Re: ATM 100% illumination and secondary sizing

["Jan van Gastel" <jhm.vangastel@wanadoo.nl>]

Mel Bartels has two programs downloadable from his website for calculating
the secondary size. These programs also calculate the 100% illuminated field
of view for a given secondary size and the drop in illumination in
percentage and in magnitude. There is also a good nwetonian design program
on the net, made by Jim Fly. I have links to al these programs (and to other
design programs) on my links page. Newt alone gives not enough information
to make a good decision.

Jan
http://www.geocities.com/janvangastel/Astronomy/

----- Original Message -----
From: "David Moorhouse" <climber@world-net.co.nz>
To: <atm@shore.net>
Sent: Friday, March 16, 2001 2:30 AM
Subject: ATM 100% illumination and secondary sizing


I'm wanting to know a bit more detail about secondary mirror sizing, 100%
illumination area and what difference it all really makes.

I'm using the program Newt. I'm building a 12.5" F5 telescope I've chosen
about 2" as my focuser plus 1" free height in focuser measurments, 14.5" ID
for my upper tube so as to be able to use it as a richfeild telescope.  I'm
only doing visual work, not photography.

Punching these numbers into Newt

2.6" secondary gives a 0.6" 100% illuminated feild.
but with a 21% obstruction..

2.14 secondary gives an almost 0" 100% illuminated feild.
with a 17% obstruction.

What real visable difference would these two different configurations
actually give me?
Which way should I leap?

Thanks from
David Moorhouse
Auckland
New Zealand