For peak performance of a Fabry-Perot device the error in plate parallelism must be much less than deviations from flatness in the plate surface. The coated plates in TTF are individually flat to and normally parallelism must be established and maintained to at least during use. To achieve this, the plates of TTF are controlled through an active feedback loop that constantly corrects the plates when small changes from plate position occur ([Hicks & Atherton 1997]; q.v. [Ramsay 1962]). Such closed loop control is essential for a device such as TTF, where plate stability could otherwise be influenced by variations in temperature, humidity and gravity on the plates as the telescope moves.
Hicks, Reay & Atherton (1984) pioneered the technique of Fabry-Perot stabilization using a capacitance bridge. Their Fig. 1 shows the components employed in the active feedback system used by TTF. Four capacitors around the edge of the inner plate surfaces (labelled in their Fig. 1 as CX1, CX2, CY1 and CY2) detect changes in plate spacing. Such changes permit measurement of the plate tilt along the direction of the x-y axes defined by the two capacitor pairs. This capacitance micrometry is capable of detecting displacements of 10-12 m ([Jones & Richards 1973]). Tilt information from the capacitors is fed to piezo-electric transducers (PZTs) which compensate for the amount of deviation. There are three PZTs, each located around the plate edge between the capacitors and separated by . An additional reference capacitor measures the gap spacing with respect to a fixed capacitor built onto one of the plates.
When the plates are parallel, capacitance will not be equal between either the CX1, CX2 or CY1, CY2 pairs. This is why the feedback system can only maintain parallelism and not establish it in the first place. Electronic offsets are applied to compensate for variations in capacitance whenever they occur. These can arise from temperature gradients across the Fabry-Perot or continual changes in the piezo dimensions due to creep in the PZT lattice structure. All such capactitance changes are continually balanced and nulled automatically by the system electronics.
We are able to introduce fixed vertical offsets ZX, ZY,
along the x and ydirections through three levels of control: coarse, fine and
software. Both coarse and fine are analogue inputs directly
through the hardware of the TTF controller. Software control allows
precision adjustment of the plates via digital input. The maximum
ZX and ZY amplitudes are 3.21 m. Clearly, it is crucial
to ensure the plates are parallel before attempting to achieve gaps
smaller than about 7m. The vertical deviation along the
x-axis from the zero-point is given by
No gap scanning is done through the ZX or ZY movements.
They are purely offsets that remain fixed unless adjusted for
parallelism. Scanning is controlled through a third parameter
Z which has the much
larger amplitude of 13.05 m. It too can be adjusted through
three levels of coarse, fine and software control,
The amount of offset is derived directly from the parallelism test and refined through successive iterations. We describe the test in the following section.