Is this noise? If so, how can I remove it?

[RichInLesta]

I'm brand new to PI and am working on my first astro image (the Andromeda Galaxy).  I started out by following Kayron's Andromeda tutorial step-by-step, and using those exact settings, to get a basic understanding of PI and the workflow.  Since then, I've gone back through it numerous times to "test" different settings at different points in the process to get an understanding of what the various settings do for the image.  Here is a link to one of the "versions": 

https://www.dropbox.com/s/17t4j81q8watey7/Andromeda_112816.xisf?dl=0

The image above has been completely processed according to Kaylons tutorial here:

http://www.lightvortexastronomy.com/tutorial-example-m31-andromeda-galaxy---dslr.html

I know there are many things I need to get better at, and that it will take time and practice.  And there are numerous things with this image that I'm not happy with (another set of questions for another day).  But my immediate question is this:

1. What is that "cloud" that surrounds Andromeda and is quite prominent between Andromeda and M110?  Is it really something that's there out in space?  Is it noise, or something that I introduced at some point in the process?
2. If it DOES belong there, I can live with that, although I think it takes away from the galaxy itself.
3. If it DOESN'T belong there, how can I remove it?  I've tried numerous settings and different processes but any that were able to reduce/remove the cloud/fog/dust also wreaked havoc on the rest of the image.

Here are the details:
This is a composite of 50 thirty-second images taken with a Canon 70D and 70-200mm Canon f2.8 lens at 200mm at ISO800.  The camera was piggybacked on a Celestron SCT with AVX mount and tracking seemed to be very good (individual images were very well aligned even before registering).

I did all the pre-processing in PixInsight, following Kayron's step-by-step tutorial, located here:

http://www.lightvortexastronomy.com/tutorial-pre-processing-calibrating-and-stacking-images-in-pixinsight.html

In addition to the 50 Light Frames, I created and used a SuperBias from 150 subframes and Master Dark from 50 Dark frames.  I didn't use any Flats (working on implementing that into my capture process).

The following image has only had DBE applied to it (after pre-processing, of course) as well as HistogramTransformation to show the image at the very beginning of the process.

https://www.dropbox.com/s/ne4ifu6l6bzxvv6/Andromeda_Closeup_112816_Weighted_DBE.jpg?dl=0

Although I'm only looking for an answer to this one question, feel free to comment on any other elements of the image so that I can learn.  But be gentle! 

Thank you, in advance, for any insight you could provide.

[RickS]

Unfortunately, no, this isn't something that's really out there in space.  My first guess is that it's a result of not doing flat correction - brightening up the vignetted corners has resulted in a central hot spot.  If that's the problem then the best fix would be to take some flats.  An alternative is to try DBE (using Division for Target Image Correction) or synthetic flats: http://trappedphotons.com/blog/?p=756

Cheers,
Rick.

[RichInLesta]

Well, that's good news (that it doesn't belong there!) because I now know that I can get a better image with a bit more work, as opposed to accepting my result as it currently stands.

I continued to experiment today and I should add that i was able to remove most of the "fog" in Photoshop by using its Dehaze function.  That suggests to me that you may be on the mark with your evaluation.  So I'll try the two suggestions you offered and see if that makes a difference.

Thank you for your feedback

[RickS]

No problem.  Good luck...

[RichInLesta]

Bingo!  Your assessment was right on the money.  I created some flats to test the assumption and reprocessed them in PI, then re-registered and re-stacked all my lights.  The end result of that was an incredible improvement from any of my earlier attempts.

https://www.dropbox.com/s/be9u3ni7ku50fz4/Andromeda_112816_with_Flats_DBE.tif?dl=0

I know there's still quite a bit of work to do, from start (image capture) to finish (processing in PI) and a fair bit yet to learn, but this removed a tremendous obstacle and "taught me a lesson" about the value of flats.

Thanks once again for the very helpful feedback.

[RickS]

That's much better.  Glad to have been of help...

Cheers,
Rick.