RE: [ATM] overshot sagitta correction stroke

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

	What method are you using to measure ROC? Using a sphereometer and
measuring a small area in the center might not reveal that the outer areas
of the mirror are changing. Going to TOT and long W or chordal stroke should
move to a longer radius but will work mostly on the outer parts leaving a
shorter radius area in the middle. When the radius gets long enough at the
outer zones you should spend some time with the short strokes to smooth the
curve into one uniform radius from center to edge. The curve when smoothed
should end up in between what the center and outer zones where before
smoothing. 
	It seems in my mind, judging from posts about missing the desired
ROC that it is always shorter than desired. I have read many who's mirror
ROC is getting shorter. I can't recall any ending up too long. Get it right
at one grade of grit and then change grit and it gets shorter. Either there
is an error of measurement (on the long side) when on rougher grits, or just
a tendency for the radius to get shorter even with 50-50 MOT TOT. It might
be both reasons. I usually measure using a light source to find the center
of curve on a wet mirror. I attribute at least some of the "shortening" of
the radius to the surface tension of the water pulling a longer radius onto
the water surface. It seems the radius gets shorter right after changing
grits and then stabilizes somewhat. My theory is the finer surface allows a
thinner layer of water to coat the mirror which in turn allows less water to
be pulled to the center by surface tension and therefore a shorter radius
than before on the water even if the glass has not changed radius.

	What radius you want to have at some particular grit will depending
on how close you want to be to the exact f5 focal length. You could get as
close as you can measure at each grit. Check frequently and not allow
departure without immediate correction. Or you can take account of what you
have seen as the shortening tendency and try to leave a couple of inches
extra at say 220 and watch closely how it changes at 320. The amount of
extra radius is a judgment call from your recent experience. Try to end 320
with half of the extra you had with 220 and so on through the grits.
Naturally the check frequently and fix immediately is more precise. And with
experience allowing better judgment a combination of both methods would
allow precision with less correction.

	Using a sphereometer with a 3" base for example, some will measure
at the center of the mirror only. If you get a short ROC reading there and
lengthen your stroke TOT the center will be the last area to change. You
should measure as close to the edge as you can get the base of the
sphereometer also. Particularly after using a stroke to change the radius
and no matter whether you where working to shorten or lengthen it, the ROC's
of the center and outer zones will probably be different, the center being
of shorter radius than the outer. Move that sphereometer around to get a
good idea of the entire surface.

Jerry




	 

-----Original Message-----
From: mahesh
 

Unfortunately, I have overshot the sagitta and landed with F=46in at the 320
grit stage. 

1) What stoke will be efficient in reducing the sagitta?

2) Since I would like to end up with an f/5 mirror, what should the focal
length be at the 220/320 grit stage?
 



_______________________________________________
ATM mailing list http://www.atmlist.net/