[ATM] Flat Testing

["aneves" <aneves@snowsoft.biz>]

Hi,

I am in the process of making an 8" flat by the 1-2-3 method and I am not quite sure on how to
check if the flats are convex or concave. When I place them on top of one another I get the expected bull eye 
fringes and when I press the edges the fringes move toward the center.  On bob mays site I found 
the following description:

"by pressing the edge of the glass toward the center of curvature of the interference 
bands - bottom edge in Fig. 2 - and noting whether the bands move toward your finger or 
away from it. If they move toward the point of pressure, the disc is convex; 
but if they move in the opposite direction - away from your finger - the disc is concave."

Is this description only for the case that you have a master flat? Can anybody describeme a method that I can use since I do not have a master flat.

thanks,

Alfredo

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