OASI: ToM 2019

James Appleton and Martin Cook coordinated arrangements for observing the transit from Orwell Park Observatory. Following customary practice, they aimed to utilise the Tomline Refractor to project an image of the solar disk, which could be studied visually and photographed, leaving observers on the balconies of the Observatory to use more sophisticated electronic imaging in both white light and Hα wavelengths.

1.1 In The Dome

Experience with previous transits [a] demonstrated the benefit of an effective sunshade in minimising ambient sunlight entering the dome, thereby improving the contrast of the projected image. On 11 November, the Sun would attain a maximum altitude of only approximately 20° so the sunshade needed to accommodate only a modest elevation of the telescope. James designed a sunshade utilising the runners introduced for the 2016 transit, using a double thickness of semi-opaque plastic flooring material fixed in position to obscure the bulk of the aperture, with a flexible fabric covering the lower portion, able to ride up and down the runners to accommodate the movement of the telescope.

Experience with the 2016 transit indicated that sunshade construction and installation could be an elongated affair, so work in 2019 began in good time. James finalised the design and purchased materials in mid-late October. Due to supply problems, he used canvass for the lower, movable part of the sunshade, and attached wooden battens which in turn would attach to runners able to move up and down the rails. Members of OASI assembled scaffolding in the dome on 30 October and began the task of assembling the fixed element of the sunshade. Unfortunately, it immediately became apparent that the plastic material would be too flimsy to withstand the rigours of a strong wind (should there be one), and an alternative design was required; the decision was quickly made to replace the plastic with a length of blackout fabric. James and Martin completed assembly of the sunshade on 10 November, and also fitted a 1 m square lightshield at the OG end of the Tomline Refractor and obscured the windows of the dome. The result was a dark interior, with relatively little ingress of ambient light. See figures 1-8.

Fig. 1. Preparing the runners prior to installation. (ANM) [b]

Fig. 2. Installing the runners. (AG)

Fig. 3. Preparing the plastic material for the fixed part of the sunshade. (ANM)

Fig. 4. Fitting the plastic material. (ANM)

Fig. 5. Replacing the plastic with blackout lining. (MPC)

Fig. 6. Adjusting the cords controlling the movable part of the sunshade. (JMA)

Fig. 7. Preparing to join the fixed and movable parts of the sunshade. (JMA)

Fig. 8. The assembled sunshade. (JMA)

On the day of the transit, observers assembled at the School at 10.00am [c], and made their way to the Observatory by 10.15am. On arrival, the weather was most unpromising, with wind, cloud and occasional rain. No-one among the party had any expectation of seeing even the Sun, let alone planet Mercury! The weather had dissuaded several would-be observers from attending.

More in hope than expectation, James and Martin mounted the camera bracket on the Tomline Refractor and attached a Canon 1100D and a Canon 6D Mk II (figure 9). They then rejoined the remainder of the group in the Belvedere and all gloomily surveyed the weather. At 11.00am, all staff and pupils from the School trooped onto the grass at the rear of the premises and held a ceremony marking Armistice Day. Those present in the Observatory looked on from a balcony, and marked the two minute silence, ended by a bugler.

By 11.30am, although there had been brief periods during which small areas of thinner cloud enabled the Sun to be glimpsed, the outlook for the transit remained hopeless. However, during the next 30 minutes, the rain ceased and the cloud gradually thinned, and it became apparent that there might be a chance to observe after all! A frantic effort then ensued to open the dome, mate the Tomline Refractor to the sunshade with the OG protruding through to the sunlit side (figure 10), align on the Sun and begin observing.

The sunshade proved so effective that the telescope did not cast any appreciable shadow, and aligning it on the Sun therefore proved tricky. Martin eventually succeeded in the alignment and the telescope drive was engaged to track the apparent motion of the Sun. The observers selected a large, ex-military surplus eyepiece (the same one employed for observation of previous transits). This projected an image of the Sun approximately 75 cm in diameter. The image was too large to fit in total on the projection screen, so the telescope was adjusted to centralise the portion of the solar limb where Mercury would first appear (figure 11) [d]. By 12.21pm, everything was in place and the two cameras were recording videos of the projected image. Martin secured a large radio-synchronised "Rugby" clock (more properly an MSF clock) to the corner of the projection screen for timing events.

Due to the low altitude of the Sun (approximately 19°), the image was very unsteady, and the solar limb in particular suffered continuous undulations. The low altitude of the Sun also meant that the contrast of the image was not as great as at previous transits. No sunspots could be seen. The observers thought at one stage that some solar granulation was visible but, on close inspection, this conclusions was very uncertain. Cloud could be seen crossing the image from time to time, causing varying degrees of obscuration (figure 12).

Fig. 9. Fitting cameras to the Tomline Refractor. (MPC)

Fig. 10. The telescope mated with the sunshade. (JMA)

Fig. 11. The projected image. (JMA)

Fig. 12. Cloud was all too common. Arrow points to Mercury. (JMA)

At 12.30pm, just before first contact, with Martin downstairs in the Belvedere, James adjusted the position of the sunshade and, unfortunately, because of its coupling to the telescope, the position of the latter also changed, slewing away from the Sun. Fortunately, Martin shortly returned to the dome and was able to reposition the telescope and, by 12:33:30 UT, with only two minutes until first contact, observations resumed. Phew – that was close!

Paul Whiting joined James and Martin to observe ingress. The observers called first contact at 12:35:47 UT. At approximately 12:36:49 UT, the black drop effect began. It ended, with the planet appearing fully inside the solar disk, at 12:37:11 UT. Shortly thereafter, the difference in size between the Sun and the silhouette of the planet became very pronounced, the latter appearing only approximately 4 mm in diameter.

Later frame-by-frame analysis of the video from James' camera provided slightly different estimates of event times. Table 1 compares the event times predicted by theory (assuming geometric contact times), recorded in the dome, and estimated by offline analysis of the video. The black drop effect, unsteadiness of the image and the small apparent size of the silhouette of Mercury conspired to make the estimates of event times very uncertain. Predicted times are calculated with the NASA JPL ephemeris DE-430.

	Event times
Event	Dome	Video	Theory
1^st contact	12:35:47	12:35:39	12:35:35
Start of black drop	12:36:49	12:36:45	-
2^nd contact	12:37:11	12:37:07	12:37:16
End of black drop	Uncertain	12:37:16	-

Figure 13 shows still images taken by Martin, using a Samsung S7 mobile phone, which illustrate well the black drop effect and the unsteadiness of the limb.