SV: ATM 18" f4.6 with a 2.6" diagonal

["Nils Olof Carlin" <nilsolof.carlin@swipnet.se>]

Carl

> OK.  Now with Newt30 I'm armed and dangerous. 

You sound so - don't hurt yoursef by being too eager :-)

> design an f4.6 (or so) 18"er with a 2.6" diagonal (for higher contrast
> and lower coma) with 100% illumination at the eyepiece.

I have a feeling that you are trying to meet too many conflicting design
goals at the same time. What is the main purpose of the design?

A smaller diagonal will not lower coma- if that is important, it may be
wise to go for a bit more focal ratio - even if it means you will have to
stand on something higher.

An 18" mirror made to the tolerances (whether you or anyone else does it)
where the performance ever would be noticeably better with 14% obstruction
than 20% canīt be easy to come by I should think. 

> Question 1: What's a minimum and a desirable 100% illumin. DIAMETER? (I
> know, you're going to say it depends.)

It does- but using f/4.6, I believe a fully illuminated field of 30 mm will
be big enough for just about any eyepiece with a useful exit pupil (such as
the Nagler II 20 mm)- so decide somewhere in the range 0-30 mm.

> Question 2: How far into the tube can I stick the base of the focuser
> without appreciable (or any?) vignetting?  

With a distance focus-optical axis of half mirror dia (9") + half the
useful field (say 15 mm=0.6") you will have no vignetting, and with 2" for
focuser, it should leave you with a fully illuminated field of 0.09"=2 mm!!


Why not a 3.1" diagonal? - it could give you 0.67"=17 mm instead, or room
for a somewhat more realistic focuser. I know it hurts, but...



> Question 4:  Am I dreaming?  Is this still Kansas, Toto?

Don't know about you but here it isn't and it isn't Oz either - it is east
of the moon, west of the sun and north of Jupiter.



Nils Olof