Time for a telescopic challenge – the dwarf planet Pluto is at opposition on June 29th. Rising as the Sun sets, Pluto is a dim magnitude 14 object. This is 400 times fainter than the faintest object you can see with the unaided eye in a really clear dark sky. Pluto is in Sagittarius, above the lid of the teapot and to the left of the open cluster M25. This is just to the east of the line of the Milky Way in the sky. This region of the sky is rich in stars, so it may be easiest to spot Pluto by observing it over the course of a week and looking for the faint star that moves against the background stars.
I will be hunting for Pluto with my 10 inch telescope. The general view is that an 8 inch or better telescope is needed to see Pluto, but some observers, with dark, clear skies and good eyes have reported finding it with 4 inch or smaller scopes. Telescopes are like buckets – the larger the diameter of the mirror or lens, the more light it can collect. A telescope less than 4 inches in size just isn’t big enough to collect enough light for eyeball observation of Pluto, although it will be fine if you can hook a camera up to it to take 20 minute exposures.
Scope calculators – there are several available online – will give you an idea if you can see down to the magnitude of Pluto. But a lot of the success of a Pluto hunt will depend on how well trained your eye is. The most sensitive part of the eye to faint light is not the cones at the centre of the eye, but the rods off to one side. Knowing where that part of your eye is can take some time at a telescope practicing what is called averted vision. In essence you look through the telescope eyepiece at the faint object and then allow your eye to move slightly to one side – fainter details can then pop into view, but will vanish again when you look directly at them! Experienced observers reckon to see twice as much detail, but since everyone’s eyes are different you have to take the time to establish the best way to avert your vision.
Get a detailed chart of the sky around where Pluto will be and tick off the stars that you can see – the one that moves is Pluto! Over the next few weeks Pluto will be moving westward, passing below M25 in late July before swinging back eastward again. But, in a few months’ time the Earth will have moved around in its orbit of the Sun and Pluto will vanish into the Sun’s glare. Next year look for Pluto in pretty much the same place – it is in the constellation of Sagittarius until 2023!