From: "mlbrown{at}everstrive.com" 
To: atm{at}shore.net
Reply-To: "mlbrown{at}everstrive.com" 


John,
 #0: Please no HTML on outgoing mail.  Very hard to read on plain text
email programs.  If using MS LookOut, go to Options/Mail Format/ Send in
this message format: Plain Text

#1: By your 3rd mirror, it is not at all surprising that you are doing the
right thing to get a sphere.  The shadows should look a lot like your 4.25"

#2: Lots of options for parabolization.  Long strokes are still my favorite
for small mirrors.  Deforming a lap works nicely, too, and by now you may
have discovered that rebuilding a lap is not that tedious of a job.

#3: Uneven figure (dogbiscuit) is often due to localized heating from
polishing too fast.  Slow down the stroke speed.  I now like to use rouge
for final figuring, and haven't had any dogbiscuit since I moved to it.
Rouge isn't all that dirty to use if you keep a bar of soap, a bucket of
water, and a towel handy.

#4: Your technique is quite good.  I keep around a pristine commercial lens
and compare the sunlight test to it.  A pen laser is also a very stringent
test.  Never heard of using the sharpie test for polish (just for contact).

=Matt




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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