expert opinion needed. is this normal ?

[Pentatonicblues]

This has become a very 'interesting' thread. What you are seeing is a stacking artifact due to the fact that the moon is orbiting Jupiter and therefore the distance it moves during your imaging run is slightly different to the rate the planet surface features move at. I see it regularly when I image using RGB filters, as the imaging run is about 2 minutes across all fliters. This means I generally have to locally realign the channels for the moons in phortshop. To see this effect, have a look at the winjupos online guides-it shows that if you derotate jupiter images where there is a moon present in the image, the moon becomes elongated. This is due to the differential rate of movement relative to the planet cloud 'surface'. I have seen this on my iwinjupos derotated images if they have a moon or shadow included.

I preferred the aliens explanation though....

Stewart