Observe Pluto This Year!

September, 2009

By Tom Koonce

Antelope Valley Astronomy Club

Lancaster, California

How many planets have you observed?  How many minor planets and dwarf planets?   Even though this month’s IYA theme is “Planets and Moonsâ€? our new Dwarf Planet, Pluto, offers an interesting challenge.  Let’s not debate the terms “Planetâ€? or “Dwarf Planetâ€?, but instead ask if you have you ever observed faint Pluto?  It’s a difficult object to see and to verify.

Pluto can be observed through an 8â€? telescope, but in my opinion it is HARD to do for an intermediate-level observer.  In Greek mythology, Pluto was named after Hades, the God of the underworld, and you’ll think about sending this challenge to the same location, but stick with it because spotting Pluto on your own for the first time is an extremely rewarding experience.

You need exceptionally dark skies, a decent telescope and a lot of patience!  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):

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*log d) + (5*log D)

where d is the aperture of the human eye in meters and D is the aperture of the telescope in meters.  So to give some examples, let’s consider a normal sky where the visual limit is around Magnitude 4.5 and using a 3-inch (76 mm) refractor telescope.  We’ll use 6 mm as an example aperture of the dark-adapted human eye (young eyes can get to 7 mm):

Telescope Limiting Magnitude = 4.5 – (5*log(0.006)) + (5*log(0.076)) = 10.0

So with a small refractor you can theoretically see down to a limit of about Magnitude 10.0 under these conditions.  Pluto however is at Magnitude 13.8 so this is well out of the range of such a small telescope.  Under very good skies with a limiting Magnitude of 7.0 and using a telescope of 10 inches (254 mm) aperture, the limiting magnitude becomes.

Telescope Limiting Magnitude = 7.0 – (5*log(0.006)) + (5*log(0.254)) = 15.1

This puts Pluto easily into “realistically observableâ€? status.  Why not set the goal of observing all the planets, and Pluto – just for fun?

Depending upon the type of telescope you have and if you have astrophotography skill, you may choose to image Pluto instead of working on the drawing recommended here.  Either way you’ll have to know where to look.  It’s recommended that you determine (and memorize) the field of view that you will use during your observation.  You can utilize the “12DString FOV Calculatorâ€? online here: (http://www.12dstring.me.uk/fov.htm) to help figure out the field of view you will see in the eyepiece.  You can use a Go-To scope or you can star-hop to the location of Pluto.  Either way you must use your telescopes’ clock drive to keep the field around the suspected position of Pluto and carefully draw the field of stars.  It is critical to spend a lot of time making this drawing because you’ll use it over the next two nights to determine which of the faint dots of light is moving and which are static.  Fixed = background stars… moving = Pluto!

You will see something like this in your eyepiece:

Amateur astronomer Chris Peterson, 12-inch telescope, Cloudbait Observatory, Guffey, CO

NOT something like this:

Pluto Image from Bill Dirk

Take the Pluto Observing challenge!  Try to observe all of the planets and at least one dwarf planet within the next twelve months!  Maybe you’ll be able to see or image Charon, Pluto’s moon!