[Merlin66 (Ken)]

Guys,
In a previous life I was the Section Director of the Photographic Section of the ASV for about ten years, in the hayday of 2415 and Hypered film etc. We had exactly the same problems as you have today! How do you get precise focus and how good are the optics.

I wrote a series of articles on astro-photography including a write-up on Ronchi Gratings; what they are and how to use them. I need to check my archives and see if I can re-find them and upload.

In the meantime I have some Ronchi gratings which I made using a slide copier and an "original" with the trustly OM-1 and 2415 B&W film. Over the years I've given most of them away to amateurs like ourselves. The 120/150 line per inch was the optimum.
Don't know if nowadays you can get the resolution by printing onto a transparent film with a standard inkjet/ laser type printer?? I've also used successfully a bit of fine mesh with similar lines/inch. I'm prepared to check out my "stock" and copy additional 35mm frames in B&W of the gratings I have and distribute them for a SAE.

The Ronchi grating acts like a Foucult knife edge tester but with multiple knife edges!! When the grating is precisely at focus, ( no eyepiece in the scope, only the grating positioned at the focus of the main mirror/ objective) the image of the star ( assuming we're testing a telescope against a star in the night sky) will appear as a grey disk similar to that seen in a Foucult mirror test... any distortions of the optical system will show up as lighter or darker grey areas against a uniform medium grey background. Because we're effectively testing the system with an infinite distant star, a perfect optical system will show an even gray.

When the grating is moved slightly away from focus, you'll see four or five dark and light "lines" or bands across the image: any distortion of the lines is bad news!! They should be straight, parallel, evenly spaced and no edge ripple or distortion. 100% perfect optics under 100% perfect skies!

Any distortion; the lines appearing to be closer at one edge or the other would indicate mis-alignment of the optics; they should be straight, evenly spaced and parallel.

If they appear thicker in the middle than the edges or curved inwards at the edges; the mirror/ objective is undercorrected and not the perfect shape!! Not much you can do about it unless you regrind the mirror/ replace the objective.

An easy tool to use: great for finding focus and setting flip mirrors etc.