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Stephane,
A few reasons for eggy stars following a dither provided your stars are round without dithering
1/ Dither aggressiveness to high
2/ Dither recovery time too short for your mount
3/ Mount has tracking and guiding issues ( backlash issues etc... )
Does NINA has adjustable settings for item 1 and 2 above ?
Are you using PHD2 ?
Make sure your dither settings in PHD2 are “Random” and dither scale is 1.0 otherwise your dithers maybe to aggressive.
Most skywatcher HEQ5 ,EQ6 and EQ6-R mounts etc...require anywhere between 10 to 30 sec to recover from dithering before you take your next exposure
Both my EQ6-R mounts require 15 seconds to recover from a dither before the next exposure it taken. APT controls my dithering.I set my dither distance in APT to 1.0 which is the lowest and dither recovery time to 15 sec , has worked ok for years.
Also the PHD2 guiding graph visually gives you an idea when the mount has recovered enough to take the next exposure.
Generally a 3 to 6 pixel dither is sufficient to mitigate fixed pattern noise but the mount will let you know it’s limits , some mounts can handle a 10 pixel dither and recover in 15 sec others only 5 pixel , they’re all different.
Hope the above helps
Cheers
Martin
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