Astrophotography
Animate the Galilean moons.
Jupiter presents a wealth of detail but tantalisingly this can be tricky to image because of the planet’s fast spin rate. Jupiter has a diameter of 139,822km making it the largest planet in our Solar System. Spinning on its axis in less than 10 hours, its gaseous form bulges at the equator producing an oblate disc through the eyepiece. Fierce jet streams rage around the planet, the atmosphere appearing banded and turbulent.
The Earth’s atmosphere blurs and distorts the disc of a planet. The best way to image one is by using a high frame rate camera: by capturing lots of sequential frames, some of the images will be less distorted than others. Using a registration-stacking application such as RegiStax or AutoStakkert!, it’s then possible to capitalise on the better frames, extracting them from the pack, registering them together and averaging the result. Cameras with frame rates ranging up to several hundred framesper-second (fps) are common these days.
Honing your planetary imaging skills on a fast-rotating planet like Jupiter can take lots of practice and matters are further complicated when the Galilean satellites become involved. Io, Europa, Ganymede and, to a lesser extent, Callisto, can all appear to interact with Jupiter’s disc, passing across it as a transit and casting their shadows on its surface in what’s known as a shadow transit.
The main issue with moon and moon shadow transits is that they take place at a different rate to the rotation of the planet below. Advanced techniques such as disc de-rotation, allow for extended capture times beyond which extensive motion blur would normally occur. They work by effectively rotating all of the frames back to a common time, undoing the rotation of the planet. When a moon transit becomes involved, de-rotation doesn’t work as well because of the relative motion of the moon.
One way around this problem is to keep your captures short. This produces noisier end results but at least everything appears relatively sharp. Taking lots of short captures will allow you to produce a sequence of results which can be added
together as an animation. The benefit of this technique is that it creates a dynamic record of Jupiter’s rotation as well as the moon and shadow transit. It also helps disguise the additional noise in each capture result.
Our step-by-step guide opposite shows how to create a monochrome animation using a mono, high frame rate camera fitted with an infrared pass filter. A mono camera could be used with individual RGB filters but this would entail a lot of work and open the door to additional issues because of the relative motion of the moons and their shadows. One big advantage of using an infrared pass filter in conjunction with a monocamera is that it can make moons such as Io and Europa clearer to see as they shine brighter in infrared than they do in visual wavelengths.
Alternatively, a one-shot, colour, high frame rate camera can be used, but with Jupiter now getting lower as seen from the UK, the best results require a more advanced approach using an optical device called an atmospheric dispersion corrector (ADC). This is used to counteract the effects of atmospheric dispersion that become more evident for objects lower in the sky.
However you do it, a successful animation of the Jovian system is a delight to watch as it really helps emphasise how the Galilean moons move in their orbit around their host planet.