NASA’s NASA’s Solar Dynamics Observatory (SDO) is hoping to use Venus’s transit of the sun on June 5 to better calibrate its instruments.
At 6:03 PM EDT, Venus will cross in front of the sun in one of the rarest of planetary alignments. Two such Venus transits always occur within eight years of each other, followed by a break of either 105 or 121 years.
Since the points at which Venus will first touch and later leave the sun are precisely known, SDO can use this information to make sure its images are oriented to true solar North.
Calibrations throughout SDO’s two-year career have left scientists confident that the instruments are highly accurate – but new observations could help make sure that its orientation is accurate to within a tenth of a pixel.
The SDO team can also use the lightless center of Venus to help calibrate the point spread function of the telescope – a measure of how much light leaks from one pixel into others around it.
Since no light is 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 will have a better idea of how to correct for that.
The measurements should also help scientists understand the black drop effect, in which a tiny black spot appears to connect Venus to the sun – an effect that confounded attempts to measure the exact position of Venus during transits in the 18th and 19th centuries.
The team also hopes to learn more about Venus’s atmosphere.