Identifying Pluto

I had never seen Pluto before, so in the back of my mind, I have had the inclination to try it sometime. Then I saw an article in the July issue of Sky & Telescope magazine, where Tony Flanders writes,

Every year like clockwork, editors of Sky & Telescope debate whether to print a finder chart for Pluto. For the last few years the decision has always been “Okay, but this is the last time. We won’t run it next year — no way!”

When the forecast called for clear skies for the next two days, I thought, Here’s my chance!

After the discovery of many other small and distant objects in our solar system, a new classification system was presented by the International Astronomical Union. According to the new system, Pluto was re-classified as a “dwarf planet,” along with Ceres, Haumea, Makemake and Eris. The number of “planets” in the solar system was reduced to eight.

Pluto is currently passing through a dense field of stars in Sagittarius– an area of the sky near the plane of the Milky Way galaxy. This means that Pluto, currently at magnitude 14 is difficult to distinguish from the thousands of nearby stars of equal or greater brightness. From day to day, Pluto appears to move against the background of stars, so the best way to be sure one is looking at Pluto is to compare where it is on at least two successive days.

My plan was to take two images of the star field where I expected Pluto to be on June 4-5. Using my planetarium program, TheSky, I determined that shortly after 3:00 am Pluto would be positioned at an altitude of 23° and in between trees to the south of the observatory. I planned to use my ST-8XME camera attached to my Takahashi FSQ-106 telescope at f/8. The star field generated by TheSky for this arrangement is shown here, with my field of view outlined by the red rectangle.

Field of view for Pluto, generated by TheSky planetarium program

Here is first image taken through the telescope in the early morning of June 4, 2011 at 3:44 am.

Pluto June 4, 2011

Now the trick was to relate stars in the image with “stars” displayed by the planetarium program, in order to locate Pluto. My best guess is shown below, indicated by red tick marks:

Is this Pluto?

Is this Pluto? The only way to find out for sure is to wait a day and take another image of the same star field.

On the morning of June 5 at 3:10 am, I took a second image of the same region of sky.

Pluto June 5, 2011

Comparing images for June 4 and June 5, can you identify which speck has moved??

Neither could I.

However, using the Blink Comparator feature of CCDSoft, it is possible to see the difference between the two images. Here is the result, shown as an animation of the two frames:Animation of Pluto in star field June 4 and June 5, 2011

Now can you spot Pluto? It’s the dot that moves a bit near the center of the frame.

So my first guess was wrong as to which dot was Pluto– it actually was the smaller, dimmer one of the pair.

The other tiny specks that appear in one frame but not another are probably due to random cosmic rays hitting the imaging chip during the exposure.

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