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Kbramley

Odd diffraction spike on part of image

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I have just finished installing a new Moonlite CR2 on my 190MN, after some collimation issues I finally got it all squared up and everything looked great, the Airy disk was reasonable (there is a strange blemish that i can't explain at the top), the corners were in focus and no oblong stars which is a huge improvement over the stock focusser. However when I came to  image some brighter stars i noticed a strange diffraction spike style effect on the brighter stars in only the bottom part of my images. see below with a 400% crop of one of them.

Any thoughts on what causes this?

 

 

Airy.jpg

m44-cropped.png

m44-stretched.png

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Off topic but I noticed you have a de-bayered PS3 Eye Cam... is that useful?

I have a modded Lifecam doing nothing so maybe I'll debayer/destroy that...

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Yes it was definitely useful as a little project, essentially I ended up with a mono cam with 70-80fps with RAW output for about £5. The de-bayering process wasn't perfect so the outer edge of the images had to be cropped a little but it worked. I have very recently upgraded to an Atik GP however which offers a greater bit depth and far more sensitivity so the modded PS3 doesn't see any light anymore :(  

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Something flat is obstructing light cone in that region of the sensor.

If you are using OAG - might be prism is somewhat obscuring the sensor (corrected by flats, but there is still an edge for light to refract off). If not using OAG - inspect focuser for flat surfaces - It should be at a slight angle compared to longer edge of the sensor - since diffraction spike is not perfectly perpendicular (if it were perpendicular that would mean - parallel to longer edge of sensor).

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thanks for the advice, I am using an OAG, the prism is parallel to the short edge of the sensor, but well out of the light path (but could be reflecting off the base of the prism holder, so it would be off to the left of this image. I will try taking this out of the imaging train to see how that changes things. 

 

Could it be something on the edge of the secondary perhaps?

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There is simple way to check where is potential problem.

Rotate camera + oag assembly and take test shot with really bright star in lower part of the image (where spiked stars are now). With bright star it should be visible even in single short exposure so no need to stack. Take one shot prior to rotating for reference and one after rotating.

If you rotate camera and spike changes orientation - then it is related to "stationary" part - ;ole somewhere on secondary or focuser - whatever did not move can be a cause. If it stays the same (in same direction) - then it is related to assembly that you rotated, either OAG or some connection / reducer / flattener, what ever you might be using.

Direction of spike is also something to consider - edge causing it is always perpendicular to direction of spike.

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