Re: [ATM] Grinding questions

["John Stephen" <atmlist.net@johnstephen.org>]

> How are you coming to the determination that your
> mirror is a "perfectly good sphere" without testing
> the reflection? Are you saying that your spherometer
> is showing  NO DEVIATIONS across the disk at some time
> and not at others? If you are seeing this, I'd
> definitely ditch the plate glass and go to PYREX,
> especially if you can afford it. Also, give the
> SHARPIE test (search the archives and GOOGLE) for the
> way to do this...

I should choose my words more carefully, but you are correct -- I'm going by 
the spherometer showing no deviations and not by testing the reflection. 
I've done the Sharpie test and the contact seems to be very good, but I do 
see small deviations after letting the mirror rest which I don't see after 
grinding.  I'm going to go up one more grade of grit so I can try a 
reflection test and see how it looks before I give up and start again.

> Actually, I don't see how anyone could NOT afford to
> go to a better grade of glass. After all, you are
> going to spend up to hundreds of hours working on it
> and hopefully thousands of hours using it. Why go
> "CHEAP" and not be satisfied? A good grade of glass
> will be easier to work and give better results over
> the years... Yes, you can save 10-25 bucks but at what
> cost in time and aggravation.

I decided on plate glass based on comments I had read elsewhere about it 
being easier to work and also having a faster cool-down time.  I think I may 
have chosen poorly :-)  I bought the glass from mirrorkits.com but they're 
not doing business for the rest of this year so I need to find someone else 
to buy a hogged-out pyrex mirror from.  I'm going to try Newport Glassworks, 
but any other suggestions would be appreciated.

Thanks,
- John 


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