[ATM] Focal Length Accuracy

["Anthony Anconetani" <Anthony.Anconetani@genband.com>]

Hi everybody,
 
I have a question in regard to focal length accuracy.
 
Background data:
-------------------------
 
I'm currently grinding a 6F6. I'm in the 220 grit stage. I calculate the
correct focal length should be 36 inches.
 
At the completion of 120, I measured the focal length with the sun at
approximately 32-33 inches. I measured the sagitta with feeler gauges at
approximately 0.069 inches. (Note, I'm still trying to find a nice metal
pulley for use as a sphere-o-meter...)
 
In an effort to make the mirror more shallow, I've done done about 30
minutes of work with TOT.
 
I now measure the approximate focal length with sun at 33.5 inches. 
 
I've measured the approximate center of curvature with my Foucault
tester. The result was approximately 67 inches +/- 1 inch. I believe
this should have been 72 inches.
 
I've measured the sagitta with feeler gauges. The result is about .066
inches. I believe this should have measured between .062 and .063
inches.
 
So, from the above data I believe the focal length is a little short. It
also doesn't seem like I'm accomplishing much correction.
 
Questions:
---------------
 
1) Is now the time to get the focal length as close to 36 inches as
possible?
 
2) If no to question 1, should I continue through the grit sequence with
TOT until the focal length is correct?
 
3) If yes to question 1, should I step back from 220 grit to 120 grit to
speed up the correction? Or should I just keep pushing on at 220 until
the focal length is correct?
 
Thank guys,
 
-Anthony
Round Rock, TX
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