Mirror testing

[Robbos30 (Peter)]

Hi everyone, I thought I'd ask this question that has bugged me for a while.
Is there anyway of testing your Newtonian mirror for its accuracy while it's still in your mirror box. Reason, when I received my primary mirror years ago, Nova optical didn't send its specks. Just wondering, for convenience and practicality sake?

[astro744]

You can do an optical bench test using either Foucault knife edge or Ronchi grating. You will likely have to remove secondary and/or spider so that the light source is not obscured.

The best method though is a star test and there are plenty of resources online as well as this book: http://www.willbell.com/tm/tm5.htm

If you want a test with a laser interferometer then I think the mirror would have to be removed.

[sopticals (Stephen)]

Originally Posted by Robbos30

Is there anyway of testing your Newtonian mirror for its accuracy while it's still in your mirror box. Reason,:

You can use a Ronchi eyepiece. 133lpi (or more). Fit the eyepiece, focus it up on any bright star. If the optics are f5 or slower and image displays parallel clean lines, (usually 4 to 6 lines focused), from edge to edge, then one can assume the optics are probably a 1/4 wave or better. If the system is faster than f5 you need to increase the sensitivity of the test by using a good 2x barlow in front of the eyepiece.

[Robbos30 (Peter)]

Thanks for the feedback, great website, appreciate it. My is an f(4.8), where would I purchase a Ronchi eyepiece from?

[sopticals (Stephen)]

Hi Peter,

two options come to mind. You can source a Ronchi Screen from RonchiScreens.com and make up your own test eyepiece by cutting the screen to size to fit into the shell of some old eyepiece you have deemed useless/unsatisfactory,(of course discarding the old lens set first). Or secondly google up: Schmidling Productions, Inc. for their Ronchi 133lpi Easy Tester eyepiece.

[Robbos30 (Peter)]

Great stuff, will do! Many thanks, really appreciate your time.
Cheers Peter.