IN
ORDER 'n 10" tested frequently