Here is a image made by taking 250 exposures at 5 sec each with a Canon R5 and Sigma 40mm f/1.4 ART lens at f/1.6.
The exposures were raw converted in a complex way because of lack of support for R5 - I went from CR3–> DNG with Adobe Raw Converter, then DNG–>TIFF with Raw Therapee using linear processing only, then TIFF–> FITS with Siril 0.99.6
Here is the final image, after processing with Startools.
Unfortunately, the stars in the lower left hand corner have been turned into short star trails. Here is a close up to show that.
At first I thought this was due to coma in the lens, but that isn’t really the case. Here is an excerpt of a similar area from one of the sub-frames. You can see that there is some elongation of the stars, but nowhere near as much as in the stacked version.
So I tried to stack the same images using Astro Pixel Processor (APP), and here is a detail from the result
The stars are MUCH better looking in the APP stack than the Siril stack.
The stars are not perfect in the single frame image - at f/1.6 there is some stretching of stars, but it is quite small.
Here is what seems to be going on. The 40mm Sigma is an excellent lens, even at f/1.6, but there is inevitably some distortion caused by the lens between the center and the edges or corners.
Siril registered the images without a lens distortion model. APP registered with a distortion model.
I believe this is a fundamental restriction Siril has, but perhaps there is a way to get the distortion corrected that I am not familiar with - I am relatively new to Siril.
The lack of a distortion model means that Siril was stacking the frames off center from each other, as one would expect for a fixed camera. The registration was likely driven by the mass of stars near the milky way. When those are in the correct position, the distortion present in the
In the case of a tracked, rather than fixed camera this would not occur if you were stacking a single frame. However, it will occur in stitching any mosaics, in the overlap region. The degree of the problem will depend on the amount of lens distortion. The 40 mm Sigma is barely a wide angle lens - with a 24mm or 14 mm lens the distortion would be MUCH worse.
Siril is not going to be able to compete with APP until the distortion model is included.
That is a shame because Siril is much faster at registration and stacking than APP. The distortion model would take some computation, but even cell phone panorama stitchers do it, so it can’t be all that time consuming.
In principle it should also be straightforward to add - the lens distortion calculation of the sort necessary is already done by any panorama stitching program such as Hugin or PTools. That said, I am not familiar with Siril internals - it’s always easy for the uninformed to think something is “easy”.