Re: ATM Ross null setup aid

[epabcc@epa.ericsson.se (Bratislav Curcic)]

Ric,

> I've seen slitless testers with razor blades that are not all 
> that straight and the bend in the blade makes it trying to find the exact 
> focal plane -where do you call it?

The point is that even with bent blade you can still use this setup
(but of course, one should use stronger/straighter blades; a blade from
carpenter's knife is ideal).
I've run George's program with lens/image distance varied over several
millimeters, with insignificant loss in residual spherical aberration, say
down from 1/300 wave to 1/100 wave (this is with a BSC2 lens of
R~20").  Main reason for this setup is that you don't have to make new
mirror-lens gauges (and some are quite long and cumbersome!) for every
new mirror. And squaring the setup can sometimes be a frustrating
experience ... this way you have lens squarely onto the mirror in seconds,
and you are automatically on lens' axis. 
Of course, with this setup you _can_ have it both ways too - just push
the back tube _in_ an inch or so, and fix your mirror/lens distance.
There should be heaps of room for you to place your tester of choice
behind ... A hole in the back will still help with cenetring/squaring.

Bratislav