From: "Jerry B. Hillman" 
To: 
Reply-To: "Jerry B. Hillman" 


Hi all,
I guess desparate situations call for desparate measures.  Not wanting to
regrind I decided to use a method most people say newbies should not
attempt.  I attacked the edge of this mirror with my thumb. Naturally it
resulted in an interesting Ronchi pattern. Instead of the lines curving
slightly just as they reached the edge when outside of ROC they formed a
zig-zag pattern over the 80-90% zones.  More work with my thumb on the 90%
zone, then on the 80-90% zone, then with a 1 1/5" pitch lap on the
70-80-90% zones, then with a 7" lap doing a w stroke from the 90% zone
into the 60% zone removed the zig-zag pattern.  Then I used the 7"
pitch lap with a parabolizing stroke with a lot of overhang, (the center of
the pitch lap going into the 90% zone) twice around the barrel. My readings
put the mirror at 1/40th wave, so I decide to star test it. I am using a 27
point flotation cell. Outside of focus the image is round, with concentric
circles all the way to the center.  They all seem evenly bright. The in
focus image is nearly round with several halos,  The inside of focus image
had a dark core with a ring around it, several additional rings and a
bright ring enclosing it all. Very slight astigmatism was evident as I
moved through the focus point. This could be attributed to being slightly
out of collimation, which I could see evident in a slightly decentered
shadow of the secondary when far out of focus. It also could be due to the
sling not being perfectly centered on the mirrors horizontal center of
gravity. The eyepiece is not the problem.  The astigmatism didn't change
orientation when I rotated the eyepiece.  I didn't try rotating the mirror
since I would have then had to recollimate the scope and I was too tired to
mess with it.  Next time when I haven't being doing farm chores all day
I'll spend more time with it. The Ronchi shows a very small outward hook
right at the edge of the mirror.Probably less than 1/8"  I can live
with that. I'll mask it off. What was missing from this test was the
extremely bright outside ring evident in the last star test of this mirror.
 And the broad hooks in the Ronchi test.  The mirror may not be perfect but
it is closer than it was. Clear skies, Jerry

--- BBBS/NT v4.01 Flag-4