Georgia, Dan Brown

The temporary SRM heater table that has been built uses a periscope to take the beam up to the central viewport on the the north HAM5 door. This then needs to be aligned to two steering mirrors inside the chamber and onto the back of the SRM. The CO2 laser on the table has been co-aligned to a red laser which we will use to do the coarse initial alignment. Given the table is quite large and the 2-inch pico mount at the top of the periscope obscures the view through the middle viewport we wanted to try a quick alignment test to see how hard it was going to be. As HAM5 is opening up next week we were also wondering if it would be worth trying to pre-align the table somehow with the doors off using beam cards and mark on the floor where the table needs to be. In summary, we don't need to align with the doors off, doing it with them on seems easy enough.

To test this we used a laser pointer strapped to the top of a tripod on a box.

The view through the small ZnSe viewport shows the first steering mirror we are aiming for. It's very reflective and hard to see through without getting right up close and blocking out ambient light. It also reflects and scatters the alignment laser light a lot. This makes it hard to shine it through and get close enough to see where it ends up without getting the laser reflected into your eye.

Luckily the viewport to the left gives us a good view of the SRM baffles. The hole on the left is the entry point of the CO2 laser, it then reflects off of SRM and out through the hole on the right into a beam dump. Blue is where the IFO beam comes through.

At the right angle and zooming in we get a nice reflection of the two steering mirrors from the baffle. Using this we managed to get the beam roughly centered on the first and onto the second steering mirror.

Finer adjustment on to the second and we were able to hit the input baffle hole. This should be easier with the actual setup as the tripod wasn't easy to adjust and the red laser on the table is ~3mW rather than ~1mW from the pointer.

The SRM heater currently has one 2-inch pico controlled mirror mount at the top and the whole periscope slides left to right a few cm for coarse adjustments. Though we're now thinking we probably need another pico mount at the bottom of the periscope to give us some finer control after this exercise. One aspect we're still not sure on is how to check the beam is aligned to the beam dump correctly, for this we might need another viewport installed on the HAM5 (possibly north door top right) which could give us a view of the red light hitting the beam dump, but we need to check that view on a solidworks model or next week with the doors off with a viewport emulator.

Aidan also brought up that the refractive index difference in the ZnSe viewport is 2.4 for 10um and 2.6 for 635nm, so the final spot position on the SRM is going to be different. Quick calculations show we'd be ~1cm different at the SRM per 10 deg of angle of incidence on the viewport.