Peter K, Sudarshan, Gabriele

As stated in the previous alog entry, we finally understood the cabling of the picomotors, and we checked that we can move all the four axis of the two ISS motorized mirrors. 

We managed to get a beam on the QPD, with a total sum signal of about 12000 counts. Since the transimpedance should be 100k, this corresponds to something like 100 uW, which is even more that what we expect from 10 mW in input of the ISS array. So we are quite confident that we have the real beam on the QPD.

However, we still don't see much power on any of the eight photodiodes. The signal is something of the order of 5-6 counts at maximum for some of the diodes. Moving the picomotors while trying to keep the beam on the QPD didn't help much.

Searching for the maximum power on the photodiode is made difficult by the lack of a real DC output: the present board has a -20 dB gain at DC, rises with a first zero at 6 mHz, to about 60 dB at 10 Hz. This means that any transient in the power impinging on the photodiodes will easily mask the improvement we should get if the beam pass over the photodiode too fast.

So we modified the signal acquisition: using a DB9 break box we directly wired the test points after the transimpedance to the ADC, skipping the whitening. This modification has been implemented for PD1,2,3,4.

Moving around the picomotors we could finally get a beam on two photodiodes: PD1 sees something like 0.2 mW and PD3 something like 0.15 mW. This is still much lower than the expected ~1 mW per diode. PD2 and PD4 see some signal, but much smaller. We don't know what's happening to PD5-8, since they're still acquired with the whitening.

In this position, the beam is not hitting the QPD.