Re: ATM RELAY LENSES

[Bratislav Curcic <epabcc@epa.ericsson.se>]

Kurt wrote :

> I am currently designing a small triplet into a Newtonian using a 6 inch
> F4 primary.  The triplet I'll probably end up using is an Edmunds of
> 16mm dia and 25mm focal length (cost $35).  I raytraced the layout on
> IRT (shareware)

If one's goal is to get a long f-ratio in a short tube, this is one way
to do it. But if main reason for this excercise is to reduce
obstruction (and count on extra contrast gains by doing that), think
again and do your ray tracing carefully. The small drop in MTF from
going from say 11% to 25% seems to me much better proposition than
introduced chromatism and extra field aberrations.  Also, ultrafast
paraboloid is a tough cookie and I'd be really surprised if relayed
f/14 performed ANYWHERE near as a well made f/8 (even with twice the
obstruction).
_If_ one really wants a planetary standard scope in compact form (with
spherical primary to ensure quality), look at Rutten & van Venrooij air
spaced triplet design from old TM magazine. Or Brixner/Jones-Bird
designs (but perhaps start with a more moderate primary).
But I'd put up with extra length any day.

Bratislav