Ronchi test

[dark star]

I have a couple of questions about the Ronchi test. I have built a Foucault/Ronchi  tester based on the one on the Stellafane ATM website. I have a green LED. The best lines from the Ronchi grating seem to be when the LED is shining through the middle of the grating on to the telescope mirror. The problem with this is that your eye is just above or to the side of the LED looking through the grating which causes problems with seeing the lines in the middle of the mirror and the LED shining in your eye!. Am I missing something obvious here?

On the Stellafane website it looks like the LED is under the square hole in the tester which holds the razor/ knife edge for the Foucault test and the Ronchi grating. But if the LED is placed at the bottom of the Ronchi grating the lines from the grating do not cover the whole mirror.

Another thing that puzzles me is that I have a lot of lines when inside ROC, i have a 120 lines per inch grating, from all the examples I have looked at on the internet I am getting too many lines.

So far using the Foucault tester and Ronchi grating I can find where the radius of curvature is, where inside ROC and outside ROC is and if a mirror is a parabola or sphere. Now I am trying to use the matching Ronchi test to get my 6 inch mirror to the correct parabola.

David

[Glasspusher]

David,

I think that you will find the answer to your questions if you have a look at my notes on ronchi testing here...

http://www.nicholoptical.co.uk/pdf/The%20Ronchi%20Test.pdf

You will find some pictures of a simple Ronchi tester here, compare them to what you are using, it should help..

http://stargazerslounge.com/topic/75093-making-a-12-inch-mirror/page-4

Hope this helps,

John