The X-Y capacitive bridges define the direction of the orthogonal x and y axes. We assume that all components are located centrally to the beam although this alignment is not critical. Only three of the four possible quadrants are needed to establish magnitude and direction of plate tilt. In any such arrangement of quadrants, we define the corner quadrant to be the reference quadrant and the remaining pair as the X and Y quadrants according to their direction from the reference.
We image an emission line by scanning through each quadrant in turn, producing a charge-shuffled long-slit spectrum for each. The amount of tilt is measured by the size of the offset between the line through the X or Y and the reference quadrant. The plates are parallel when no offsets exist in either direction and the emission line appears simultaneously in all three.
Figure 3 shows how adjustment of the capacitive offsets optimises plate separation. Here, identical central strips have been taken from a full charge-shuffle image (such as Fig. 2). The X, Y and reference quadrants are labelled. Each shuffle step represents an increase in plate spacing of 8 Å or . Panel (a) shows a small offset in Y only (arrowed). This implies the plates are tilted in the y-direction but not that of x. In the following section we demonstrate how this offset in gap is proportional to the offsets input to compensate. Note that the calibration source seen in Fig. 3 is poorly diffused over the entrance aperture and differences in illumination between quadrants are present.
Panel (b) shows the appearance of the lines when the plates are more poorly aligned. Not only are the lines misaligned in this case but much less distinct as well. Preliminary adjustments such as those between (a) and (b) are used to establish the directional relationship between offset voltage and tilt. Having the pupil masks telecentric with quadrant edges aligned to the x-y axes greatly simplifies the process.
The remaining fine adjustment of the plates is an iterative process whereby piezo offset voltages are adjusted through software control until the wavelength offsets between all quadrants are zero. Alignment of all lines in (c) indicates the plates to be parallel. This ensures that the effective finesse and throughput of the instrument are now optimised.