Re: [ATM] What shape is this mirror?

[Jim Burrows <burrjaw@earthlink.net>]

At 2006-03-20 18:33 -0500, Bob wrote:

>I am about 8 hours into polishing my 8" mirror (first attempt at mirror
>making).  Per the Stellafane web site I have been fine grinding and
>polishing using their "Normal" stroke (1/3 COC - no side or W movement).
>
>However, several
>hours of polishing later (perhaps only 2-3 hours away from finishing) my
>readings have not changed much and appear to be SO FAR away from
>spherical that I'm wondering if I can fix this without going back to
>grinding.

Sixtests input file:
Bob
        8
Foucault
        0
y, inch  X, inch
    1.379        0
     2.24    -0.24
    2.837   -0.395
    3.326   -0.445
    3.752    -0.43
*
       -1        0
        0
      122
INCH

>Negative is towards the mirror, so obviously oblate spheroid but way
>beyond a conic value of 1 (my software says best fit conic is greater
>than 7).

Yup, Sixtests agrees:  b = 8.497!  Surface RMS 334.5 nm relative to 
parabola R = 3089.8.  Shame on Stellafane for pushing a COC stroke - theory 
says it can push the mirror beyond a sphere into oblate spheroid territory, 
as you have.  Texereau's normal is 1/3 COC PLUS 1/4 W (that's a LOT of 
experience talking - he didn't need to know the theory).  You probably 
don't have to go back to grinding - try Texereau's parabolizing strokes A 
I, II, or III (p. 97) while finishing the polishing - should reduce that big b.

         -- Jim Burrows
         -- mailto://burrjaw@earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84)  

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