Ganymede Occulting Io – My First Galilean Moon Occultation!

UPDATE: See below for a new colour image of Io and Ganymede.

This morning I was able to capture my first Galilean Moon occultation, when Ganymede passed in front of Io, covering about 40% of the volanic moon.

The seeing was reasonably stable, maybe 6.5/10 and the morning started off beautiful and clear. Some cloud bands came through right at the wrong time, ruining some of the sequences right in the middle of the occultation. Of course it cleared completely after the event was all over.

Despite the annoying clouds almost ruining my chances of capturing anything of the occultation, I was able to image enough of the event to put together this 32-frame animation, covering 33 minutes of elapsed time. Detail and albedo features can be seen on both moons. Click the image to play the animation.

Ganymede Occulting Io - click to play

Ganymede Occulting Io - click to play

Continue reading to find about my capture and processing settings for this event, and why it’s even so special at all.

Every 6 years, Earth passes through the plane of Jupiter’s ring/moon system so from our vantage point on Earth, the Galilean Moons will all pass back and forwards in a straight line as they orbit Jupiter. It’s during this time, that we can see occultations and eclipses between Jupiter’s Moons.

Imaging techniques and equipment have improved immensely over the past 6 years, so this is the first year where amateurs are recording these events and producing detailed, high-resolution images of the phenomena. 

Anthony Wesley produced the first high-resolution image of an occultation this year, with Europa passing in front of Ganymede on the 8th May. He’s also since captured Europa occulting Io on the 12th May.

Paul Maxson produced a lovely animation showing Ganymede occulting Europa on the 18th May.

A full list of mutual moon events for this year can be seen at the Astronomical Almanac.

I used my normal Planetary Imaging Equipment, and recorded each run for 30 seconds using the red channel only, at 30fps (900 frames). I left 10-20 seconds between each run in most cases, though when clouds covered Jupiter at times it was a minute or two between captures.

For processing, I used Registax to stack the best 200 frames for each image and wavelet filters for mild sharpening. Photoshop was used to layer each image, with minor levels adjustments to set the black point and white point.

The animation was created using Jasc Software Animation Shop 3.

Before the occultation started, I was imaging Jupiter itself with the Moons when they were further apart. I still need to process that data, but will post more in the next few days.

Here’s a new colour image of Io and Ganymede, taken with the RGB filters before I concentrated on the red filter alone.

Io and Ganymede in Colour

Io and Ganymede in Colour

Thanks for looking.

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