OASI: C/2017 T2

Comet C/2017 T2 (PanSTARRS) was discovered on 02 October 2017 by the PanSTARRS-1 survey telescope in Maui, Hawaii, when it was over 9 AU distant from the Sun. Astronomers hoped initially that the comet would achieve naked eye visibility; unfortunately, that did not happen, but it remains suitable for imaging, and in early 2020 is very well placed for observers in the northern hemisphere.

At the start of the year, the comet is at high altitude in northern Perseus heading towards the Double Cluster on 26-27 January. From there, it starts to loop back and head towards Ursa Major, reaching perihelion on 04 May at a distance of 1.6 AU from the Sun and 1.7 AU from the Earth. (At perihelion, the comet is predicted to be almost 10 times brighter than it appears in figure 2 below.) Three weeks later, on 24 May, it encounters M81 and M82. The comet then moves south during June, July and August before becoming too low in the sky to observe. Reference [1] provides its orbital elements and a map of its path. Figure 1 shows a screen shot from the Guide 7.0 planetarium program showing the position of the comet in relation to M81 and M82 on 24 May. The circle is 3° in diameter.

Figure 2 shows the first image by Mike Harlow of the comet, taken on 29 November 2019 when it was just 3° west of Capella, α Auriga. The image includes in the foreground Apollo asteroid 99248, which just happened to be passing at the time! Field of view 16x16 arcmins for each pane. 30.5 cm f/3.6 astrograph, 47 x 30 s exposures with CCD and luminance filter.

In early December 2019, C/2017 T2 was at an altitude of approximately 85° at midnight, well-placed for observing. All images and videos from this time onwards are by Nigel Evans. On 06 December, Nigel attempted to image the object. After sheltering indoors from the cold for an hour while the telescope tracked the comet, he was horrified to find that all the stellar images appeared as doughnuts! With the telescope pointed to high altitude, the camera had slowly slid out of the eyepiece holder by about 3 mm and only the first two images were sufficiently in focus to stack, presented as figure 3 below. Equipment used: 200 mm, f/10 Celestron Edge HD Schmidt-Cassegrain instrument and SBIG 8300 camera using 2x2 binning.

Figure 4 is a composite of 315 minutes of exposures from 19:07 on 18 January to 00:34 on 19 January 2020, with the comet moving at less than 1 arcmin per hour through the stars of Perseus. The LH panel is processed to show the passage of the comet against the background stars; the RH panel is centred on the comet and shows the stars sliding by. The tail is rather unimpressive! Equipment as figure 3.

On 28 January, the comet was close to the Double Cluster but the field of view of the Celestron 8 EdgeHD was too narrow to show the two objects together: that was a job for a 200 mm telephoto lens. The comet was at high altitude (approximately 75°) but, from Nigel's observatory in town, light pollution is a big problem and he had no LP filter for the lens. The result is singularly unimpressive (figure 5): the comet is bright but the tail is small and faint. The image taken with the Celestron at the same time has a smaller FoV and sharper stars. Dark skies don't improve the view much, as demonstrated in https://apod.nasa.gov/apod/ap200130.html (although the latter is a much nicer picture!)

Figure 6, also taken on 28 January, shows in the LH panel the passage of the comet against the background stars and in the RH panel the motion of the stars appearing to slide past the comet. Equipment as figure 3.

Clear skies on the evening of 03 February provided another opportunity to image the comet. Nigel used a Megrez 90 mm refractor to capture both the Double Cluster and the comet in the field-of-view (figure 7). He had the benefit of a light pollution filter, but the first quarter Moon contributed to overall sky brightness.

Fig. 1. Position of the comet close to galaxies on 24 May 2020.

Fig. 2. 29 November 2019, Mike Harlow.

Fig. 3. 06 December 2019, Nigel Evans.

Fig. 4. 18-19 January 2020, Nigel Evans.

Fig. 5. 28 January 2020, Nigel Evans.

Fig. 6. 28 January 2020, Nigel Evans.

Fig. 7. 03 February 2020, Nigel Evans.

Fig. 8. 24 March 2020, Nigel Evans.

The video immediately below is formed from the images used to create figure 4. The video below it is formed from the images used to create figure 6; it follows the comet from an altitude of 77° all the way down to 30° whereupon clouds rolled in. The tracking left something to be desired at times, and a narrow band of high altitude cloud passed over around 20:30 UT.

The final video was taken on 01 March 2020, with the comet still at high altitude in the evening evening sky in Cassiopeia (indeed, it was circumpolar and visible all night). Clouds forced an end to observation.