NASA's Solar Dynamics Observatory (SDO) had a ringside seat to yesterday's transit of Venus across the violent surface of the Sun --something it has done only seven times since the invention of the telescope. This transit is among the rarest of planetary alignments and it has an odd cycle. Two such Venus transits always occur within eight years of each other and then there is a break of either 105 or 121 years before it happens again.
The SDO team used the lightless center of Venus to help calibrate what is called the point spread function of the telescope. This function describes how much light leaks from one pixel into others around it. Since there is no light emitted from the very center of Venus as it crosses the sun, it serves as a perfect test case for an area of the image where the pixels should remain black. By measuring how much light bleeds into those pixels from the rest of the sun, the SDO team had a better sense of how to correct for that.
These measurements also help to understand the black drop effect – in which a tiny black spot appears to connect Venus to the limb of the sun -- that bedeviled scientists' attempts to measure the exact position of Venus during transits in the 18th and 19th centuries.
The Daily Galaxy via NASA SDO