Inspired by Geoff’s results I tried using the automerge in photoshop on my pictures of the Heart and Soul Nebulas. Oddly it only matched them if I selected the Soul first! Unfortunately it doesn’t rotate the images and my two sessions were at quite different angles for some reason.
I have combined images in photoshop before, but only on top of each other, never as a mosaic, but i decided to follow a similar path:
First I doubled the canvas size for the Soul image an imported the Heart as a layer on top.
I set opacity (opposite of transparency) of the heart image to 50%, and guided by stellarium to find the rough alignment it was easy to find a small asterism on both images. Zoomed in, I overlapped one of the stars in the asterism as accurately as I could. Circles of smeared stars around this point gave an idea of how much rotation would be needed.
using the select tool I then moved the centre marker of the Heart image over the centre of the aligned star and went to the edit menu to select ‘free transform’. This opens boxes where you can alter things like horizontal and vertical size and skew the image – not needed for images at the same scale. Instead I just used the ‘rotate’ box – this lest you enter angles in increments of +/- 0.05 degrees. This may sound a quite big step, but actually I’ve found it is small enough to align stars across a whole image. It’s easiest to step in whole degrees, then tenths of a degree and finish by using 0.05 degree to get the best possible result. It’s very obvious when you are aligned as the overlapping area suddenly looks much sharper.
I then changed to the magnifying glass tool – this brings up a box asking you if you want to apply the transformation – click yes if you are happy!
I then restored the Heart image to 100% opacity. It was clear it was less contrasty and a bit paler background. I used the ‘levels’ dialogue to alter gamma and black point until it looked a closer match to the Soul – you could also do this with curves. the whole thing might have been better if I had originally process the two images the same way.
Once they matched there was still an area of overlap with poor quality in the corner of the Heart image. I used the lasso tool with a feathered edge to remove most of this corner. A small patch still appeared too pale, so I lassoed it and changed its levels to match better.
This is the result, not perfect but not bad either: