Some good news...

As planned, attempts were made today to take samples of the contaminant from the gap between the shell and its reference. 

• The first pass was made using shims with the shell resting on the reference. The gap was too tight and it was not possible to get much out of the gap.
• The second pass was then initiated: forces where applied on the whole mirror, as already done quite a few times over this recovery period, in order to open the gap at the periphery of the shell and ease the collection of some samples of the contaminant. 

The good news is that suddenly the shell separated from the reference, at which time the application of forces on the shell was stopped and the shell secured. An unexpected, but much welcome, turn of events!

Next step is to remove (some use the word "drop" but it is a scary one!) the shell and look at the state of the various components. It will give us a first feeling of the extent of the damage. The operation requires a full AdSec-savvy crew on the mountain. It will be for early next week. 

We won't list all the options we had on the table to force the shell to separate from the reference... Some of them were definitely scary ;)  

If you wonder how a frosted AdSec looks like, here you are! 

AdSec-DX frosted - Apr 14, 2013

About the gap contamination

This sticky compound is clearly a sticking point in our efforts to move forward and it must be looked at more closely! Two actions are currently underway:

• Taking samples directly at the source, i.e. in the gap between the reference and the shell. We are prepping for this today, finding the right tools for minimizing the risks involved. Hopefully, we will have samples in hand by the end of the day tomorrow. Contact has been made with the Chemistry Department to have them analyzed.
• Creating in the lab the conditions which arose on AdSec: applying current between a silver-coated plate and an aluminized plate separated by a thin layer of water. The goal of the experiment is to understand how the white contaminant observed in the gap is formed and how strong is the bond it can produce.

 Some of the contamination material can be seen in the picture below.

 

Status at the start of this blog...

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