Bahtinov diffraction pattern not right

[Alan Snook]

Hi Folks,
Last night I put a Bahtinov mask on my Altair Astro 130EDT refractor. The diffraction pattern I am seeing at point of focus is not the expected cross with an intercept. Any suggestions on what the reason may be please?

• This topic was modified 4 weeks ago by Alan Snook.

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[Dr Paul Leyland]

Are you sure that the scope is in focus?

The vertical line is usually outside the cross when it is severely out of focus. Otherwise, I have no idea at present.

[Alan Snook]

Thanks for getting in touch Paul. Yes, that is the point of best focus. It occurs to me, I had a (decent) Barlow in, I’ve seen a suggestion that non-APO Barlows can mess up the diffraction pattern. So the first thing to try this eve is see what happens when it’s left out.

[Alan Snook]

After another session last night I think I am on top of what’s going on now. The expected diffraction pattern is very apparent when you look at it through an eyepiece. When I was looking at it on screen via the camera I evidently did not have anywhere near enough exposure/gain. In consequence I was only seeing what I suppose is the brightest “1st order” pattern alongside the target star. With more exposure the camera is now recording the 2nd & 3rd order (and beyond) patterns. Why it takes until the 2nd order pattern to ‘settle down’ I do not yet understand. I shall try and discover why.
Alan.

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