Back in mid-April, the right-side adaptive secondary mirror of the LBT (nicknamed AdSec-DX) experienced a strong over-cooling due to a faulty valve remaining open on a glycol line. It resulted in most of the hardware of the secondary unit getting much too cold and ending with frost all over... With the system warming up, frost became water and the gap between the shell and its reference body got at some point filled with water too.
The secondary had to be removed from the telescope and stored in the clean room in the mountain lab. To make a long story short, forces were applied to separate the shell from its reference to no avail. The gap is very small (up to a few tens of microns). Water even partially filling the gap would create a very strong adherence of the shell to the body. Thanks to the chemistry involving a silver coating on the reference, an aluminum coating on the shell, water in between and electric currents between the two, the water eventually dried up and left a sticky layer which now makes the shell now firmly stick to the reference.
The last attempts to "easily" unstick the shell were made last Thursday to no avail.
- First, a manual torque was applied to the shell, which is 911mm in diameter but only 1.6mm thick! The shell did not move at all.
- Applying forces to the actuators to push on the shell and apply a tensile stress in an area of the shell where there was a boundary between an open gap and a saturated gap did not work either. The sticky layer was found to be highly elastic and confirmed how stuck the shell is.
The team is now preparing a list of actions and will propose options which will have carefully looked at. There is obviously much pressure to get AdSec-DX back on the telescope, as it is badly needed for the development of LBTI as well as the LINC-NIRVANA Pathfinder experiment.
We will keep you informed on a regular basis on the status of AdSec-DX, so please visit this blog or subscribe to its posts if you want to follow the progress of our recovery efforts.
If you want to know more about the LBT adaptive secondary mirrors, you can find good information in a article following this link: http://lbtwww.arcetri.astro.it/tech/adsecLBT03.pdf
