B. Bland, J. Kissel
The results from the overnight TFs that Betsy started last night are attached. The lame news: the SUS is not yet ready for chamber close up. However, over the phone, Betsy informs me that more items on/around the suspended elements have changed and some other tweaking occurred today since the measurement and more will change tomorrow. Though this set of TFs is informative academically, we have to wait until we're "done" with mechanical stuff before a "final" close out measurement (which will hopefully show a clean bill of health).
From what we can [remember / piece back together / guess], the state of the chamber is:
- HEPI Floating, but OFF
- BSC-ISI Floating, but OFF
- QUAD
- Free, OFF
- One-off, wire loop, prisms, and glass optic main chain; Production ThinCP fully laced
- EQ stops brought into 0.75 mm gap
- One set of ECD magnets is removed (Betsy doesn't recall exactly which, but we think lower cluster below M0 F3)
Here's what I see, and what I think is going on:
Main Chain
Although the diagonal TFs (LtoL, TtoT, etc.) look OK, its obvious from the off-diagonal terms that there's a serious amount of cross coupling (see 2012-05-07_H2SUSITMY_M0_ALL_TFs.pdf). My guess is that this is because of the missing weight from the absent ECD magnet cluster. As mentioned in G1100865, pg 4, one of the possible VtoP / PtoV cross coupling mechanism is a static offset in pitch. The fact that PtoP looks "clean" (though a little bit low in overall magnitude), and both VtoV and RtoR show the 2nd pitch mode, my guess is that the missing ECD magnet cluster brought whichever corner of the mass up, creating both a static roll and pitch in the mass, causing the exact cross coupling described in G1100865.
We should get that ECD magnet back on there, do any other final tooling additions and removals, and measure again.
I use the "wire" model for this one-off, wire loop, prisms, and glass optic main chain, and it seems to predict it pretty well. In allquads_120507_H2SUSITMY_ALL_TFs.pdf I compare it against other metal mass wire SUS, and other than the cross-coupling mentioned about, and slightly less magnitude, it matches up pretty well.
Reaction Chain
Here, the VtoV transfer function shows something new, fun, and bad. Notice there's only two resonances? My gut reaction, is that the lower two modes have shifted up in frequency due to increased stiffness, and the lower two masses are stiffened to the point on moving as one; cabling's the only thing I can think of off-hand. Either that or something's interfering with the lower blades. Remember, the higher frequency the resonance, the more it involves lower stages. Also, because other modes, and other degrees of freedom show little to no reduction in Q, implies that whatever's stiffening up the vertical DOF, likely isn't at the top stage. Looks like the high-frequency Roll / Trans modes have changed shape too, likely due to the same vertical stiffness issues.
I'd check around the blades, and re-assess cabling on this one.
