Posted: Jan 18, 2022 3:50 pm
by newolder
All mirror segments bar A3 & A6 now unlocked and ready for fine tweaking. The only additional info. on A3 & A6 I found by a poster at Reddit so, ...
See 2018 SPIE paper by Wolf et al. "JWST mirror and actuator performance at cryo-vacuum" (find it by searching on "JWST A3 and A6 position sensors").

See sec 1.2 and 2.4.2 of the paper.

It turns out that the mirror segments at A3 and A6 have "faulty"linear variable differential transformers (LVDT) (position sensors). Each LVDT is suppose to have two coils to cancel out differential thermal effects. In each of A3 and A6, one of the two coils is faulty.The engineers had to work out a way of getting accurate readouts using only one coil in each LVDT. So they developed a different readout procedure that could use only the one good coil in each LVDT.

I would guess that they wanted to use a uniform readout procedure (the nominal procedure). to get the 16 segments with good sensors in place, before switching to the modified procedure for A3 and A6. Otherwise, switching back and forth between readout procedures could increase the risk of using the "wrong" procedure. I do not know why it was impossible to replace the faulty LVDTs before launch. It probably had to do with the extensive test time required to re-certify the the whole primary mirror if two new LVDTs were installed.

Jan 19
All segments deployed. Woohoo! Now on to L2 insertion in the coming week and a few months of mirror tweaks and instrument calibrations thereafter. And then... First light!