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Old 29-10-2019, 10:58 PM
Martin Pugh
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Bahtinov Mask - That's weird?

Hi all
I am using a Bahtinov mask on the RASA in conjunction with the QHY600.

I thought I was making progress....got two corners showing 3 clear spikes and the opposite 2 showing 2 spikes.

But then I rotated the mask, just out of interest, and the 2 spikes turned into 3. I understand that displacement of the diffraction pattern will be reversed if you rotate the mask 180 degs. But I am pretty sure 2 spikes should remain 2 spikes regardless of the orientation of the mask.

Is that assumption correct?
cheers
Martin
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Old 30-10-2019, 04:32 AM
glend (Glen)
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Well in my experience three spikes are the norm, but this is a focus aid, not something that might be suitable for debugging tilt or rotational issues imho. In rotating the mask you are assuming rotation is exact and the tolerances may not support that assumption. You are also effectively 'off axis' as you move further away from the optical centre, and RASA design, with its high speed, does not behave the same way as typically slower designs.
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Old 30-10-2019, 06:45 AM
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Atmos (Colin)
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The way I’ve found best is to focus on the centre, find the worse corner and then rack the focus in or out and see whether it’s inside or outside focus. If you can put a digital focused on the RASA you can calculate how far out it is in microns.
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Old 30-10-2019, 07:06 AM
Stefan Buda
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I think the changes you see when you rotate the mask are due to the fact that the the entrance pupil is displaced relative to the center of the mask for stars in the corner of the field. The orientation of the mask should not matter for the center of the field.
I find that a Bahtinov mask is surprisingly sensitive device for detecting on axis astigmatism. If you focus on a star in the center of the field and then check the focus for different orientations of the mask you will see differences if astigmatism is present.
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Old 30-10-2019, 11:04 AM
Martin Pugh
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Thanks for the replies.

I have been reading about a tri-bahtinov mask that is supposedly good as both a focusing and collimation aid. Would be interesting to try. Anyone making them here?

cheers
Martin
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Old 30-10-2019, 11:52 AM
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h0ughy (David)
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I found that your cables get in on the act as well so you get an enhanced image where they cover under the mask
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Old 30-10-2019, 01:27 PM
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multiweb (Marc)
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Quote:
Originally Posted by Martin Pugh View Post
Thanks for the replies.

I have been reading about a tri-bahtinov mask that is supposedly good as both a focusing and collimation aid. Would be interesting to try. Anyone making them here?

cheers
Martin
That's pretty cool. Will have to try that for star collimation. I can never judge the diffraction rings clearly.
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Old 30-10-2019, 02:12 PM
Sunfish (Ray)
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Thanks
Looks like a useful project
https://github.com/cytan299/tribahtinov
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