Im constantly reminding my employees at the optic shop, "When working with Bk-7, be careful using heat, it is thermal sensitive...."
So, I decided to heat up my 14 lb slab of Bk-7 to mount on a aluminum tooling to slice down the middle, so I could end up with two plates, as opposed to milling down a 1.5" thick slab to make a 1/4 or 1/2 inch corrector or window.
Heated both slab and tooling on the same stove, to the same temperature, picked up the tooling to apply mounting wax, must of cooled just enough, so when I attached it to the big slab, It instantly CRACKED in half....like a knife plunged deep into my soul!!
Suddenly a picture comes to mind: Elihu Thompson had poured the big 60" fused silica blank, working with George Hale on trying to figure out how to make a 200" mirror. Someone opened the annealing oven to soon, and when they examined the disc, it was cracked!!
Supposedly, Elihu turned to A.L. Ellis and all he said was "Better Lay down another 60" disc. We have proved we could do it"
Not quite the same analagy, but I pulled myself out of the deep pit of despair, got on the phone to Glass Fab in N.Y., got our salesman on the line and asked if he had anything close to a 9" dia plate of BK-7.(I rather use fused silica, but at 6x the price, can't rationalize it.)
Low and behold, turns out they had a 11" dia x 1/2" thick plate, and a 15" dia x 1/4" thick plate, both for not much more than I paid Ohara for the big slab. So, the cracked slab goes back on the inventory shelf, where it'll become thousands of dollars worth of small optical parts, and the two plates are on there way from glass fab....
So, what to do now...1) Have a 8" f5 primary ready for polishing 2) Have a 11"X1/2" blank, perfect for a optical window for a 10" mirror (I do have a 10" mirror blank sitting around" 3) Have a 15"x1/4" blank Bk-7
Obviously, the 11" could be trimed to 9", but I thinks that's a waste, and I should use that with the 10" mirror.
The 15" plate has some defects on the edge, but could be trimmed to 13" for use with a 12.5, but by the time its ground and polished, it'll probably be more like .200", which maybe to thin for a schmidt corrector. Besides, I promised the ol lady to not build anything that size till we're settled down permanately.
Would .200" be two thin for an 9" optical window (plano/plano), I know it dosn't meet the 1/20 ratio, but what would the effects be??
I'll come to some decision at some point, I'm still educating myself in design mathametics, polishing the 8' primary to perfection (hopefully), building a much more accurate knife edge tester, etc, so no hurry...
Comments welcome as always...
Bill Marriott Forest Knolls, Ca. USA btk@ix.netcom.com