Interesting Images of Pluto

In terms of quality, this image is no masterpiece. Although basic calibration has been carried out, there is an obvious colour cast to the background and some of the star images don’t look right.

Some of this however is intentional. This is a composite image comprising eight individual frames taken over a 10-day period in July 2020, showing the planet Pluto moving through the stars of Sagittarius. (Yes, I said planet: don’t go there).

There are two 15-minute images taken through each of my red, green, blue and clear filters. Typically the images are 24 hours apart, except for a couple of gaps due to poor weather. I have marked the position of Pluto in each image: it is the ‘star’ directly above the date number.

The first thing of note here is how much Pluto moves in just a day (the frame is about a quarter of a degree wide). Pluto takes 248 years to orbit the Sun (that’s over 90,000 days) so to see it move noticeably in just 24 hours I find remarkable.

You will see that most of the stars show a range of colours, mostly white, yellow and orange. These colours are not strictly correct because I have not properly colour-calibrated the image; but the images of Pluto itself taken through the colour filters appear pure red, green and blue (other than those taken on the 14th and 15th, which were taken through a clear filter). This makes it much easier to identify Pluto in the images as it is the only object to appear pure red, green or blue.

The reason for this is that Pluto only appears in each image at the position it occupied at the time of that exposure, so for example on the 23rd it was imaged through the red filter. Hence there is no blue or green image of Pluto on that date in that position, so Pluto appears pure red in the colour-combined composite. The same is the case for the green and blue images.

The date numbers also show up in the colour of the filter because the images were annotated while still in monochrome mode, so when they were assigned to their colour channels the numbers took on the colour of the filter.

One other item of note concerns the blue images. You will notice that Pluto is rather faint in blue, the image of the 19th being quite difficult to see. However there are two brighter blue ‘stars’ nearby and at first I mistook these for Pluto, until I realized that they were not at its expected location. 

I put ‘stars’ in inverted commas because I am not sure what these objects are. They are certainly not cosmic ray hits, and zooming in shows a perfectly round Airy disk about 3 arcseconds across – about the same size as nearby stars. However their perfect roundness sets them apart from the nearby stars (and Pluto) which are slightly elongated due to small errors in the telescope guiding. This tells me that they were transient events, much shorter than the 15 minute exposure time. It also means they were much brighter than the other stars to record as many photons as they did in a much shorter time interval.

The objects only appear on one image.

I have seen many defects in CCD images but I have no idea what these could be.