Jupiter opposition

About now, Jupiter is at its highest, and Saturn is rising in the evenings. It’s at this time that we begin to think about looking at the planets.

Planets are surprisingly challenging things to get a good look at. The problem is that they're so small. It’s only because they’re so bright that we can see them with our naked eyes. The large planets, Jupiter and Saturn, are similar in apparent size (if you include Saturn’s rings), but smaller planets like Mars are tiny.

To get a decent look at a planet, you need buckets of magnification. A telescope's magnification is calculated by the ratio of its focal length to the eyepiece’s focal length. In order to see a disk on Jupiter or Saturn, you’ll need around 100 times. My wide-field refractor's focal length is 560mm, and my only eyepiece is a 20mm. That only gives me 28 times (560/20), so I've got no hope. All I'm able to see is a bright dot.

The "beginner" telescopes we sell have focal lengths around 700mm, and you’d need the 4mm eyepiece that comes with the scope to get a look at the disk of the planet. Intermediate scopes are larger - about 900mm. Here's a simulation of what Jupiter might look like with one of these scopes and its 10mm eyepiece. As you can see, the planet is going to be small but recognisable.

FOV calculator from 12dstring.me.uk

A 127mm Maksutov-Cassegrain (with a focal length of 1500mm) coupled with its 10mm eyepiece will begin to get you some better views, but if you want a real look at the planet, you need a Schmidt-Cassegrain. I haven’t tried it out yet, but on our upcoming open night we’ll bring out our Celestron CPC925. With a focal length of 2350mm, we should be able to get views of the clouds on Jupiter.

Of course, I can’t stop at just looking at a planet – I’m an obsessive photographer, which brings a whole set of different challenges. This week, I'm going to take the 127 Mak home to see what I can get (the CPC925 won't fit in my little VW). I hope I'll get a better shot than last year's effort, which is below.