LROC Heaters and Focus

What does the LROC team need to do before imaging can commence? First, we need to dry out the NAC telescope structures so the cameras will be in focus.

The LROC Narrow Angle Camera carbon-fiber structure absorbs water under ambient conditions (Earth's atmosphere) resulting in expansion, which puts the camera out of focus. Once in the vacuum of space the camera outgasses, the structure shrinks, and presto, the camera is in focus. Sounds a little scary - build a camera that is out of focus then hope it is in focus once it is in space. Well, not quite. The LROC team did test the "in space" focus of each NAC by putting the cameras in a vacuum chamber and heating the structure to 65 degrees C (149 F) for 5 days. In the vacuum chamber there was a special collimator and target that allowed the MSSS engineering team to confirm that the design was indeed correct. And yes both NACs were in focus after the vacuum bakeout. However, as the cameras left thermal vacuum they soon began absorbing water again. Shortly after launch the LRO Mission Operations Team commanded the LROC structure heaters to turn on to expedite bakeout. Except for a few hours during the LOI burn the heaters have been keeping the telescopes at about 50 degrees C (122 F). It should take the heaters about two weeks to bring the cameras into focus. Focus test images acquired in orbit will tell us when the cameras are truly in focus.

Published by Mark Robinson on 23 June 2009