Pinpoint stars

Hi Emmanuel, after seeing your images (which are pretty nice!), I can tell that you've stretched the entire histogram on some of your images instead of the middle centered portion of it- this causes the transition from the stars core to the darkness of space to be sudden and makes stars look like they were put in ontop of the image, it also bloats them out past the normal transition line. If you use only the black point and grey point sliders, the stars cores stay sharp, and the transition is very nice.

I image at F/7 and 1422mm focal length, so I have to be very careful as to not explode my stars, but what I do is I never stretch the entire histogram of an image, using only the black point and the grey point sliders in GIMP (what I use for processing), and never touching the White point slider unless absolutely necessary. Additionally, sharpening images can reduce star size, but not that much.

It is possible to have perfect optics, conditions, no wind etc as well as near perfect guiding, equal in RA and DEC  according to PHD2 which - typically -  is sampling  at say 2.5s intervals BUT  still end up with elliptical stars.

That is because some mounts  suffer high frequency (too fast for PHD2 to track) micro-excursions as they track in RA which occur on the sub-second timescale.  For example I have seen short sharp 2 arcsec back and forth movements in RA occuring about once a second.   This creates elliptical-looking  stars that are fatter along the RA axis .

My CGEM II has this- but at a bad level, and it forces me to use 0.5s exposures to offset this issue. Thankfully, my guidecam is at 2"/px so I dont get any seeing issues, stars appear as pretty much a solid cuboid-ball the entire time. Gets me 0.8" guiding on a bad self-tuned CGEM II though, so I can't complain.