Re: [ATM] Flat grinding the back of the primary mirror

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Mel Bartels wrote:
> For small and medium mirrors, or mirrors that are 6:1 ratio of diameter to
> thickness, do not grind the back flat.  There's nothing to be gained.

I think there is something to be gained, mainly for "medium" size 
mirror, which I take to be 6"-12" in size.

If the mirror is to be glued in, I find that glue sticks better to 
smoother, finer ground surfaces, as they are more easily cleaned.

If the mirror is NOT being glued in....

If the back is flat and smooth and the mirror is de-wedged, the mirror 
will maintain collimation better.  I find that mirrors often rotate in 
their cells during transport, and that throws off collimation.  A 
flat-backed, de-wedged mirror can also be put in a cell with closer 
tolerances, so the mirror will rattle around less.

Also, mirrors with smooth backs are freer to move in the cell, sliding 
on their support points.  Free movement means the optic is less likely 
to get pinched.

Maybe I am a purist, but these are the reasons why I grind the back 
flat through fine grinding.  I don't worry about removing all pits, 
though, since I just want a smooth, almost slick back surface, and a 
few larger pits don't matter.  A slightly curved back surface is not a 
problem, I just want it smooth.

      Mike Lockwood

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