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I'm looking for some advice. I have never successfully created a mosaic of M31 that I am happy with. For some reason I can never quite match the intensity or the colours across the panels which you can see from the image below ( the Lum is usually ok). I'm not so worried about the seams as I know I can sort that but I do need help with the rest please.  This year I made sure to take each filter on the same night. there is approx 1.5 hours of 300s in each RGB panel (x4) although i lost a when sifting them, and the Lum has 3hours in each panel My work flow is:

1. PI - Crop the edges

2. DBE each panel

3. Transfer to APP for mosaic construction - repeat for all RGB

4. DBE agan in PI

5. Register

6. Combine, background neutralisation, colour calibration

7. Noise reduction

8. Linear fit 

9. Stretch

Any advice would be appreciated - I've included both the stars and starless versions.






Edited by steviemac500
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Hi Steve

Seeing as you are cropping in P.I before moving to APP, i presume that means you are using P.I for stacking the individual panels? If so, then i strongly recommend you do all the stacking in APP instead. One of the major selling points of APP is it's mosaic capabilities, and to fully take advantage of that you will need to use it for the stacking too. Once you have the registered Lum and RGB images, you are then free to move on to any program you wish for your usual post-processing workflow. 

Here's a great video guide, and a summary of the main settings, if you're not sure:  https://www.astropixelprocessor.com/part-3-register-normalize-integrate-the-mosaic-of-the-mosaic-tutorial-milky-way-to-rho-ophiuchi-by-mabula/

The really important bit, and it is one that might need a bit of experimentation with the settings (No. of Degrees and Iterations) is the LNC settings in the 6) Integrate tab when you come to create your 4 individual mosaics for each channel (Lum, R, G, B). With the right settings, the brightness of each panel should blend together really, really well. I did a mosaic myself recently, and i used 2nd Degree and 10 iterations for a 6 panel Ha mosaic and i thought it worked really well. I did have some issues at first, but i tracked it down to using LNC on each stacked panel. Once i turned it off, and only applied it at the mosaic creation stage, the results improved a lot. Of course, YMMV! 

Additionally, i would try using APP's 'remove light pollution tool' (i'm going to abbreviate that to RLP) instead of DBE. A lot of folk seem to think it does a better job. I can't say for sure myself, but seeing as you're in APP anyway i think it makes more sense to keep this stage 'in-house' so to speak. I also wouldn't run RLP on each individual panel. I would only run it on each 4 fully-completed mosaic channels, and then again on the combined RGB mosaic. 


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Totally agree with @Xiga, APP can do a really good job of blending the panels on a mosaic, but you will need to try out different combinations of the LNC and MBB settings at the integration step.

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Agree with all you wrote @Xiga, APP does a great job at mosaics.

16 hours ago, Xiga said:

i tracked it down to using LNC on each stacked panel. Once i turned it off, and only applied it at the mosaic creation stage, the results improved a lot.

How do you turn this off for the panels?

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...And then it automatically gives you the option to turn it on for the mosaic as a whole then?  I normally get that window that pops up and says do you want to do LNC again - is that the option to use for the mosaic only?

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