Wednesday, 17 April 2013

Shooting Jupiter's Moons

It was a nice clear night, so I cracked the camera out to see how clear a picture I could get of the moon.

 Not bad, considering my 5 year old camera really isn't a telescope!

It really wasn't much of a challenge, though in some ways you wouldn't expect it to be. The moon is quite close to Earth in the grand scheme of things; 'only' 249,986 miles on this particular night. It is also normally the biggest and brightest object in the night sky, running at about magnitude -10 on that night.

So I set myself a more difficult challenge... How about aiming for a picture of some moons around a different planet? Jupiter is the obvious choice because it has four huge moons (the Galilean moons Ganymede, Callisto, Io and Europa) and sits relatively close to the sun. Jupiter is also nice and bright and easy to spot in the night sky, although is still about 200 times fainter than our moon.

The problem with taking a picture of Jupiter's moons lies in their sheer distance from Earth. Jupiter was 531,833,620 miles (over half a billion miles) from Earth on that night. This causes two problems: Firstly because the moons are over 2000 times further away from Earth than our moon they appear much much smaller in the sky. Secondly Jupiter's moons are also about 5 times further from the sun than ours, which means they are illuminated much more weakly by the sun than our moon... Together this means that Jupiters moons appear about 2-5 million times fainter than our moon in the night sky. A proper challenge!

So what kind of picture can you get of Jupiter and its moons? It took some tweaking to get a good picture (I had to capture 20 images, align them and average them together to remove the background noise) but here it is:

Huh, that looks plausible... A big blob (Jupiter) and four smaller blobs (its moons?). It was easy to check where Jupiter's moons are expected to be:

The positions of the Galilean moons: 16/04/2013 20:58

A perfect match! Even down to the brightnesses of individual moons with Ganymede appearing brightest and Callisto faintest. This really is quite incredible; with a standard, modern, off-the-shelf camera and lens you can get a clear picture of the Galilean moons. In comparison in 1610, when Galileo discovered these moons, he was at the cutting edge of optical technology. 403 years for technology to go from a cutting edge revolutionary idea, to a cheap consumer commodity.

At the moment astronomers are just about able to image some planets around other stars. Now imagine in 403 years time, the year 2416; will people be able to buy some consumer camera, pop out into the garden one evening and take a picture of planets around another star?

Software used:
ImageJ: Image processing
Stellarium: Simulated images

The geeky details:
Canon EOS 450D
Sigma 18-200mm f/3.5-6.3 DC OS HSM
The lens was used at 200mm, maximum aperture (f/6.3), with focus set manually to infinity. 20 images of Jupiter were captured at ISO 800 with a 2.5 exposure time. Short exposures and high ISO have to be used because through a telephoto lens the stars drift rapidly through Earth's rotation. The moons were visible in the raw images, but to get a clear and less noisy image they had to be aligned and averaged in ImageJ.


  1. Well I don't know if you don't have any readers or if there's just nobody here who appreciates this stuff - but it's fucking awesome. Considering how long it took for *anyone* to even know those things were there... wow. And you nailed those moons pretty damn good if you ask me.
    But that's part of what's wrong today. People take things for granted they either cannot, or do not care enough to try and understand.

    Great blog though, keep it up!

    1. Thanks, I really appreciate your kind words :D