Observing Pluto

July 28, 2008 — Leave a comment

Last night I fulfilled a longstanding ambition: I saw Pluto. The dwarf planet is extremely hard to see. You need exceptionally dark skies, a decent telescope and a hell of a lot of patience! I had previously doubted that the Meade ETX90 could even see it, and rightly so. There is an equation to help you work out how far down the magnitude scale you can get with a telescope (Remember big magnitudes = fainter objects):

Telescope Limiting Magnitude = Visual Limiting Magnitude – 5*logd 5*logD

, where d is the aperture of the human eye and D is the aperture of the telescope. So to give some examples, let’s consider a normal sky where the visual limit is around Mag 5.5 and you have a little 3-inch refractor telescope. We’ll use 3mm as the aperture of the human eye.

Telescope Limiting Magnitude = 5.5 – 5*log(0.003) 5*log(0.07) = 12.3

So with a small refractor you can see down to a limit of about Mag 12. Pluto however is at Mag 13.8 so this would not suffice. Here is the maths for my own situation. Last night the sky was so dark I could make out Mag 7.0 stars, this is my visual limit and it is about right for very good observing conditions. The telescope I am using has a 9cm aperture.

Telescope Limiting Magnitude = 7.0 – 5*log(0.003) 5*log(0.09) = 14.8

This puts Pluto into the realms of the feasible, which was great news since Pluto from my location is well above the horizon and unobscured by light pollution. Also, this week I have set myself the goal of observing all the planets, and Pluto – just for fun.

Pluto can currently be found in Sagittarius at the painfully dim magnitude of 13.8. Along with its sister body Charon and two satellites Nix and Hydra, it is a very cold, dark and far away place. Even the best images ever taken of Pluto reveal little more than a patchy ball of rock. I was unable to take a picture myself, far too dim, but Googling reveals a handful of amateurs have succeeded.

The image above is taken from Mauna Kea and the one below comes from an amateur named Bill Dirk.

If you want to try and see Pluto, I can now recommend a few things to help you along. Firstly, check that you have dark enough skies. This isn’t trivial, I have rarely had as good conditions as last night. Unless you have a very big telescope (more than 15cm) you’ll find Pluto is beyond your reach in anything other than exceptional skies.

Secondly, know where to look. Using the Meade with a good calibration means that getting into the general vicinity of Pluto is fairly easy. you need to know your stuff if you’re using a regular, manually guided telescope. Whatever happens, you need tracking, other wise the objects you find will vanish before you can see them at all.

Thirdly, get a good map of the planet’s location and memorize the patterns of stars around it. Once you’re looking at a star-field in the eyepiece, it will look the same as every other star field, unless you know what you’re after. Most planetarium software will give you this, the trick is figuring out the field of view you will be looking at through the eyepiece. I recommend using the 12DString FOV Calculator.

Finally, have patience. This will take time and several attempts. Even if you do find it, if you’re like me you’ll feel the need to verify everything twice anyway. But I think it was worth it!

Having finally seen Pluto I am now compelled to complete my planetary observation collection. All of the planets are visible during the night at some point from my current spot. I will not be here much longer so I need to get out and see them all. I have never yet seen Neptune or Uranus and I have only seen Mercury twice.

I’ll report back later in the week.

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