[ATM] It amazes me...

["Shane LaPierre" <shane@ct-astronomer.com>]

...how sensitive the star test is.  I just finished an 8" 
<f5 mirror at 39" FL tonight (this a.m. actually).  I used 
foucault & ronchi for testing.  Ronchi showed nice shape, 
foucault told me 10% overcorrected (tested very flat & 
regular if you need a 110% parabola).  Anyway, star test 
is very sensitive to overcorrection.  I star-tested w/ 
Vega (nice & high) to verify... yep.  Overcorrected.  I 
bring it back a little... mirror is still in good shape 
and star test looks better, but not perfect... still a bit 
of overcorrection.  So I repeat the process.  To boot, 
since the seeing is good, the scope is tracking and the 
mirror stable I take a knife-edge and ronchi to the 
telescope.  Knife-edge is just not possible visibly... 
right at focus there's just too much turbulence.  Ronchi 
is amazing.  Perfectly straight lines... closer to focus 2 
lines... perfectly straight... 1 line... straight.  Edge 
looks great.  No zones that I can see.  Still the star 
shows the mildest overcorrection.  No big deal.  I know 
it's a good mirror.  I pointed it at Jupiter and uncoated 
it looks great.  I can see craters in Plato on the moon.

It amazes me how sensitive the star test is.  I really 
have been wanting a nice optical flat for an 
auto-collimator...  But I think it's possible that the 
testing result may not satisfy the star test.  I'm 
wondering about a knife-edge on the auto-collimator.  I 
bet that and the ronchi as nulls would be the best chance 
of getting the correction dead-on and getting that perfect 
star-test.

Shane LaPierre
http://www.ct-astronomer.com
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