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

[Dave McCarter <McCarterD@claven.fanshawec.on.ca>]

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

It certainly does. Let us start by agreeing it has to be somthing! Now, how
big will the object be that you most want to look at. The moon? The Helix?
The Ring? M6 and M7? M57 is certainly small, and in a 18" F4.6 scope
someone can (not me, yet) figure out what area it will cover. I usually
work backwards from the eyepiece field stop, as I think the field in a
moderate focal length should be mostly covered and I am prepared for some
moderate fall off in a wide field eyepiece. So let's say give it 0.3 inch
or so.

>Question 2: How far into the tube can I stick the base of the focuser
>without appreciable (or any?) vignetting?  I'm guessing a 19.5" tube ID,
>and hoping to put the base in 0.5" (-0.5" for the "Tube Thickness"
>parameter in Newt25=Newt30), or 0.25" from the projected primary edge.

That would be fine on axis, but for low power the visable light cone at the
edge of field might get closer to the base. Then there are those pesky
bolts and nuts you will have to trim flush and paint black.

>Question 3: Is there a focuser in the house that I can get away with a
>total of 2" for height PLUS travel??

Many of the low profile 2" Crayford styles are like this, but it might be
difficult to mount them on the inside of the tube. Making your own to suit
your needs is not impossible.

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

Success is defined as having a dream and making it happen. As for Kansas I
cannot say.


David McCarter   McCarterD@claven.fanshawec.on.ca
R.A.S.C. London Centre VP & Observers Chair,
Amateur Radio VE3GSO