Star halos are prolonged backward. What is causing this, and how to fix it. 
Star shape error
What caused this: Collimation, lack of.
How to fix it: Collimate the scope.
How to fix it: Collimate the scope.
Without any information about your rig I can only guess that it's a combination of off axis aberration and possibly some drift error.
Well Written
Ed I agree with Andrea that collimation is the culprit. Double check it and try again…
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Yes, confirming that the collimation is correct is the first place to start. Based on the example it certainly looks like the collimation is off.
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could be a combination of lacking collimation and deviation from optimal back focus
cheers
Gernot
cheers
Gernot
To the OP. As you can see there there are a lot of possibilities. Without more information we're all just guessing. Please describe your optics, is the frame from the center or a corner, what camera and accessories are you using. This is basic information and should be included in any post like this.
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A quick perusal of the OP's gallery would answer those questions, wouldn't it?
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Come on andrea, it's common curtesy to provide the basic information needed for us to properly evaluate the image. Why should the burden be placed everyone else to go to the page, figure out which image is being referenced, zoom in and find the field and then check the imaging stats? Even just providing a link to the image would have been very helpful.
Well Written Concise
andrea tasselli:
A quick perusal of the OP's gallery would answer those questions, wouldn't it?
Not really. They could be testing out a new scope or camera and certainly don't know where in the frame it is. Rather than having a bunch of people spending time trying to deduce the info from looking at a bunch of images to see if they have more than one scope/camera, if the OP wants useful help, providing that information will get a better answer more quickly
Helpful Insightful
To the OP: Notwithstanding the last few posts, if you collimate your scope I’m confident that you’ll find a remarkable improvement in your stars. ✨ Sometimes additional information is helpful as noted above but your example image is enough to get you started down the right path. 
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I dont mean to create brawl here 
thank you people for posting regardless of post contents.
Btw, it is not my image so i cant provide details. These deformed stars are everywhere in the image. Found a culprit already.
thank you people for posting regardless of post contents. Btw, it is not my image so i cant provide details. These deformed stars are everywhere in the image. Found a culprit already.
Ed, don't leave me hanging on… What was the culprit?
Thanks and CS
Tim
Thanks and CS
Tim
Tim Ray:
Ed, don't leave me hanging on... What was the culprit?
Thanks and CS
Tim
Hi Tim, the image is my friend's. I asked him to send me over the telescope for collimation. I found small bulge of flocking material covering the focuser tube. I collimated and cut iff this bulge. Now waiting to bring it under the sky if it is okay.
That stray piece of flocking will cause it. I am happy it is such an easy fix! Congrats!
Clear Skies and good hunting!
Tim
Clear Skies and good hunting!
Tim
ed bat:
Good find on the obstruction in the light path.
How was the collimation when you checked it?
Let us know how tings turn out.