J. Garcia, J. Kissel

We've measured the M1 to M1 (TOP to TOP) pitch transfer function after the rehang, electronics turn on, OSEM installation, to complete the story we had last heard about two months ago. 

The executive summary (for those who have been following the story) is that the first pitch mode is now at 0.47 Hz, slightly higher than before the wire length change, so we are happy (given that we're trying to hit 0.48 Hz). However, before we give a complete blessing we must measure the remaining degrees of freedom of the top stages. The data is in the can, we're now processing the data -- hoping to have an answer by the end of the week (although initial glimpses at the results are very encouraging).

A recap of the story thus far: a requirement/goal for the BSFM is to have its first pitch mode "reasonably far away" from the first longitudinal mode, such that control for these two DOFs can be reasonably decoupled. Initial modeling of the suspension predicted the first longitudinal mode at 0.41 Hz, and the first pitch mode at 0.48 Hz. However when we had hung the BSFM for the first time (many moons ago), the pitch mode was *below* the first longitudinal mode. Several as-built parameters were then changed, to get the pitch up to 0.44 Hz. Finally, two months ago we had played around with the suspension physical parameters further, and ended up with the mode at 0.46 Hz, which was deemed good enough. The answer we had come up with to get 0.46 Hz was to lower the M2 blade tip heights by 3 mm (or a d1 of 2 mm below the COM). However, to recover the overall height decrease of the masses below (specifically M3, or the optic) from this blade tip height decrease, we needed new wire lengths. 

Cut to this week where we've installed the new wire lengths, suspended and turned on the suspension, and measured the same TOP to TOP transfer function. Here, we hope for at least the same first pitch frequency (0.46 Hz), but -- as the attachment shows -- we've surpassed it, to be that much closer (0.47 Hz) to the original model (0.48 Hz). W00T!

What to look at (in both pages): 
BLUE - Nominal model of the BSFM in pitch. This is what we would like to see out of the suspension, and served as a reference point for our physical parameter exploration.
GREEN - Modified model to try and explain the 110621 measurement. It's unclear whether having d4=0 was actually physically happening (since it would result in an unstable suspension), but it explained the measurement.
RED - 110621 measurement, that had decreased blade spring tip heights (at 23.6 mm), but wire lengths that were therefore too long, since the original design (and therefore this suspensions') lengths were for a blade tip height of 26.6 mm.
CYAN - 110823 measurement, with 23.6 mm blade tip heights and new wire lengths.

Stay tuned for analysis of remaining degrees of freedom!