In order to confirm the molecular layer formation at cryogenic temperature, I started another measurement with modified spacer.
The spacer has a slit to let residual gas molecules impinge on the folding mirror.
The incident angle is about 22.5 deg.
I installed it and pumped down.
Then I did some measurements to characterize the mirror property.
I turned on the cryocooler at 16:08.
The finesse of the cavity was about 22000, which is better than before.
It seems that the beam hit the defect before.
At the defect, several tens ppm optical loss exists in folding mirror.
The frequency shift between p- and s-pol was about 20 MHz which is almost consistent with the theoretical calculation.
This measurement enables characterization of coatings.
I will monitor the finesse and splitting frequency for one or more weeks.