[Atmswap] Large Optical Glass Blank for Sale, 23.25" dia, 2.125" thick, SK16, $750

[Sidor . Kurt]

I have finnally decided to part with this large piece of glass.  It has been
sitting in my closet for 5 years and I have admitted to myself I will never
get around to it. 

FOR SALE:

23.25" outside diameter
2.125" edge thickness
concave side originally polished to ROC 58.57"
convex side as molded -64.73"
Glass Type SK16
INDEX: Nd 1.6204
ABBE: Vd 60.29
Weight: 113 pounds
Price: $750

According to my research the two largest Maksutovs in North America are:

Max Bray 20" F#10 at the Skywatchers Inn in Arizona:

http://www.communiverse.com/skywatcher/telescopes.html

and the 22" James Baker Mak at Stamford:

http://vsophotos.topcities.com/fall01.html

Theoretically this piece of glass could make a Maksutov even bigger than
these, (even if just barely!)

This piece was ITEK surplus unused and it had a second piece just like it.
The second piece had some extensive edge damage and only 20" of the diameter
would have been usable.  I sold the second piece to a fellow ATM-er 5 years
ago when I originally acquired the pair.  He did end up designing his glass
into a Maksutov Cassegrain in ZEMAX and it looked quite promising.  As far
as I know construction has not started yet.  I myself have not actually
designed a prescription using this glass but seeing this ZEMAX file was
proof for me, this piece of glass is the correct proportions to make a
Maksutov corrector.

Here are many pictures showing its condition:

It is 23.25 inches O.D. as shown here:

http://images.andale.com/f2/115/106/3663062/1025109587019_OUTSIDEDIAMETER.JP
G

Here is the concave side:

http://images.andale.com/f2/115/106/3663062/1025109562371_CONCAVE.JPG

The concave side is polished with some black "crud" spattered on it, most
comes off with a little elbow grease.  A few of the bits of "crud" appear to
have pitted the glass very shallowly, only a few thousands of an inch.  I
have no idea how old this piece
is but SK16 is obsolete glass and the polish does show some signs of
"weathering", the piece was porely stored in a barn. The concave side ROC is
58.57".

There are a few edge chips on the concave face all very shallow (I'm
guessing 0.050 ish):

http://images.andale.com/f2/115/106/3663062/1025109566502_CONCAVECHIP1.JPG

http://images.andale.com/f2/115/106/3663062/1025109569924_CONCAVECHIP2.JPG

http://images.andale.com/f2/115/106/3663062/1025109573762_CONCAVECHIP3.JPG

There is one larger chip with a very slight internal fracture that extends
down
into the edge thickness:

http://images.andale.com/f2/115/106/3663062/1025109590392_SIDECRACK1.JPG

http://images.andale.com/f2/115/106/3663062/1025109593611_SIDECRACK2.JPG

The edge thickness is exactly 2.125":

http://images.andale.com/f2/115/106/3663062/1025109604737_THICKNESSTAPEMEASU
RE.JPG


http://images.andale.com/f2/115/106/3663062/1025109601482_THICKNESSCALIPERS.
JPG


Here is the convex side, it appears "mottled-ish".  I'm guessing this is as
molded:
The striped thing in both photos is a pillowcase.

http://images.andale.com/f2/115/106/3663062/1025109577528_CONVEX.JPG

http://images.andale.com/f2/115/106/3663062/1025109582078_CONVEXCLOSEUP.JPG

The convex side has an ROC of -64.73"

Here is the glass type, SK16, engraved in it:

http://images.andale.com/f2/115/106/3663062/1043245957949_sk16closeup.JPG

There are other numbers but they do not relate to index info, I think they
refer to the batch #.

My 1956 Schott catalog lists Nd for SK16 as being 1.6204 and a Vd of 60.29.
If you want more numbers let me know.  Also I want to add that the glass has
a "weather" rating of 3 on a 1 to 5 scale with 5 being the worst.  KzFS and
LaF glasses rank a 5, BK7 ranks a 2.  For long life of this corrector you
may want to consider getting a layer of SiO put on it or a single layer AR
coating of MagFlouride.

I checked the internal stress using crossed polarizers and the results are
very good, no internal stress observed.
My filter wasn't big enough to test the whole piece at once so here are
sections.  The little sopt of light in the lower left corner is where a
small piece of the filter had been cut away.  The light at the top is
unpolarized coming from my light table.

Here is the central section of the blank:
http://images.andale.com/f2/115/106/3663062/1043245935878_23inchmiddle.JPG 

Left:
http://images.andale.com/f2/115/106/3663062/1043245929564_23inchleft.JPG

Right:
http://images.andale.com/f2/115/106/3663062/1043245941749_23inchright.JPG 

I thought I noticed some small stress near the outside diameter:
http://images.andale.com/f2/115/106/3663062/1043245947534_falsestress.JPG 

When I rotated it to the top so my camera was perpendicular the effect
dissappeared (notice the position of the small edge chip in the glass in
both photos, on the left):
http://images.andale.com/f2/115/106/3663062/1043245953114_falsestress2.JPG 

If you, your club, or anybody else you know may be interested in this rare
chance to own one of the worlds largest pieces of optical glass please let
me know.

I am asking $750 plus shipping.

Thanks!

Kurt Sidor
Mechanical Engineer
Dynamics Research Corp.
Encoder Division