2020 August 6

Observer’s Challenge – Re-discovering Pluto

Eliot Hall and Richard Miles

Clyde Tombaugh discovered Pluto in 1930 by blinking two images of the same part of the sky taken on January 23 and 29 of that year.  Tombaugh used a 13″ (0.33-m) astrographic telescope to make approximately 60-minute exposures onto large (14″ x 17″) glass photographic plates.

Nowadays with modern equipment, Pluto is easily imaged with a medium-size telescope in conjunction with a CCD camera. But how many amateurs have seen it visually or have imaged it with a DSLR camera and lens? 

Pluto reached opposition on July 15 and so is now relatively conveniently placed for observation in the evening sky.  You will however need a low southerly horizon as it is currently located between Jupiter and Saturn at a declination of –22˚ in a crowded region of the Milky Way in Sagittarius, and so may be difficult to separate from nearby stars.

Unlike the brighter asteroids, Pluto is very distant being >33 au from Earth. As such, it is relatively faint at 14^thmagnitude and very slow moving at just 3 arcsec per hour. So, like Clyde Tombaugh, to properly detect its motion will require exposures to be made on different nights.

In the vicinity of Pluto, you can also observe several main-belt asteroids that you could use to practice on, given that they are brighter and their motion can be registered in a single night.

Visual Challenge

During mid-August, Pluto will be found near R.A. 19h 39m, Dec. –22.5˚, i.e. roughly midway between Jupiter and Saturn. So locate this area of sky using suitable planetarium software or a star chart. Draw the pattern of stars in this region over several nights and, if drawn accurately, inspection of the patterns should reveal the dwarf planet, Pluto as it tracks past nearby stars. We look forward to seeing your drawings.

DSLR Challenge

For this, you will need a driven mount / telescope on which to attach your DSLR camera. It should be relatively easy to record stars down to magnitude 15V if you use a long focal length lens such as a 300mm. But as part of this challenge, how about trying to image Pluto over several nights using as small a lens as possible – maybe even using a standard lens?

Things to consider:

Pluto is in a crowded field so if the image resolution is not high enough stars blend in together.  Timing may also be important as Pluto can often pass close to other stars.

Focus is extremely important with such short local lengths, both for maximising resolution in a crowded star field and ensuring the highest possible signal-to-noise ratio in the final image. You will probably have to stack several images to be able to detect Pluto. Try and point the camera with Pluto near the centre of the field of view otherwise lens aberrations towards the edge of the field may distort star images especially at fast focal ratios. Stopping down the lens by one or two steps might also help image quality and be beneficial overall.

A careful balance between exposure length and number of frames secured will be needed – remember that image quality is all-important so it may be better to take more shorter exposures to ensure no trailing is evident.  Your actual choice will depend on a number of factors including the camera, ISO speed used, and the background sky brightness. Try doing a dark subtraction to reduce noise in the image: some cameras can do this automatically.

Another tip is to remember you are trying to image faint stars so the brighter ones may become saturated, but that’s OK. 

With Pluto sitting between Jupiter and Saturn (they are approx. 8° separation), it should be easy to point your camera. However, watch out for internal lens reflections as Jupiter especially is very bright – otherwise you may think you have discovered a new comet!  What isn’t quite so easy is to find the tiny image of Pluto inching its way through a dense star field.

Please post your observations on the BAA members’ pages or e-mail them into the Asteroid and Remote Planets section via arps@britastro.org