Re: ATM Prime focus

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

Tom Mchugh wrote :

>> Is this to say that prime focus for an SCT is indeed somewhere in the
>>  neighborhood of the convex secondary mirror? If so, it almost makes me
>>  sound correct in the first place, which might have a tragic effect on my
>>  ego. :-)

> The short answer is 'yes'. The prime focus point would be near the secondary
> mirror, but beyond it (that is, in a direction away from the main mirror),
> outside
> of the telescope.  As such, the prime focus is not used except as a
> 'landmark'
> for the placement of the secondary in the design of the instrument since
> the secondary intercepts the light that would have gone to the prime focus,
> and directs it back to the SCT focus.

Hey, hold your horses ! If we are supposed to apply the same logic you
outlined for convex primary mirrors (e.g. "virtual primary focus"), why
are you skipping the first element in the optical chain of an SCT, the
CORRECTOR?  I can tell you that corrector will form a REAL focus
(albeit very long one), as in order to minimize chromatic aberration
they are almost always given significant curvature before applying the
aspherization.  So in that case your SCT 'primary' (which is in fact
SECONDARY element) will form a SECONDARY focus in the vicinity of the
secondary (which is alas, the third element in the optical chain).
Pretty standard stuff, eh ?

Can you define prime focus of these telescopes :

1) Schuppman 
2) Maksutov (first element is a negative meniscus)
3) Yolo (primary tilted a LOT)

The key point (and misunderstanding) is that I'm talking about 
TELESCOPES' prime focus, you're splitting telescope into constitutional
elements, and not even being consistent at that too.

Bratislav

PS just to debunk the "very odd chance" of having a convex primary,
have look at real world examples of working telescopes at Dave
Stevick's home page 
http://bhs.broo.k12.wv.us/homepage/alumni/dstevick/herrig/newtct.htm
Here we have not just a convex primary, it is TILTED as well. It also
bounces rays TWICE before forming the image. What's prime focus for
this one ?