Hirata-san and I investigated the best BS/SR prism design:
Fabian calculated that the sapphire prism for the BS should be 6.28 mm tall to match the existing clamps on the intermediate mass. We have a drawing for a 6.28 mm prism ready to go.
The SR prism was originally going to be 1.3 mm tall as for the PR. Because of the failures of the PR prisms, it was decided to make the SR prism substantially larger so it would be more robust. A provisional height of 5 mm was chosen based on the LIGO HSTS prism which was made an used successfully. This turned out to require some modifications to the recoil mass, but is workable.
The prisms for the BS and SR will have the same groove separation and groove shape. It would be good to make them exactly the same to create a larger production run, (probably) reduce cost, and simplify sparing.
The SR cannot easily be made 6.28 mm tall without needing to widen the groove in the recoil mass an inconvenient amount. Therefore we have explored the possibility of making the BS prism 5 mm as well.
If the BS prism is reduced in height by 1.28 mm and nothing else is done, the wires will be slightly off-centre in the slits in the recoil mass, but this leaves plenty of room - see Picture1.
If nothing else is done, the wires holding the BS will be slanted inwards at the bottom by 0.13°. This is probably insignificant, but it is trivial to correct by milling 1.28 mm off the wire clamp base blocks at the top.
If the clamp base blocks are reduced and nothing else is done, the wires holding the RM will then be slanted outwards at the bottom by 0.13°. To correct this would require entirely remaking several more parts and is not worth it.
Therefore we recommend making both the BS and SR prisms 5 mm, and reducing the BS clamp base blocks by 1.28 mm.
