Re: ATM "Flats" for Autocollimation Null Testing

[iss@pvtnetworks.net]

Dave Rowe writes:

>Now, it turns out that a 10" diameter AM with a radius of curvature of 200
>meters would have a sagitta over 0.001" (over 50 waves).  Using a decent
>spherometer, I can grind two pieces of glass together so that each one has a
>sagitta of less than 1/10th this much, i.e., less than 100 microinches.  After
>fine grinding, the two pieces of glass should be quite spherical if good
>contact has been maintained throughout.

I tried making an 8" full thickness f50 mirror for a solar telescope,
& believe me, at this radius, these surfaces did not want to maintain
spherical contact. I think you'd need a carefully adjusted grinding
machine to do this. I was grinding Pyrex on a plate glass tool, though,
which may explain some of the difficulty. Still have the tool/blank
if anyone wants to fool with it, contact me off-list.

>Now, the real questions.  <snip>  On the other hand, if it picks
>up, during polishing, a turned edge, a hole in the center, or any one of the
>numerous other possible defects, then it will not work as an AM.  How can one
>tell?  Is this the real reason that people make flats for autocollimation null
>testing?  Because a flat can be tested against two other flats, cyclically, to
>quantitatively determine flatness?

Hypothetically, you could polish both the tool & mirror & fringe test
them against each other, both centered & off center. If they're truly
spherical the fringes would remain parallel with the pieces off center.

 
Andy Saulietis / ISS Alt-Az-Fp Drive Systems
HDPE Worm Gears, Custom designs & Machine work
39 Silver Fox Trail
Cloud Country Estates
Mayhill NM 88339
505-687-3067 Voice
505-687-3021   Fax
e-mail: iss@pvtnetworks.net
32 54 13 N  105 31 44 W 7300' elev