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BCAM Flexure

Mounting Brackets for lower BCAMs

Here is a picture of one of the invar mounting brackets which attach the lower BCAMs to the primary mirror.

These brackets are designed to minimize flexure subject to a constraint on total mass (so as not to contribute significant force on the mirror).

Discrepancies

Flexure of the BCAM mounts should show up as a systematic difference between measurements taken from different BCAM pairs. Since there are 4 lower BCAMs (attached to the primary mirror) and 4 upper BCAMs (attached to the FCM), we have, in principle, 4 * 4 = 16 different pairs of BCAMs we can consider. However, which upper BCAM we pick makes absolutely no difference to the results, indicating that if something is flexing, it is likely the lower BCAM mounting brackets.

Shown here are difference in hexapod corrections between the NW and SW BCAMs:

Similarly, the difference between the SE and SW BCAMs:

These are all well-fit by a one-dimension function of altitude.