[ATM] [atm] secondary mirror of gregorian

[Malcolm McDonald]

----- Original Message ----- 
From: "seyyed mahmoud ghasemi" <modeler72 at yahoo.com>
To: <atm at atmlist.net>
Sent: Wednesday, August 15, 2007 6:32 PM
Subject: [ATM] [atm] secondary mirror of gregorian


> Dear sirs,
>  in testing procedure of  secondary mirror , if  i put  a  LED light  with 
> a thin  slit   as light source , at the shortest  focal point   instead of 
> ( puting a polished steel ball for reflecting the light from the electric 
> bulb located out of the range of sight )  .., is it better  or i have to 
> put only the steel ball.....please guide me .

Your LED light should work if you can make it small enough but I think the 
ball wold be better. The reason for using a ball bearing is that it is small 
and won't obtruct the small lenz being tested, the light reflected is small 
like a star and being a sphere it is easy to align.The instructions 
suggested by Franck Giere at 
http://bobmay.astronomy.net/johnpierce/part9.htm are a good guide.

If you can obtain a length of 'fiber optic' cable that makes an excelent 
pinhole lite. If you can befriend a technition in a telephone company he may 
be able to help you. The ends of the fiber needs to be ground and polished 
just like your mirror, but on a microscopic scale.


>  meantime  i will be grateful  to  communicate   to  someone who has 
> experienced   making this type of telescope  ......please let me get 
> aquainted with them....
>  faithfully yours,
>  s.m.ghasemi

I have made two Gregorians, an 8 inch and a 20 inch. I was not going to get 
into this discssion because although they worked I never consider them a 
success. So I would hardly claim to be 'experienced'. I only looked at 
Jupiter once in both scopes, and in both cases the image was so large it 
looked like a fuzzy basket ball and the field of view was so small that, 
without a finder, I couldn't find anything else. Some day I may go back and 
set them up properly.

I wish you good luck with your project.

                                                                   Malcolm.