From: "Dan Chaffee" 
To: 
Reply-To: "Dan Chaffee" 


>The numbers for the 12.4" showed close to
>1/20th wave over-correct, and the star test showed a bright ring appearing
>JUST outside focus with no such sharp ring inside.  Seeing wasn't good
>enough to see detail further from focus with an un-coated mirror.

Sure it was cooled down? Mirrors look over corrected while lagging ambient
temp significantly. Overall correction errors of even .07 wavefront RMS are
very easy to detect in all but the absolute worst seeing.

 >   Of course I tested the 8" at the same time and found only one real
>difference inside vs. outside.  Overall this one's way better than the
>12.4", but I noticed that the innermost ring around the secondary shadow is
>slightly brighter outside focus than inside.  Everything else looks the
>same.  My question is, does anyone have experience interpreting the test at
>this level?  Dick Suiter's book has nothing close to errors this subtle.
>And just how good might this mirror be?

Minute undercorrection; maybe just the innermost zones. My mirrors star
test very similarly and in very good seeing, deliver 50x+ /in. handsomely.

>I'm thinking it may be better than
>1/30th wave P-V wavefront.

Putting a wavefront on it in this catagory is not saying very much, and
basing such a value on the look of a star test is really pointless, IMO.
Best to stick to foucault for the numbers, RMS in particular. I have seen
mirrors that have strehls of .98 according to the foucault give star tests
just as you describe( again, my own:-)

>The star test is supposedly good down to 1/50th wave, but I've never read
>anything about what it looks like at this level.

It depends on the aberration; but in my experience, overall correction errors
smaller that what would make a strehl of .97 or higher are very tough to be
sure of, and the seeing must be superb if you even hope to be that
discrimminating. Tiny zonal errors, or higher order spherical aberration
can stand out more  than third order correction, but I'm reluctant to say I
can confidently detect a zone  that deviates less than 25 nm on the
wavefront from the paraboloid of best fit. I simply cannot varify such an
evaluation with anything at my disposal.

Dan Chaffee

--- BBBS/NT v4.00 MP