OASI: ToM 2016

James Appleton and Martin Cook coordinated arrangements for observing the transit from Orwell Park Observatory. After some debate, they decided 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 the transit of Mercury on 07 May 2003 and the transit of Venus of 08 June 2004 demonstrated the need for an effective sunshade to minimise ingress of ambient sunlight into the dome and thereby improve the contrast of the projected image. The sunshade used in 2004 comprised essentially a lengthy home-made blind attached to the dome via nylon cords passing through eyes screwed into the wooden framework around the aperture. The end of the telescope holding the object glass (OG) protruded through the sunshade. By adjusting the length of various cords in a coordinated manner, the sunshade could be raised and lowered to match the altitude to which the telescope was directed. The sunshade was effective to an extent but did not fit well the curvature of the inside of the dome and thus allowed the ingress of considerable ambient light. James and Martin therefore planned a new, improved sunshade.

Planning started several weeks before the transit. Initial thoughts were simply to fabricate a sunshade similar to that used in 2004 but with more cords enabling it to be held closer to the curvature of the dome. Eventually, however, a more elaborate scheme evolved, based on a pair of runners fitted to the interior side of the aperture of the dome, with the sunshade held in position between the interior of the dome and the runners. Matthew Leeks constructed the runners from 15 mm copper pipe and brought them to the Observatory on Wednesday 27 April where, together with Alan Smith, he shaped them to the correct curvature. The following Wednesday, James, Martin, Matthew and Alan erected scaffolding in the dome and secured the runners in position, using brackets fabricated by Martin (figures 1 and 2). Two days later, James constructed a sunshade from 10 m of material secured to wooden battens; despite leaving work early to begin the task, practical difficulties meant that work continued late into the night! The following day, James, Martin, Matthew and Alan attended the Observatory and fitted the sunshade; as the Sun was shining in a clear sky, they were able to test its effectiveness. Unfortunately, it was immediately apparent that the material was just too thin and the degree of obscuration was far short of what was necessary. With the aperture of the dome turned in the opposite direction to the Sun, the blue of the sky was clearly visible (figure 3); conversely, with the aperture turned towards the Sun, the material allowed enough sunlight to enter the dome to cast a distinct shadow of the telescope on the projection screen, and the solar image had poor contrast (figure 4). The personnel withdrew to consider the best course of action.

The following day, James and Martin returned to the dome determined to improve matters. Enhancements included doubling the thickness of material used for most of the length of the sunshade (figure 5), obscuring the arched windows at the base of the dome, and fitting a 1 m square shadow screen at the OG-end of the telescope. After many hours of work, the task was complete and the dome was ready! The new sunshade worked well: figure 6, taken on the day of the transit, looking along the length of the telescope, shows the considerable attenuation of direct sunlight provided by the sunshade, and shows too the shadow screen. Collectively, the light-exclusion measures considerably reduced the ambient illumination in the dome. The Tomline Refractor produced an image of the Sun 50 cm in diameter, with good contrast, and no hint of a shadow of the telescope on the projection screen.

Fig. 1. Erecting scaffolding in the dome. (JMA)

Fig. 2. Fitting the runners for the sunshade. (ML)

Fig. 3. Material of the sunshade provides limited obscuration. (JMA)

Fig. 4. Solar image with very poor contrast. (JMA)

Fig. 5. Enhancing the sunshade. (JMA)

Fig. 6. The sunshade and shadow screen. (JMA)

On the day of the transit, James, Martin and David Murton were first to arrive at the Observatory, at approximately 09.30am. After helping to transport David's imaging equipment to the south-west balcony, where he began setting up, James and Martin went to the dome to prepare the Tomline Refractor. They selected a large, ex-military surplus eyepiece (the used for observation of the transit of Venus in 2004). The new sunshade obscured the aperture of the dome so well that initially it was difficult to align the Tomline Refractor on the Sun. Eventually the pair succeeded, whereupon two sunspot groups (Active Regions 2542 and 2543) together with a few isolated sunspots were immediately visible. There was no evidence of granulation or plages. Martin fitted his Canon 1100D and Samsung WB750 cameras to the mounting bracket at the eyepiece end of the Tomline Refractor (figure 7) and centred them on the region where 1^st contact would occur. He secured a large radio-synchronised "Rugby" clock (properly an MSF clock) to the corner of the projection screen for timing events. Once the preparations were complete, he took some photos of AR2542 (figure 8) to test the cameras.

Fig. 7. Cameras fitted to the Tomline Refractor. (JMA)

Fig. 8. AR 2542, 09:00:48 UT. (MPC)

Bill Barton and Paul Whiting arrived at the observatory at 10.00am, and John Wainwright shortly thereafter. Preparations complete, the observers settled down to wait for ingress to begin. The weather was promising: the sky was clear! To provide some light entertainment, Martin employed an industrial thermocouple to measure the temperature of the Sun's rays concentrated by the eyepiece of the Tomline Refractor. After a few seconds at the focus, the thermocouple glowed bright red and the temperature reading went off the scale at 750°C! (Figure 9.) Shortly after 11.00am, more observers arrived: Adrian Cubitt, Tina Hammond, Richard Stansfield and Mike Whybray. Figure 10 shows some of the observers studying the projection, waiting for the transit to begin.

Fig. 9. Demonstrating the heat of the Sun's rays. (DM)

Fig. 10. Studying the sunspots. (DM)

Approximately 15 minutes before the transit started, some thin cloud passed in front of the Sun, causing a loss of definition of the projected image. Fortunately, the cloud passed after approximately five minutes, and definition returned. The observers gathered around the projection screen and Martin set the cameras so that the 1100D took a still image every approximately 4 s and the WB750 recorded a video (which terminated shortly after 2^nd contact). A handheld "Rugby" clock was placed briefly in the field of view of the WB750 to provide a timing reference for later, off-line analysis of the video. The observers then concentrated on the image, waiting for 1^st contact.

The observers recorded the following event times (UT). Paul was first to detect the barely-perceptible indentation of the silhouette of Mercury on the Sun's east limb, at 11:12:45. Once he indicated the phenomenon, other observers were immediately able to see it too, thus all observers were reasonably confident that they had observed the silhouette of Mercury within a few seconds of the instant of 1^st contact. However, there was no such certainty in the estimation of 2^nd contact due to a prolonged teardrop effect. At 11:13:32, Paul detected the teardrop, but this was not confirmed by other observers. At 11:14:20, Paul called out an observation of 2^nd contact, but again this was not confirmed by other observers, and was later thought to be an error. Paul called out the instant of 2^nd contact again at 11:14:36 and for a third time at 11:14:39 and several observers in the dome agreed that 2^nd contact occurred somewhere around these times. However, other observers were of the opinion that there was still a pronounced teardrop effect at this time, and the silhouette of Mercury had not detached from the solar limb. Nonetheless, all present agreed that 2^nd contact had certainly occurred before 11:15:16.

Later, James and Martin independently analysed the video recording and produced off-line estimates of event times. The following table compares the various empirical estimates together with predicted contact times calculated using the NASA JPL ephemeris DE-431.

Estimate	1^st contact	2^nd contact
Observers in dome	11:12:45	Maybe 11:14:36 - 11:14:39. Certainly by 11:15:16.
James Appleton (video)	11:12:38	11:15:35
Martin Cook (video)	11:12:38	11:15:33
Predicted	11:12:23	11:15:35

Figure 11 is a montage of the images from the 1100D during the period 11:12:37 - 11:16:15 UT, from approximately the instant of 1^st contact until after 2^nd contact. It shows the very pronounced teardrop effect extending from approximately 11:14:09 - 11:15:27 UT. (Note that the timer triggering the camera was not completely accurate, and a few images deviate from the intended 4 second periodicity.) Observers with handheld cameras also recorded the teardrop effect: see figure 12, compiled from images by James Appleton taken with a Canon PowerShot SX220, 11:13:24 - 11:15:48 UT (note that the images are not evenly spaced in time).