Pixinsight grainy M81 - Forums

Started processing 18hrs of M81 data in Pixinsight but noticed a patchy grainy look to the images after colour calibration, BX & NX.

The Lower images (Lenhance)shows a reddish cloudy look that doesn’t look right where in the top image which is just IR cut filter the flux looks correct.

With the Nx I reduced the settings from 90- 65 and 3 iterations but the image doesn’t look smooth.

111 × 180s with darks and flats on an Asi533mc

Any thoughts or am pixel peeping?

Campbell

📷 Screenshot 2026-03-30 at 07.25.10.png📷 Screenshot 2026-03-30 at 07.24.30.png

The L-Enhance filter, while it’s bringing out the Ha, it’s not nearly as deep as the UV/IR image and the background certainly isn’t correct. I would extract the Ha from the bottom view and incorporate that into the deeper UV/IR image.

I have the same when using the zwo 533 with the ZWO 2" IR/UV Cut Filter had to process it out impost.

Tony Gondola · Mar 30, 2026 at 06:48 PM
The L-Enhance filter, while it’s bringing out the Ha, it’s not nearly as deep as the UV/IR image and the background certainly isn’t correct. I would extract the Ha from the bottom view and incorporate that into the deeper UV/IR image.

Thanks Tony, that was my plan but I don’t think I can use that back ground at all, not had that problem before. I blinked all the images in the Lenhance stack and removed a few clouded ones but the rest looked fine.

Wayne Emery · Mar 30, 2026 at 06:54 PM
I have the same when using the zwo 533 with the ZWO 2" IR/UV Cut Filter had to process it out impost.

It seems to be there in both but worse in the Lenhance Stack.

You got flattening wrong.

andrea tasselli · Mar 30, 2026 at 07:11 PM
You got flattening wrong.

What do mean? I took the flats after each session, I usually use asiair to auto the flats. I did wonder about the flats but couldn’t think what would be wrong.

Wayne Emery:
I have the same when using the zwo 533 with the ZWO 2" IR/UV Cut Filter had to process it out impost.

To be fair m81 is hard to process

Campbell Muir:
What do mean? I took the flats after each session, I usually use asiair to auto the flats. I did wonder about the flats but couldn’t think what would be wrong.

*I'm talking about the flattening of the L-ENH image. Either the flats are wrong or you have a light leak (which makes them wrong anyway) or you flattened the stacked image with other tools in PI, which often results in similar artefacts when not done carefully.

andrea tasselli · Mar 30, 2026 at 07:39 PM
Campbell Muir:
What do mean? I took the flats after each session, I usually use asiair to auto the flats. I did wonder about the flats but couldn’t think what would be wrong.

*I'm talking about the flattening of the L-ENH image. Either the flats are wrong or you have a light leak (which makes them wrong anyway) or you flattened the stacked image with other tools in PI, which often results in similar artefacts when not done carefully.

The scope has a black shower cap on the back and the flats are done at night with a light panel, the two images have only been stretched so far and yet to be combined. Might check all the flat frames in blink

The new tool multiscale adaptive stretch might be able to help with this. I was able to clean up a lot of old data with it.

A black shower cap is nowhere near black enough, I can tell you that. In fact not even two are. I had to make one with double thick felt cloth to be "dark" enough.

Quixotic · Mar 30, 2026 at 07:54 PM
The new tool multiscale adaptive stretch might be able to help with this. I was able to clean up a lot of old data with it.

Thanks, that sounds promising. I’ll take a look at that tonight.

Using Graxpert, by any chance?

andrea tasselli · Mar 30, 2026 at 07:58 PM
Using Graxpert, by any chance?

No, the pixinsight version

These are screenshots of the linear images, correct? Is this the regular screenstretch or using the 24-bit LUT?

Applying NXT on linear images can show very strange artefacts if you don’t look at it using the 24-bits LUT. This is only the screen stretch though. How does the image look if you stretch the image (MAS, GHS, or something similar)?

Furthermore the lower image looks like the gradient correction settings were not completely right. Again not sure how much you will see of that after normal stretch.

Campbell Muir:
andrea tasselli · Mar 30, 2026 at 07:58 PM

Using Graxpert, by any chance?

No, the pixinsight version

Of course, the "pixinsight version". That is the one possible source of the incorrect gradient correction, normally seen as a dark halo around galaxies.

Campbell Muir · Mar 30, 2026 at 08:08 PM
andrea tasselli · Mar 30, 2026 at 07:58 PM
Using Graxpert, by any chance?
No, the pixinsight version

Any chance there was a meridian flip during imaging? The stripe is right in the middle. The red areas seem flipped. Was wondering if a strong moon was out and caused a shadow which flipped when the meridian flip occurred. Just spitballing. I had noise issues that resolved when I bought a better filter wheel.

isn’t this just the IFN in this area that you’ve captured? the area is definitely not smooth if your image is deep enough. it actually looks OK to me.