From: "Jerry" 
To: "'Jon Bishop'" ,

Reply-To: "Jerry" 




-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net] On Behalf Of Jon
Bishop
Sent: Monday, August 18, 2003 1:37 PM To: ATM
Subject: ATM some questions/ideas

Hi,
        I am working on my third mirror, an 8" F5.25. I am halfway through
polishing and have been doing some preliminary knife edge testing. Yes, I
know it is way too early to do this, but I find it quite interesting, and
it lets me measure the exact focal length of the mirror so I can start
designing the rest of the scope.
Here are several thoughts/questions...







My suggestions........................



#1) Already Spherical

        I have had the same experience with all of my mirrors being quite
spherical during polishing. But it does not last until completion of
polish. You may end up close to a sphere but it won't be as good as now. I
had asked on one of these boards if others shared my experience but no one
replied. But I would say that it shows you have done very good job in
grinding at least as far as getting a good spherical shape. You have planed
the tops of the peaks off and the sphere is good but when you get the peaks
completely removed down to the valleys (fully polished) you will see the
shape start deviating at least a bit from a sphere. My opinion of what is
happening. I offer no proof. And what would it really matter if I did.

#2) How to Parabolize

        Pick one. Different people will suggest different ways and some of
them will work. My opinion is only graduate or otherwise change the lap
when you find that you cannot get all the way to the parabola with a normal
lap. I don't think with your mirror you will have any problem with a full
size normal lap. In my experience small laps, graduated facets and star
laps produce rougher surfaces than a full size lap. Not saying the other
laps aren't ever the thing to do. But I always get smoother with a full
size lap. I don't think I am alone. I will just say that that if you are
making a thin mirror let someone else answer. I haven't made a thin mirror.
But if it a typical 1 to 6 thickness to diameter blank I would say do the
classical Mirror on Top long W as in Texereau. That is probably in the
other books But I like Texereau.



#3) Uneven Figure

        At least from your description there is nothing to worry about yet.
A little irregularity is probably no problem. (or even a lot of irregularity)

Some would tell you not to even bother to put it on the test stand until
polished. I would say go ahead and look (it is fun and instructive) but
don't worry much about what you see as far as shape and smoothness. And
don't let putting it on the stand to look at it slow you from polishing.



#4)  When to Stop Polishing

        I'll let others tell you this. I check mine just a bit different
than what I have seen posted on the net. I have always gotten fully
polished but if I told you how I do it and you didn't get fully polished I
would feel bad. I will say that all I have helped on the first mirror
thought theirs was fully polished before it was and I had to nasty to get
them to keep on polishing.

Jerry

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