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Alnitak Meridian Flip Artifacts

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#1 Oyaji

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Posted 05 March 2021 - 04:32 AM

Last night I did my first images ever of the Horsehead and Flame Nebulas.  Due to various equipment and weather issues, the total time on target was way too short--only about 24 minutes before a meridian flip and 56 minutes after the meridian flip.  Clearly, I'll have to take another run at Horsehead/Flame.  

 

Still, a few subs are better than none.  Tonight I processed he before-flip and after-flip images separately.  Cropped pics showing just Alnitak and part of the Flame Nebula are below.  

 

In the after-flip image, there are pronounced and asymmetrical artifacts around Alnitak--they look to me like curved diffraction spikes.  Although the imaging train was exactly the same, these spikes are absent in the before-flip image.  I realize, of course, that processing has resulted in different colors, contrast and black point in the two pictures, and the scale of the two images as cropped by me isn't exactly the same, but the difference is also plainly visible in the unprocessed subs.  

 

Can somebody help me figure out why these spikes are present in the after-flip image and not the before-flip image?  

 

Image train:  120mm f/7.5 Refractor--->Baader UV/IR Cut filter--->Celestron OAG--->ASI294 MC Pro Camera  

 

If it might be germane to the issue, the temperature was dropping gradually--maybe 4 degrees F in total--through these two sequences.  And I stopped imaging when a cloud got between me and the target. 

 

 

Before the flip:

 

Horsehead And flame before MF  Cr

 

 

After the flip: 

 

B33 NGC2024 Horsehead And Flame Nebulas after MF Cr

 



#2 Ethanwyh

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Posted 05 March 2021 - 06:15 AM

Are these processed images? To confirm this issue, you must always look at the raw subs first.
EDIT: just saw your 2nd/3rd line, Do post up your raw fits here so that CNers here can help you.

However from your pictures, it seems to me that it's a processing issue (could be my eyes but i see ringing around Altinak in the 2nd pic)


From experience, i did get star halos on one side of the piper, none on the other side with a Lum filter and it was usually caused by stray light from nearby light sources. Reason is that: scope was pointing in the direction of the light source present on one side of the pier, but not on the other side (as it points away from the light source)

-Ethan


Edited by Ethanwyh, 05 March 2021 - 06:16 AM.


#3 Oyaji

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Posted 05 March 2021 - 04:51 PM

If I can figure out how to upload a FITS file, I'll do that. Or, I'll find a file hosting site and post them there. 

 

Your theory about stray light is interesting.  I suppose if stray light impinged on the IR/UV cut filter it might have an effect.  But I wouldn't expect it to be a big blue spike!  Alnitak is well known for producing a halo because it's so bright, but the spikes, if you want to call them that, in the pre-flip image are straight and symmetrical, whereas the big spikes are curved and asymmetrical in the post-flip image.  

 

I have only experienced this phenomenon once before, again with a very bright star, Antares, in a picture I took with my FSQ85 of the neighborhood of Antares and M4.  I attributed that to an issue with a light pollution filter (I had forgotten my UV/IR cut filter on that trip to New Mexico).  That pic was under Bortle 1 skies with NO stray lights.  Here's just Antares:  

 

Antares spikes


#4 Oyaji

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Posted 05 March 2021 - 10:10 PM

OK, I opened a Onedrive account and created two shared folders, each of which has 3 subs in FITS format, unprocessed.

 

The first folder has subs after the meridian flip:  https://1drv.ms/u/s!...XDIRcZ?e=6bj420

 

The second has subs before the meridian flip:  https://1drv.ms/u/s!...-2l0hY?e=L4TGxX

 

Thank you for any light you can shed on this matter.  




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