To: atm{at}shore.net
From: Jim Burrows 
Reply-To: Jim Burrows 


At 13:29 2003-03-27 -0800, Don Bates wrote:

>According to this, I also have a TDE. But wouldn't a
>TDE show up as being bent UP on the graph, indicating
>over-correction? I am confused!

I don't think it's a good idea to talk about over/under correction of the
edge.  To find the mirror's correction, you make an optical best-fit of a
conic to the whole mirror's surface, then the resultant conic constant b
tells you the correction (b > -1 = undercorrected, etc.).

Back to the edge.  TUE/TDE depends on your reference.  For the final test,
just before the mirror goes to the coater, you plot the deviation of the
surface from the best-focus parabola (minimize the surface RMS), then if
the edge is below that reference, it's TDE.  However, if you're still
figuring, it might be easier to push the surface toward a different
parabola.  For example, the edge relative to a parabola with a RoC a couple
of tenths of a millimeter longer than best-fit shows a huge TUE.

>Also, when you take out the chamfer on my mirror, the
>diameter is 9.75" instead of 10". This makes the
>mirror a F/6.77 instead of F/6.6 for the full 10
>inches. Should I consider the mirror a F/6.77 for
>purposes of locating the secondary mirror?

Locating the secondary mirror is just a ray-trace geometric exercise - use
the optical diameter and the measured focal length.


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

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