Aso, Sato, Akutsu

Investigated how to deal with the light beam.

• First, we peeled off the aluminum foils that covered the flanges to make the beam path (Fig 1 and 2). Using two KOACH filters, one of which was locate in-between IFI-IMM chambers, and the other was located in-between IMM-PRM cahmbers. Then the laser beam from the IMC came into the chambers.
• Then, Aso-san somehow aligned PRM so that the light beam reflected from PRM could come back to the IFI and go to REFL. Note that the aligment IMMT1 and 2 would not be perfect (we have no idea; to be known when the main interferometer will be fully aligned in the future), but this time we just tweaked PRM to imitate the reflection beam. The QPD of the oplev for PRM was just set according to the current PRM default tilt (see 15540), so Aso-san iterated (1) to move the reflected beam spot on the IFI to overlap it onto the in-coming beam by tweaking PRM and (2) the oplev beam is about to out of range of the QPD so adjust the QPD by micrometers.
• We checked several ghost beams including transmission beam from IMMT1. I tried to check the transmission beam from IMMT2, but I leaned that the transmissivity of IMMT1 and 2 are different about 10 times, and the one for IMMT2 was weak accordingly.
• With the reflected beam from PRM, we could see two tansmission beam for IMMT1 (going to and back from PRM).
• Fig 3 would be the ghost beam from MCo as expected. (But seems to have strange shape...)

To see ghost beams and so on, we asked to increase the laser power input (1.3-1.7W now) so that we could have clearer views for the ghost beams, but there was a trouble to increase the power, and Nakano-kun and Tanaka-kun will take care of it later. Anyway, we could check some ghost beams that had been already known.