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M31 What's causing this gradient? Full Moon?


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Hi all,

I snapped this pic of M31, 10 x 5 min subs using an asi2600mc-pro on an SM90, with a flattener/reducer. I'm a relative beginner... one thing I'd love to know, is what's causing the light gradient here, i.e. the image is 10% brighter on the left than the right.

What typically causes this? Could it be the moon light creeping in (pretty much a full moon), or a tilt on my sensor, or something else? I was at a bortle 4 site, so I don't think there was much light pollution at play. This isn't the first time I've seen such a gradient, but then at other times I've had images with zero gradient like this, so it's bit of a puzzle, something that shows up for time to time. It could be also be my flats setup, for which I used an ipad + flashlight-app + white t-shirt + elastic band. It could also be something up with the camera (amp glow?).

Or am I just overthinking this, and that a full-moon is always going to give a proper challenge?



If I do a dynamic background extraction, I can make the above image significantly better:


Any thoughts welcome!





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Probably the Moon I would think. What date and time was this?

RC Astro GradientXterminator gave me this with one click, but it's not free, or you could try Graexpert for free. 


My effort with that in PS





Edited by Laurieast
Replaced colour image
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A free alternative to removing gradients is Siril. This is what I could do, but it should yield better results with a proper tiff file instead of the jpeg I downloaded:


The iPad flats should be fine (I do the same), but a full moon is bad news for galaxies! I would try again on a different night.

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As has been said above, likely to be the moon but might be light pollution from local town / house. I would suggest that 5 minute subs is probably a lot longer than necessary with the 2600 unless you have very dark skies. Probably 1 or 2 minute subs would give a better result.

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My money would be on the Moon. Personally, I don't worry at all about whether the Moon is up when I'm imaging as modern processing tools remove the gradients really effectively, as aptly demonstrated by the examples in this thread! Full Moon or new Moon, I say just get out there and collect photons!

It sounds like you're using PixInsight, so here's something to try: use AutomaticBackgroundExtractor with Function degree set to 2, and Correction set to subtraction. I find this combination does a very good job of removing gradients. Of course, your experience may differ 😁


23 minutes ago, Clarkey said:

I would suggest that 5 minute subs is probably a lot longer than necessary with the 2600 unless you have very dark skies. Probably 1 or 2 minute subs would give a better result.

Clarkey is wise!

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As others have said, it's some kind of directional light pollution, be it the moon, street lights, or whatever.

You've got a good amount of background to play with, and it looks like a fairly simple gradient, so you should be able to remove it entirely with proper placement of sample boxes. 

I'd advocate for DBE over ABE as it's more effective in my opinion - it really helped remove some hideously complex gradients in an image of M101 I'm working on.

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Thanks all for the great feedback / answers.

@Laurieast it was around midnight 13/14th August 2022.

@Clarkey thanks. 5 mins subs has been my go-to 'safe zone', but I will heed your advice in future.

@Laurieast @Felias @Ratlet @Lee_P - thank, great to learn there's a bunch of options for dealing with gradients. I'll definitely have another go at it.

@The Lazy Astronomer @Clarkey @Laurieast - general consensus is the moon! Fair to say then, that directional light pollution is a common reason for gradients. Looking forward to trying again on a moonless evening.

Thanks @Laurieast and @Felias for processing the images I uploaded. It's amazing to see what dealing with the background does to the image.

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Hi All,

I took another picture on a recent moonless night, and there's NO gradient, so I can conclude the original gradient was directional light pollution, as suggested on this thread. Thanks! Any other feedback would be appreciated, but I'm happy with the result. As auggested, I also went down to 3 min subs and took 30.




SM90, asi2600mc-pro

Edited by Jon Pither
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