Re: [ATM] What shape is this mirror?

["Jerry" <wa4guu@verizon.net>]

Keep polishing until it is fully polished.

You could ignore the shape now, and just continue polishing until it is
fully polished. Then it should only take 15 minutes or so to get close to a
sphere. That is why you don't want to waste time "pseudo figuring" as you
polish.

You would expect to be a little oblate at this point. Your mirror is very
oblate.

You don't have to, but if I were making that mirror I would adjust the
polishing stroke a little to reduce the oblateness. You can adjust your
stroke to work the oblate curve toward a sphere as you polish by making the
stroke a little longer or wider, or both. Only make slight adjustments and
see what it looks like after a full polishing session. Don't think of it as
figuring and don't let it slow you down. You are not figuring so don't go to
short sessions. You don't need to run a full Foucault test. If it turns into
that, you will be wasting time that would be better spent polishing. You
just check to see the general shape. If you are digging a central depression
you reduce side motion or length of stroke. If it's a central hill, you can
add side or stroke length.

Once you have found the polishing stroke that you like, you will probably
not have to make adjustments. It will work much the same for all mirrors.
 

Jerry   



-----Original Message-----
From: Meeks Robert-PT1784
 
Hello ATM list -
 

8 hours into polishing my 8" mirror 


shape was no where near spherical and kept going.

Several hours of polishing later 
readings have not changed much

appear to be SO FAR away from spherical that I'm wondering if I can fix this
without going back to grinding.  
 
Here are the latest readings (in inches).  These are offsets using a
Couder mask:
 
ROC = 122"
Moving Light Source
 
Zone radius   my reading   Ideal (for Parabola)   Ideal (for Sphere)
--------------------------------------------------------------------
1    1.379      0.000            0.000           0.000
2    2.240     -0.240            0.013           0.000
3    2.837     -0.395            0.025           0.000
4    3.326     -0.445            0.038           0.000
5    3.752     -0.430            0.050           0.000
 


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