Re: ATM Mirror Blank Imperfect Circle...

[mdholm@telerama.com]

>Is there any limitation with regard to a round
>mirror blank , but not a very accurate circle.
>Lets say, diameter in one side measures 8" it may
>measure 8.1" in some other direction. Will such
>glass blank be acceptable to grind a Mirror ? 
>What problems may arise if I had to use such a blank ?

There should be no problem at all during grinding, or during final use of the 
mirror in a telescope.

The only possible problem I can forsee is that, when you are polishing, you 
will have to be careful to keep the pitch lap trimmed to less than the smaller 
diameter of the glass.  If the pitch lap overhangs part of the mirror while it 
is pressing (Becoming conformed to the shape of the mirror, a frequent process 
during polishing) any portion of the pitch that extends beyond the surface of 
the mirror will not be pressed down.  This could cause a lot of trouble if you 
were unaware of it, and did nothing to prevent it.  The simple prevention is 
to trim the pitch so that it nowhere extends beyond the mirror surface, no 
matter the orientation of the mirror on the lap.  Use of laps smaller than the 
mirror size is a common method.  The experienced polishers on this list can 
give plenty of advice.

Oh, I just thought of another very small problem.  Your mirror cell will have 
to be designed to accomodate the slightly noncircular shape.  For some cell 
designs, there will be no change at all.  For others, some minor modification 
will be needed.

Basically, a slightly non circular mirror will not cause any serious problem.

Mark Holm
mdholm@telerama.com