Moonlight

Las Campanas, Chile (Apr 26, 2002)

A moon rising photo taken from the terrace of the Magellan telescope at the Las Campanas observatory in Chile, soon after sunset. This photo is in fact the combination of two photos with different exposure times. I took one photo with a longer exposure to capture the details of the mountains, and a second one with a much shorter exposure to capture some details of the moon. Then I combined the two photos by cloning the overexposed moon in the first photo with the good one in the second photo. A similar technique is used all the time in professional astronomical imaging to obtain very high dynamic range images combining bright and faint astronomical sources in the same frame (in some cases I have combined thousands of individual images resulting in one image showing at the same time far galaxies and a star one million times brighter in the same field of view). One peculiarity about the photo I posted today is that I took it with a point-and-shoot digital camera, with little or no manual control. Since I couldn’t set manually the exposure time, I used the trick to exclude the flash in the first shot, and set it in the second: this forced the automatic software of the camera to have a long and a short exposure in the two cases.

Las Campanas is one of the possible sites where one of the next generation giant telescopes will be built. The Giant Magellan Telescope (GMT) will have a primary mirror composed by seven individual mirrors each 8.4 meters in diameter, acting like a gigantic telescope with a 25 meter diameter primary optics. The telescope is still in the design phase, and still not completely funded, but the manufacturing of the individual mirrors has already begun, as there is only one place in the US (at the University of Arizona) that can actually build such large mirrors, and each of the 8.4 meters mirrors requires more than one year to be built.