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Elongated stars

Sky-Watcher EQ6-R Pro ZWO ASI678MC Acquisition Deep Sky Artifacts
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Carlo Paschetto avatar
Carlo Paschetto avatar
Hello everyone,

I've noticed for some time that the stars in my photos appear slightly oval, with the elongation oriented in the same direction for all stars despite a nearly perfect polar alignment and well-calibrated guiding. It doesn’t seem to be a typical backfocus issue, due to the elongation appearing parallel.

Until recently, I could easily handle exposures up to 300 seconds, but now I'm observing this phenomenon even with shorter exposures. I'm using an EQ6-R mount and a 115/800 refractor. I started noticing this issue a few weeks ago when I began using a small ASI678MC: it's supposed to be more suited for planetary use, but I've been having fun using it to try to photograph small nebulae and galaxies that are nearly unreachable with my usual Canon. However, I don't think the camera has anything to do with this issue, even I've not yet moved back to my Canon for a counterproof, as during these weeks it's quite useless with galaxies. 

Where I should look at to investigate this issue?

Thank you,
Carlo
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andrea tasselli avatar
andrea tasselli avatar
Example please.
Carlo Paschetto avatar
Carlo Paschetto avatar
Here are a couple of light frames taken in different sessions. Elongation is very slight, but there is, particularly from right to left in the first example, from left to right in the second one. Depending on the moment, sometimes elongation is clearly visible, sometimes just approximate. Actually, the last pics published in my gallery, all of them taken on the last months with the above mentioned equipment, show stars not perfectly rounded.
andrea tasselli avatar
andrea tasselli avatar
With this image scale (totally unsuitable for DS with a small refractor, it shall be said) you have a bit of guiding error but nothing really out of the ordinary.
Carlo Paschetto avatar
Carlo Paschetto avatar
Thank you Andrea for your opinion (I was thinking something like periodic error or things like that, given the apparently regular pattern). Indeed, these are just experiments to try something different from my ordinary set-up and gaining confidence with planetary camera, which requires a total different approach from what I'm used to with DSLR. It's like starting again from scratch.
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Scott Badger avatar
Scott Badger avatar
Even if total RMS is pretty good, if DEC and RA are substantially different, that can make stars oval and I think all oval in the same direction would be consistent with that.

Cheers,
Scott
DanyJrt avatar
DanyJrt avatar
Elongated stars might also come from a back focus that isn't optimal.

Look at this diagram and analyze your own picture against it. If you see a similar pattern then this might probably be the issue. 

If all stars are oblong in the same direction across the frame, then a guiding error or wind might be the reason.
You can use BlurX in correct only mode on your final stack to correct the shape of the stars.
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Carlo Paschetto avatar
Carlo Paschetto avatar
Elongated stars might also come from a back focus that isn't optimal.

Look at this diagram and analyze your own picture against it. If you see a similar pattern then this might probably be the issue. 

If all stars are oblong in the same direction across the frame, then a guiding error or wind might be the reason.
You can use BlurX in correct only mode on your final stack to correct the shape of the stars.

Stars oblong in the same direction, this is why I excluded a back focus issue. Based on all of your comments, I think then it's just a guiding error, the only remark is that it seems it's been happening on a regular basis for a couple of weeks now
DanyJrt avatar
DanyJrt avatar
Here's what I would do :
- Make sure you're very well balanced in DEC and RA
- Lower your DEC and RA correction agressivness in your guiding settings
- Refocus your guide cam
- Try not to choose a guide star which is too bright and activate multi-star guiding
- Make sure to run guiding calibration before starting to guide. Calibration should be done near Zenith, on the same side of the Meridian than your target.

Based on your screenshots, I believe BlurX can easily handle the star shape correction. Give it a try and you should end up with nice round stars across the entire frame.
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Jeffery Richards avatar
Jeffery Richards avatar
If using an OAG, check to make sure everything is tight with no movement. I recently had a slightly loose stalk of the OAG (the camera holder) that would allow for a very small amount of movement of the guide camera.
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Carlo Paschetto avatar
Carlo Paschetto avatar
Here's what I would do :
- Make sure you're very well balanced in DEC and RA
- Lower your DEC and RA correction agressivness in your guiding settings
- Refocus your guide cam
- Try not to choose a guide star which is too bright and activate multi-star guiding
- Make sure to run guiding calibration before starting to guide. Calibration should be done near Zenith, on the same side of the Meridian than your target.

Based on your screenshots, I believe BlurX can easily handle the star shape correction. Give it a try and you should end up with nice round stars across the entire frame.

BlurX works great! Thank you!
dayglow avatar
dayglow avatar
This looks to me like the result of differential flexure.  Is guiding being done through a guide scope or OAG ?
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Carlo Paschetto avatar
Carlo Paschetto avatar
This looks to me like the result of differential flexure.  Is guiding being done through a guide scope or OAG ?

Guide scope
Brian Puhl avatar
Brian Puhl avatar
Load your subs into Blink (pixinsight) and play them sequentially.   It will generally tell you a more accurate story.   Flexure is definitely an option, but you're using 2um pixels on an 800mm scope.   That's 0.51 arc second image scale.    Your EQ6 isn't going to reliably keep up with that unless you have amazing seeing conditions.

Your guiding can be the absolute best an EQ6 is capable of and you still wont have reliable images at that sampling.  RMS means nothing.   You're oversampled and undermounted.   Get something with bigger pixels.
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