by Rod Pommier
Telescope: Celestron Super C8 Plus with Fork Mount. and Byers drive, f/6.3 focal reducer-corrector. Camera: Canon EOS 20D DSLR. Exposures: ISO 800, 1/1000 second, all exposures. Other equipment: Thousand Oaks Optical glass, full aperture, type 2 solar filter.
Location: Haleakala, Maui, Hawaii.
Time: 12:28:50 HST.
The black drop effect associated with 2nd contact of the transit of Venus of June 5, 2012. This is an enlargement taken from a full disk image.
The black drop effect is notorious for having thwarted attempts to derive accurate timings of the transits of 1761 & 1769 in order to measure the distance to Venus and the Sun, and thus determine the scale of the solar system. It was clearly drawn by Captain James Cook, in Tahiti as well as by his assistant, Mr. Green. It was again recorded by multiple observers, including Ellery, Moerlin, Wilson, and during the transits of 1874 and 1884. It was hoped that photographs taken by the United States expeditions would solve the problem, but the black drop effect was very prominent on photographs. It is now known that it is due to a combination of 3 factors, the Sun's limb darkening effect, diffraction of light in the gap between the edge of the Sun and Venus, and atmospheric seeing effects. It can last seconds or minutes.
The black drop effect showed prominently in my images. It lasted for over a minute at Haleakala.