I removed WFS2 from the AP table (after placing markers so I can put it back in ~the same place) so that I could take some reflectivity as a function of angle measurements for aLIGO WFS design stuff.
I was dismayed to discover, upon glancing at the diode itself, that half of the diode is covered with some kind of oil!!!. The oil is mostly confined to quadrants 3 and 4, which explains the confusion with their quantum efficiency measurements, as well as why the readback values on the MEDM WFS Head screen for WFS2 don't really make sense.
The WFS QPD has a piece of glass protecting the diode itself, and the oil seems to be on top of the glass, so I'm going to use some lens tissue and clean it off.
Pre-cleaning photos are on Picasa.
Update: I tried scrubbing the glass with a Q-tip soaked with Iso, and then one soaked in methanol. Both of these failed to make any improvement. I am suspicious that perhaps whatever it is, is underneath the glass, but I don't know. Rana suggested replacing the diode, if we have spares / when we order some spares.
Problem with 2 quadrants of WFS2?
While doing all of this, I noticed that quadrants 3 and 4 of WFS2 seem to be different than all the rest. You can see this on the MEDM screens in that all 6 other quadrants, when there is no light, read about -0.2, whereas the 2 funny quadrants read positive values. This might be okay, because they both respond to light, in some kind of proportion to the amount of light on them. I ended up getting QE of ~72% for both of these quadrants, which doesn't make a whole lot of sense since the spec for green is 70%, and silicon is supposed to be less good for infrared than green. Anyhow, we'll have to meditate on this. We should also see if we have a trend, to check how long they have been funny.