This is a log that I was going to post on the 18th...
Ushiba, Miyamoto, Izumi
We managed to find the interferometer beam at EYC by steering the BS.
This time, we intentionally displaced the entire baffle PD structure so that one of the baffle PD was centered with respect to the ETMY mirror.
Transimpedance of the baffle PDs were increased
Carefully examining the PD gain and so forth, we noticed that the gain needed to be higher than what it was. This was mainly due to the fact that FGA21 has a smaller active are than PDA100A by a factor of 25 in terms of the surface area. Also, the transimpedance gain was low by a factor of 20 or so. So we decided to increase the tranimpedance gain to 7 Mohms from 20 kOhms. Note that we were using the low gain feedback resister at the beggining. In addition, the parallel capacitor to the feedback resister was changed to 270 pF from 0.1 uF to maintain a high bandwidth of 100 Hz.
One of the baffle PD is centerd to the test mass
In the morning yesterday, Akutsu-san reminded me of the existence of a view port in the beam path with an aperture size of 100mm in diameter nearby the EYA chamber. Calculating some numbers, we immediately learned that this aperture was so small that the beam would never be able to hit the baffle PDs unless the gate valve (which holds the viewport) is opened. The baffle PDs are placed at points along the circle with a diameter of 240 mm which is way too larger than the viewport size. The view port seems to be afar from the test mass by roughly 40 m.
Ushiba-kun and Miyamoto-kun subsequently displaced the baffle PD structure such that the upper right PD (as seen from the BS side) is centered to the test mass.
Even though this is going to occult most of the beam incident on ETMY, we became able to detect the light incident on ETMY.
The mirror seems dusty
See the screenshot below. This is a picture taken by the Tcam in its commissioning mode. The bright bulk part is the reflection from the baffle structure, illuminated by the interferometer beam. Right below it, a number of spots are seen which we think are due to some dusts on the HR surface.
