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Old 22-06-2013, 07:37 AM
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Sensor Alignment with a Bahtinov Mask

I use the diffraction patterns produced by a Bahtinov Mask to check my sensor alignment.

I usually take a long exposure and move the mount by jogging 30' arc over the range +- 1,5 degrees. Both horizontally and vertically. Of course the Bahtinov Mask needs to be rotated by 90 degrees for each direction.

This is a stack of six exposures with the maximum pixel value ending up in the final stack. 3MB

http://d1355990.i49.quadrahosting.co.../AlignBagt.jpg


The centre pattern is two Bahtinov diffraction patterns at 90 degrees to each other superimposed. They indicate near to perfect focus at the centre.

The other four are the Bahtinov diffraction patterns 1.5 degrees away from centre.

For perfect alignment ie orthogonality of the sensor to the optic axis the four satellite patterns should be identical. They differ to the central patterns due to slight field curvature. This is what is expected in a field of 3.5x3.5 degrees at F3.

All these six exposures were taken without changing focus.

Bert
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Old 22-06-2013, 09:18 AM
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Bert

A change of focus - due to deviations of the sensor plane with respect to the plane that is orthogonal to the optical axis - would surely manifest itself as changes to the FWHM. Further more, the shape of the point spread function will deviate and become asymmetrical as one measures it outwards from the centre of the image (noting that for an aberration-free optical system the FWHM should be the same for all point sources in the system regardless of apparent magnitude).

Surely one image suffices and does not require a Bahtinov mask?

Thanks for a thought-provoking post.

Mark C.
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Old 22-06-2013, 09:27 AM
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Quote:
Originally Posted by mjc View Post
Bert

A change of focus - due to deviations of the sensor plane with respect to the plane that is orthogonal to the optical axis - would surely manifest itself as changes to the FWHM. Further more, the shape of the point spread function will deviate and become asymmetrical as one measures it outwards from the centre of the image (noting that for an aberration-free optical system the FWHM should be the same for all point sources in the system regardless of apparent magnitude).

Surely one image suffices and does not require a Bahtinov mask?

Thanks for a thought-provoking post.

Mark C.
FWHM does not lend itself to a visual sense of what direction to adjust the alignment. The Bahtinov diffraction pattern gives you a direct visual indication of which way to adjust. Keeping in mind of course that the image is inverted.

Bert
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Old 22-06-2013, 09:54 AM
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But surely if orthogonal alignment is off, then the PSF will not be circular and will appear coma-like - depending on position from centre of image - hence visual cue (I acknowledge that FWHM alone does not allow for correct interpretation of deviations from orthogonality).

What I suggested will probably require more software assistance than looking at a Bahtinov pattern - but I still feel that one image without a Bahtinov mask provides the necessary information. (I'm curious - maybe even ignorant - how can you correct for orthogonal deviations?).

The "inverted" bit requires system-specific interpretation and doesn't discriminate between your or my perspective on possible approaches - it applies to both scenarios.

Mark C.
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Old 22-06-2013, 10:25 AM
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I prefer to get instant feedback without resorting to complex computer analysis to adjust what is to me a trivial problem.

My system is fully adjustable. Is yours?


Bert

Last edited by sheeny; 23-06-2013 at 08:12 AM. Reason: inappropriate comment
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Old 22-06-2013, 07:21 PM
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I believe that my responses to your original post were reasonable posts in the context of an astronomical forum in which people post their observations etc, and the community learn together from the resulting interaction.

My posts were in the spirit of sharing knowledge - the motivation of your post - and the responses you expected are unclear. Your last response does not conform to my understanding of the spirit of this forum.

My contribution to this thread is terminated.

Mark C.
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Old 25-06-2013, 03:14 PM
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i think the purpose of Bert's first post is to document how he goes about getting sensor orthogonality for his system and what to expect. this is, not only to document his progress publicly, but also to demonstrate to other forum members how he does it. For others struggling with sensor alignment or just want to know another way to get things square, this is invaluable information. Thankyou Bert for sharing.

Josh
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