Re: ATM Re: Windows

[Bob Luffel <bobl@gr.hp.com>]

> >
> 
> Make the window as flat as practical on the first side. Turn it over, grind
> and polish the second side. Notice how the first side changed power... that
> is because the second side now has surface tension that it didn't have before. 
> Op[tically contact and polish flat..... thats it.....
> Bob
> 
> 

When I was looking into using a window for a 10" f/6.4 mirror I made
last year, I got lots of advice (some from a local optician) against 
doing it.  Evidently it is difficult (read, expensive) to get a good
enough piece of glass (variation of index across the piece) that wouldn't
degrade the wavefront significantly.  Spiders add some diffraction (and
thermal effects), but don't affect the wavefront in this way.

How hard (easy) is it to get good enough glass? (what is good enough, another
question...).  I went to some pains to get my mirror into the 1/8 wavefront
p-v level, don't want to add a window that knocks this down.



Bob Luffel