A lunar transit of the sun observed by SDO in the 171 Angstrom wavelength. Credit: NASA/SDO
As expected SDO experienced another lunar transit on November 6, 2010. This time the moon crossed the southern hemisphere moving from the south pole towards the northeast (left side) edge of the sun. The transit took about 30 minutes and during the transit, sunspot group AR11121 was covered up.
Observations of lunar transits are very useful for SDO. They are used to better understand SDO’s pointing. Also, transits allow the SDO scientists to fine tune the telescopes. One problem with telescopes is stray light. If you look at the lunar disk it should be completely black. Close inspection reveals that is it not completely black. This is because a small amount of light not directly from the sun has made its way into the inside of the telescope. By studying how much the lunar disk deviates from being black, SDO scientists can measure and ultimately account for how much unwanted scattered light is making its way into the telescope.
