KAGRA Logbook

K. Nagano, Yokogawa, Ko. Yamamoto, Sugimoto, Akutsu

Summary

Continued/learned the alignment of BRT of TMSY using the green beam
Brought several optical parts and the holders from the PSL room for the DC Gouy phase telescope.
The green beam got sometimes crazy probably due to BS suspension's something (no idea!).

Preparations

A diaphram (or iris) was set after the 2nd BRT lens; let's say iris-1. The iris was used in Mitaka to align the BRT. Because the post holder of the iris was loosened after delivered to Kamioka, I reset the height by using a laser marker (Fig.1). The foot plate of the iris automatically aligns the iris's horizontal center to the optical axis defined (scratch drawn) on the BRT base plate. So this iris can be used as a reference for aligning the BRT. We added another iris (iris-2) in-between the 2nd BRT lens and the 2nd reflection mirror (3inch mirror).

Alignment of the BRT

Fig 2 shows the first look today; just the result of the alignment trial reported here; at that time we only did pitch alignment. It is easy to see how much the green beam is still out of the optical axis thanks to the iris-1. Then today we did yaw and X-shift of the BRT, along with the pitch again and the height (or Z-shift), and the result is shown in Fig 3 (iris-1) and Fig 4 (iris-2). To do so, we refered the irises and also checked the reflection position on the 3inch mirror. In Fig. 3 and 4, even though the green beam is still slightly out of the optical axis, the beam position seems improved to approach the axis.

According to the wedge angle of the current ETMY and the location of the EYT chamber, the estimated amount of the yawing or the shifting to be required would be about 0.xxx mrad and 0.x mm order, respectively. So, considering the typical precision of the chamber locating is around a few mm, we thought it is almost impossible to do the alignment by using absolute mechanical manufature precision. Instead, we use the green beam as a reference (checking by TCam and our intelligence in many aspects), and try to align the BRT to that.

This kind of alignment should be repeated later, after the IR (or green) optical axis for bKAGRA phase 1 is defined/determined, so we stopped the alignment today with this accuracy level. Learning how to align the BRT (today: yaw, X-shift, and Z-shift, the other day: pitch) today would be a milestone.