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Potential Focus Issue - spikes on stars at the images edges

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#1 seanboon

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Posted 17 January 2021 - 07:03 PM

Just finished stacking frames from a session on the Iris Nebula and I'm noticing that as I get closer to the edges of the image, I am seeing "spikes" coming off my stars in a particular direction.  My setup is as follows:

 

William Optics GT81

Flat 6AIII (.8 reducer)

Nikon D750

 

I attached an example of what I'm seeing.   I'm interested to learn what would cause these spikes.  I'm assuming it's a focusing issue, or is it just inherent with the type of scope I have, flattener and size of my camera sensor.   Could it be that my backfocus is off?   I do use the Bhatinov Mask that came with the scope to focus and feel I'm doing a reasonably good with that.   My stars towards the center of the image generally look good.   

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#2 james7ca

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Posted 17 January 2021 - 07:48 PM

That looks like typical optical aberrations which could result from incorrect spacing of your reducer (or just not that great of performance). Does it appear more or less equal in all four corners? If so, you may be lucky since that would probably eliminate tilt in the optical train. You might also want to focus part way between the center and edges of the frame.



#3 astrodom

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Posted 17 January 2021 - 07:56 PM

This looks like coma towards the edge of the field on a lens or telescope with some abberations.  Tell us more about the scope and optical train.  



#4 erictheastrojunkie

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Posted 17 January 2021 - 08:08 PM

With the full frame sensor your backspacing is going to have to be perfect for good stars in the corners, your spacing is probably a little bit off. Try making some small adjustments with the Flat6Aiii, based on the stars radiating away from center the sensor is probably slightly too close and needs to be pushed further out.
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#5 DJL

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Posted 17 January 2021 - 08:08 PM

I have the same scope and flattener. At first I didn't realize I had to actually adjust the flattener. Unscrew it to 7.1mm and then lock down the gold ring.

Numbers for other scopes on this page: https://williamoptic...nt-not-included


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