From: Jon Bishop 
To: ATM 
Reply-To: Jon Bishop 




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...
 
#1) Already Spherical
I was quite surprised to see that the mirror is very spherical. Am I
deceiving myself? I see that when I move the knife edge only a few
thousands of an inch, either closer or further from the mirror, the shadow
is distinctly entering from the left or right. So the question is if one
has been careful to maintain short strokes during polishing, and you have a
reasonable pitch lap, is it normal to end up very close to spherical?
My pervious mirrors have been a 6" F8 and a 4" F6.5. I
remember being
able to see more dramatic knife edge tests, so am a little suspicious I am
doing something wrong. I was under the impression that the lower the F
number the more obivous the shadows would become.
#2) How to Parabolize
So after I do 2-3 more hours of polishing I will begin to parabolize
the mirror. I have read Texereau, Richard Berrys book, and even ATM 1
(still my sentimental favorite). There seem to be a large number of
opinions on how to do this. What have people found to be the most reliable
way to do this? I am tempted to using the "graduated facet"
approach, but of course that means destroying my lap.
#3) Uneven Figure
Another issue is that under the knife edge test the surface or the mirror
looks a little uneven, with very slight shadows randomly distributed over
the surface I am using CO to polish with, and have alway done some cold
pressing before beginning polishing. Also the lap seems to behave very
smoothly, without any sticking and slipping. Only a nice steady drag. Maybe
the CO is too strong a polisher to give a nice smooth finish. What
is the remedy for this?
#4)  When to Stop Polishing
I am using the technique of focusing the sun on the surface of the mirror
using a magnifying class. When the image of the sun can no longer be
detected then polishing can be declared complete. I also like to examing
the surface with a loop and look for the pits. As expected there are more
pits on the edge than the center, in other words the center polished
faster. Has anyone tried any other methods for checking this? Is the
"sharpie" test of any use?
Thanks in advance for your help...
Jon Bishop
 
 
 

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