Hallelujah - the jury is in!!!

[[1ponders] (Paul)]

Merlin you've obviously been exposed to this sort of technology before, would you consider doing a short write up about how to go about the testing or sites you are aware of that could provide similar information. I know of a certain C11 that I have never been happy with and I wouldn't mind betting this could be what is wrong with it.

[asimov (John)]

Originally Posted by Merlin66

A 35mm slide size piece of 120lpm grating and a bit of blu tac on the end of the focusser will do the job

Could you please elaborate on this. How does one get the lines on the slide? Or do ya buy them or what? I'm not opposed to trying the flywire lol... I've got miles of that stuff!

I made a ronchi tester 25 yrs ago but that involved glueing wires on by hand.

Cheers.

[bird (Anthony Wesley)]

Matt, sorry to hear about your problems, but I know *exactly* what you're feeling.

I'm going to order one of those ronchi eyepieces as well. Looks like a very useful bit of gear.

regards, Bird

[matt]

Hey Anthony!

Great to hear from you.

Thanks for your kind words.

We've all been wondering how you were doing with the move etc?

I've got a new 9.25 "in the mail". Should arrive tomorrow or Friday.

Fingers crossed this one will be a "keeper".

Cheers

[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.

[matt]

Good info Merlin

[asimov (John)]

Merlin I just finished making one, but it has between 80-90 LPI would that be sufficient to give a successful test conclusion?

I'm making another one soon that has 110 LPI.

[Merlin66 (Ken)]

As long as the edges of the grating are reasonably smooth and regular then it should work OK. The reason for the 120 LPI was to get easier lead into to the focus; as the number of lines decrease closer to focus the more lines you have in the grating (up to a max of about 200 lpi) the easier to see the transition into uniform grey.

[asimov (John)]

Thanks for that Merlin.

Not sure how smooth & regular mine is. It's actually made out of an old kithchen sink tap filter comprising of a 20mm round piece of fine brass mesh, as mentioned, about 85 LPI.

The other one I want to make comprises of extremely thin sewing monofiliment.

Tube currents in an SCT would no doubt give you a false test conclusion here do you think?

[Karls48 (Karl)]

Hi
This software - Make your own diffraction grating - may by useful.
http://www.coaa.co.uk/software.htm

[asimov (John)]

Yep, good for people that can print onto transparency. I can't do that.

Cheers for the link.

[Merlin66 (Ken)]

Asimov,
The grating will indeed show ANY deformation of the image from what ever the source; tube currents, upper atmosphere, secondary spiders ( both alive and dead!!) etc etc etc.
Basically what you see is what the camera, CCD, MkI eyeball sees.

[asimov (John)]

Well I'm a happy chappy. Just tested the C9.25 after allowing it to cool to ambient & I see perfectly straight lines with no deviation anywhere.

My mind is at rest & I can get on with it!

[Merlin66 (Ken)]

Good news; pleased to hear that.

[davidpretorius]

i heard from asi that something was wrong and am very much relieved that there is a happy ending.

well done bintel!!!

and congrats to matt for perservering!