Re: ATM Unobstructed Optics Viewing

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

Mel wrote :

> I have tried off-axis masks ala offset apertures in three positions: 
> at front of tube, on mirror, and just below eyepiece.

Strange as it may seem, but aperture _IS_ an active optical 
element !!! And it _is_ important where you put it. Playing with
a good ray trace program can be very revealing.

> The front of tube sometimes allowed the 
> wrong part of the mirror to be involved in the image (turned edge) 
> and the 'below' eyepiece caused field illumination problems.

You'll  find that off axis stop 'below eyepiece' doesn't just cause
field illumination problems, it will introduce field aberrations as
well (coma and astigmatism).

Bratislav