Re: ATM 18 inch telescope

[Ed Taychert <elt@irony.com>]

"Amelia Thomas" <amelia_thomas@hotmail.com> wrote:

> Date: Mon, 31 Dec 2001 08:40:34 -0500
> From: "Amelia Thomas" <amelia_thomas@hotmail.com>
> Subject: Re: ATM 18 inch telescope
>
> Hello All
> I haven't seen any comments on this email so I'd like to ask, besides a
> larger central obstruction and more difficulty collimating are there any
> drawbacks to this "folded telescope" design. How difficult would collimating
> be?

With a "small" fold, practically none of that matters. I folded my f/4.5 20"
30 degrees off perpendicular using a 4.25" inch diagonal. That's just over
a 21% central obstruction ... sure, it could have been smaller so, yes, it
is "larger" but it's not "big."

As far as collimating, it autocollimates with a laser exactly the same as
any other newtonian. My mirror was cored in the mirror so I couldn't
autocollimate it; I used a holographic laser.

Startesting is a different matter, because I used a "normal" secondary,
its optical profile wasn't round... after fussing with that for a while
I came up with the idea to put a 5" round piece of cardboard
in the center of the diagonal mounting bolt ...Viola! Round obstruction
for testing and then take it off because it doesn't matter in-focus.

Michael Lindner <mikell@optonline.net> Wrote:

> One I haven't seen mentioned yet is baffling. With the eyepiece aimed up at an
> angle, a larger baffle is needed to prevent stray skylight from entering the
> eyepiece directly.

This directory is full of plan (.gif) and actual (.jpg) images of my
folded 20-incher ...
http://www.irony.com/Ed/astro/20inch/

These images, in particular, shows the upper cage baffling:
http://www.irony.com/Ed/astro/20inch/shield1.jpg
http://www.irony.com/Ed/astro/20inch/shield2.jpg

Note the small piece of shielding sticking out from the spider;
that saves about a foot of extra shielding that would be needed
on a lower baffle

---
I am working on a 20" f4 version of that now. I'm just at the
paper-planning stage right now but looks like a 20 degree fold on
that will let me build it on an equatorial platform and still keep my
feet on the ground.

 - Ed.