ok - this is a 3x2 mosaic of M31 - Andromeda Galaxy.

As always with Mosaic's i had trouble with differing levels of image intensity

i used a combination of dnaLinearfit and John Murphy's PhotometricMosaic scripts

between the two of them i managed to get a reasonable image balance

All processing done in Pixinsight

Workflow

For Each Panel (After stacking)

Crop

Script:ImageAnalysis - So can maintain coordinates for later

Channel Extraction / Linearfit on lowest colour (blue) / Chanel Combination

DBE

Photometric ColourCalibration

Script:MosaicByCoordinates

dnaLinearfit: left and right panels with their associated center panel

Then used Photometric Mosaic to join the top panels into a single row

repeat with the bottom 3 panels

use dnalinear fit to get top and bottom panels nicer

then use Photometric Mosaic to join the Top and the Bottom panels together

Now i had a mosaic

Crop - take away the edges caused by the mosaic panels not being that accurate from night to night

I then performed two sets of Noise Reduction using Multiscale Linear Transform - first attacking chrominance then luminance (most of the effect was on Luminance)

Stretched using HistogramTransformation and ScreenTransferFunction - i tried MaskedStretch but the resultant image was too washed out (although i always try both as sometimes the results are better)

after stretching i performed HDRMultiscaleTransform - but it was only a gentle level with 8 layers - any less and the image started to wash out and become too brown

I then did LocalHistogramEqualisation - with a constrast limit up at about 2.5

I masked using luminance and then used curves to raise the Luminance and Saturation a little bit to make the image a little brighter.

I did not perform any star reduction as i quite liked this image with it's stars

Anyway - overall fairly happy with this image. I am sure i could do some more saturation etc but i didn't want to over do it. I know if i double stretch the image in pixinsight i can tell the panel join image intensity and background removal is not perfect- but at the intended intensity level it looks ok so i am happy with that