Re: [ATM] secondary mirror help

[Michael Lindner <mikell@optonline.net>]

On Thursday 30 September 2004 12:20 am, Oakley wrote:
>    I'm well into my first telescope building project, and I'm a little
> stumped on the secondary mirror.

Rule of thumb is secondary size can be from about 16% to 25%. If you want to 
be more precise, download the "diagonal" program from Mel Bartels' site:

http://www.efn.org/~mbartels/tm/software.html

To find the correct distances do NOT assume your mirror has a 48" focal 
length, MEASURE it. If you have no equipment to measure it, do the following:

Prop the mirror up facing horizontally. Take a flashlight and put a rubber 
band of small piece of tape across the front, so it makes a shadow. Aim it at 
the mirror, and catch the reflection on a card held right next to the front 
of the flashlight. Move around until you find the sharpest focus of the 
shadow of the rubber band on the card. Measure carefully the distance from 
the flashlight and card to the mirror. Half that distance is the focal 
length.

Once you have the tube size and thickness, and the focuser height, feed the 
info into Dale Keller's Newt program:

http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm

That will give you precise positions for the holes in the tube. If you guess, 
guaranteed you will be cutting a second set of holes.

Clear skies.
-- 
Michael Lindner
http://www.starastronomy.org *** http://home.att.net/~mikel
http://www.atmsite.org *** http://www.atmlist.net

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