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Help! Please help identify source of circular gradient in IC 6960, Veil Nebula?

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9 replies to this topic

#1 Oscar Szentirmai

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Posted 22 September 2021 - 06:26 AM

Hi Everyone

 

Yesterday I imaged NGC 6960, and during processing I noticed a very obvious CIRCULAR gradient on my image?

 

I have attached a picture. I intentionally OVERDID the RGBK in Curves to highlight it more?

 

Is this from bad flats?

 

Is it from the filter?

 

Imaging setup yesterday:

 

WO Star71

294MC Pro

L-eNhance filter

65x300s

FULL MOON

 

These flats were taken 2 days ago, and did not produce any similar gradient on any of my Soul or Heart nebula images? Mount & OTA have not been moved or touched for the past 2 days, so did not take flats yesterday night?

 

The image on screen is after: DBE, background neutralisation, PCC, EZ DENOISE, HISTOGRAM STRETCH AND FINALLY STARNET.

 

Many thanks

Attached Thumbnails

  • Veil Gradient.png

Edited by Oscar Szentirmai, 22 September 2021 - 04:23 PM.


#2 Oscar Szentirmai

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Posted 22 September 2021 - 06:46 AM

I went back to the raw stacked unprocessed image, and just did a Background Neutralisation to remove the green cast, followed by a Histogram Stretch and RGBK curves. Did not do ANY postprocessing.

 

The circular halo/gradient is there, I think, but much less?

 

Is it my processing?

Attached Thumbnails

  • Veil Gradient raw SS.jpg


#3 Oscar Szentirmai

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Posted 22 September 2021 - 12:03 PM

Any thoughts at all?

#4 DubbelDerp

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Posted 22 September 2021 - 12:19 PM

I've seen a very similar pattern when using a shorter focal length lens with moonlight shining into the objective, but not hitting the sensor. Where was the moon in relation to your target? I'd guess you might have had some moonlight coming in from the side and causing a reflection.


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#5 Oscar Szentirmai

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Posted 22 September 2021 - 04:22 PM

I've seen a very similar pattern when using a shorter focal length lens with moonlight shining into the objective, but not hitting the sensor. Where was the moon in relation to your target? I'd guess you might have had some moonlight coming in from the side and causing a reflection.

Should I just avoid imaging when the moon is full, or should I use a shroud like the ones we use on SCTs?

 

Are you sure it's the moon?

 

IC 6960 is very much near the Zenith at my latitude, and the moon is also high up in the sky at SW.



#6 DubbelDerp

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Posted 23 September 2021 - 09:30 AM

No, not sure it's the moon. Just my best guess based on a similar pattern I've seen in my own images. I wouldn't avoid imaging with the moon up, but a longer dew shield may help, if it is indeed caused by stray light. Or if there were some way to shield it so the moonlight can't shine into the objective. I've set up on the north side of a building if I'm shooting to the north, so that my gear stays in the shadow of the building, and that seemed to help.



#7 dswtan

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Posted 23 September 2021 - 09:41 AM

Despite some optimists in this thread, I would never do any imaging outside of true narrowband (<7nm) near a full moon. Because of exactly this sort of problematic gradient issue that can arise.



#8 Oscar Szentirmai

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Posted 24 September 2021 - 07:29 PM

I’ve been advised on SGL that this might be condensation on my sensor.

Did a full night of imaging yesterday night with the full moon up and I was again imaging near the zenith.

This time I engulfed the camera and filter drawer with a dew heater band.

The end result shows NO circular gradient at all.

So I think it must be indeed the condensation and not the moon?

#9 RogeZ

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Posted 24 September 2021 - 08:22 PM

The 294MC is known for difficulty to calibrate with flats.

#10 Oscar Szentirmai

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Posted 25 September 2021 - 11:50 AM

Well, I've imaged the past two nights with a dew heater band on my camera and filter drawer.

Same full moon.

No circular gradient.

Looks like it was condensation after all?




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