Matt H, Rick S, Peter K

This will be a quick summary alog which hopefully Rick or Peter may expand upon (correct my mistakes)

The ISS array work is mostly done now (I know I said that yesterday as well). Yesterday we had the array aligned so that there was readouts on all 8 photodiodes (couldn't see a QPD signal though). Unfortunately because the steering mirrors are either side of the lenses, this meant that the beam was not going through the center of the lenses anymore...and in fact was probably half way between the center and the edge of the lens. After some discussion we decided to see today if we could get it better. So we put the beam back going through the center of the lenses (by adjusting back the beamsplitter before the lenses), and then by a combination of adjusting the last mirror before the ISS array to center the beam on an aperture we put in the input aperture of the ISS array (confusing I know) , and twisting the ISS array itself to get the beam onto the photodiodes, we were able to get as good DC readouts on the photodiodes as yesterday (around 50mV each photodiode for 200mW PSL power),

How do we get DC readouts for the photodiodes you may ask. Well (hopefully I get this explanation right)  we took the lid off one of the in-air amplifiers and using clip leads to pick off the signal before the whitening etc..which is also done on the board (thus enabling us to have a higher gain) we were able to see a signal. We simply altered the ISS array until we got a maximum signal.

One thing of note is that either the top bank or the bottom bank set of photodiodes reads slightly different to the other bank (I cant remember which way around it is). One bank has max DC levels of 45-50mV the other bank of diodes 50-55mV. Should simply be a case of ever so slight misalignment that should correct with picomotors once all hooked up.

Rick also had an idea on seeing something on the QPD's. He proposed the following to Rich A:

I wonder if we can increase the QPD gain from 1e3 to 1e5.

Here is my logic:

We get about 250 mW on the ISS array for 200 W into the mode cleaner, let’s call it 200 mW for 200 W for simplicity.

Yesterday we were running with 200 mW (rather than 200 W) and for the near future the most power we plan to use is 20 W.

200 mW into the MC -> 200 microW onto the PD array

200 microW /2 (for the upper and lower bank) /100 for the 1% transmission of the upper HR mirror -> 1 microW onto the QPD

250 nW onto each quadrant of the QPD, with 1 k ohm transimpedance -> 250 microW, which explains why we didn’t see anything on the QPD signals.

With 100 k ohm transimpedance we would have had 25 mW per quadrant yesterday.  This would give 2.5 V/quadrant with 20 W.

The decision was made to change the resistors (Rick/Rich will have to say from what to what and on what board), but after this was done we actually saw counts on each segment. Something that I don't understand was making there seem to be some gain somewhere adjusting the levels from a couple hundred counts to 1x10^9 so we didn't see anything on the QPD screen until we put in a gain of 1x10-9 and then we see a signal mostly centered on the QPD (its jumping around a but due to the purge air buffeting the optics slightly). So some troubleshooting still needs to be done, but we see something.

Lid was placed on, but when I was putting the baffles back in place, I ever so slightly brushed the cables, and four of the connectors fell off. Luckily we could get them back on correctly and we checked that they photodiodes were all still reading out, but I think we really need some kind of strain relief on these cables, as there is nothing to stop them potentially popping off at anytime...maybe even under vacuum.

Also it appears that the photodiodes sense the halogen lights when we have them turned on or off. This is actually a blessing as I strongly suggest that after done all the cleaning, etc in chamber, before put on the doors, you put the halogen lights on and see if still get a readout from all 8 photodiodes.

(Sorry no pics)