Re: ATM Star testing with a smaller secondary

["Bill T." <twentiethwave@hotmail.com>]

Jim Reid wrote:

> I'm planning on building a testing rig to star test the 16" mirror I am
> working on (about F/5.3). Is it ok to use a smaller, less expensive
> secondary for star testing than I plan to use for the actual scope?
>
> If so, what are the limits of what I can get away with.

Jim,

A secondary too small for the mirror will prevent the effects of the
critical outer portions of the mirror from showing up in the star test.
This could mask such beasts as a turned edge.  You'll wind up judging only
the interior portions of the mirror.  Better to have a secondary too big,
rather than too small for star testing.  Just make sure you can see the
whole diameter of the mirror reflected in the secondary, when your eye is at
the focal point.

Bill T.