Re: ATM Re: Windows

[epabcc@epa.ericsson.se (Bratislav Curcic)]

Kevin,

> I plan to do initial polish using plastic pads.  Can someone put a _number_
> on how close the tool RofC needs to match the work when using pads?

After finishing grinding with 5 micron, my tool and window were
measuring +2, +2 and -2 (in microns). In other words, there was
about 4 microns difference in saggita between tool and one surface.
I don't know for more, but pads will easily accomodate for this
much.

> I decided to flip
> window over each trip around the barrel (same wet) and see if I could match
> them up better, as a wet of 5 micron seemed to move curvature significantly
> (so I thought) and leave the other side of window an "odd-man".

This is just asking for trouble. Do 1/2h on each side with 5 micron and
go straight to polishing. Pads/cerium will cut glass too (I'm not sure
about rate, but it is definetely in range of several microns per hour).

> The negitive lens
> tends toward a Wright design, (Flat field, low coma) while the positive lens
> has NO point in its favor.

With weak radii we're talking about, there is ABSOLUTELY no difference
whether your lens will be 1 mile focal length convex or concave. The
effect on field curvature and coma is negligible. But there is reason
(from a practical point of view) to have sides ever slightly convex
before polishing.

Make sure you wear gloves or have something to insulate the work in the
last stages of grinding (and later in polishing too). Crown will expand
much more than pyrex, and if you hold your window by the edges, you're
bound to end up with TDE.

Do as much polishing the same day you finish grinding, and call it quits 
with pads (even after only 1 hour of polishing on each side you will have
perfectly transparent window). Clean up the residual pits with pitch. I 
wouldn't trust (anymore) plaster tool over night.
Another way to go is to pour a layer of pitch an have it formed flat using
window and wax paper (NO cerium, pads won't stick). Then glue pads onto 
the pitch and you're safe. Do frequent cold pressing as usual. After
getting a good polish, scrap the pads and re-form the pitch and do finish
with Rouge, preferably. Or better, make another pitch lap (you can use 
plywood disc as a base for that one).

Bratislav