Jupiter in the evening sky, with Ganymede and Europa's shadow in transit (Niall MacNeill)

Jupiter in the evening sky, with Ganymede and Europa's shadow in transit

I realised from the WinJUPOS Ephemerides that there would be a nice apparition with two of the moons of Jupiter. Unfortunately this occurred in the evening sky. Nevertheless I attempted to image it in the hopes of recording the event. Ganymede and the shadow of Europa were in transit, as was the GRS, whilst Europa itself was moving away to the right in the image. The seeing was difficult, but for two RGB runs it was reasonably good and that allowed me to produce this image.
I have generally found that imaging the moons of Jupiter against the black of space to be very difficult due to the edge diffraction effect, otherwise known as the Mars Edge Rind Effect. This tends to create a bright annular ring and wipes out detail near the limb of the moon. The smaller the moon, like Europa, the worse the effect, since the diffraction ring's size is independent of the size of the object being imaged. As a real diffraction effect it is very difficult to prevent and is worse the smaller the objective diameter and the longer the imaging wavelength, consistent with diffraction ring size being proportional to wavelength and inversely proportional to objective diameter.
I have had success when the moon is transiting the planet, because the bright background of the planet reduces the sharpness of the brightness drop off largely eliminating the effect. Unfortunately that means that whilst we can get decent images of the anti-Jovian Hemisphere of the moons, we cannot do so easily for the pro-Jovian Hemisphere, given all 4 large moons are tidally locked to the planet. Here you can see reasonably good detail, consistent with the seeing, on Ganymede right to the limb.
However, with Europa being against the brighter sky, the diffraction effect was not apparent. Although Europa was moving in front of the planet and thus we see the anti-Jovian hemisphere, imaging this and the other moons when behind the planet, during the evening hours maybe an expedient method of resolving better detail to their limbs for their pro-Jovian hemispheres. The challenge is usually the seeing in the evening sky, although around sunset when the inversion reverses it can be possible to have decent seeing.

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