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A possible explanation for the slew drift.

As illustrated in Fig. 3 (top), the TTF plates lie parallel with the ground when the telescope is pointing at zenith. At first glance, we would expect the sag problem to be the same whether we tilt the system one way or the other. This is not what we observe in Fig. 1 (Sequence C).

A possible cause is the way the plate spacing is monitored and controlled by the piezos and capacitors around the outer perimeter of the TTF aperture (see Fig. 3). The Z-ref capacitor, and the Z-spacing capacitor, are located at one specific place on the perimeter. It may be that when the system is slewed in the direction of Z-ref, the capacitor faces are pushed closer together than when the telescope is slewed in the opposite direction. This could explain why the magnitude of the slew drift is roughly symmetric about zenith. In other words, we are seeing flexure in the extended piezo stacks. I originally pushed QI (late-80's) into developing these for TTF-style work and there was a concern at that time on possible flexure problems. (Action: could EJP let me know orientation of TTF capacitors wrt telescope horseshoe? If this is a problem, I can do it next time I am up.)

The necessary displacements are tiny. Since the FSR in Z-units is 350 at the Ne line wavelength, which corresponds to a physical movement of $\lambda/2$, a shift of 1 in Z amounts to $\lambda/700$ displacement modulo some geometrical factor. Since this amounts to no more than a total physical movement of $\lambda/70$, the line FWHM is not expected to show signs of degradation (instrument finesse = 35). We did not observe any systematic line profile broadening.


 
Figure 3: Front and side elevation of the TTF module.
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next up previous
Next: Thermal time constant for Up: Stability tests of TAURUS/TTF Previous: Recommendations
Joss Bland-Hawthorn
2000-07-24