Our club is interested in correcting a 16" f/4.5 Meade primary mirror that suffers from astigmatism, and a "less than optimal" figure (see FigureXP analysis). I had refigured a similar mirror for our observatory, which performs very well, using Foucault. We swap out the good mirror into our dob for travel to star parties...most recently to STARS OVER KATAHDIN...man, it's dark out there, one of THE DARKEST sites east of the Mississippi, and we're fighting to keep it that way.
In this case, there is astigmatism showing up in the star test, and it is certainly on the primary. I refigured the secondary to VERY flat, and the "good" mirror #1 shows zero stig.
I have a full thickness 16" dia. perforated Pyrex "flat" that has some power and some zonal issues, shown in the pics.
To "catch two fish on the same line" (rather than killing birds with a rock), I thought I'd do some touchup on the flat and then DPAC the mirror.
The quick test on half of the flat shows 2+ waves (4-5 fringes) CONCAVE, using our 10" Henry Paul 1/10th wave flat. I assume it's more like 6-8 fringes over 16". I'm assuming that the power of the flat is negligible, but the irregularity, lack of smoothness is not. The flat must have been used for some collimation tests..."flat" on one side, f/7 sphere on the back.
In the community's expert opinion, is this flat suitable for retouching, and how would I best go about it? I'm concerned about the irregularity beginning at the 5-6" radius zone. I assume it is useless as-is for correcting a paraboloid DPAC. The stig issue will be handled by polishing the mirror with a good sized lap and using pinhole Foucault technique.
Edited by ccaissie, 31 October 2019 - 04:59 AM.