Because of physical restriction on the TMS ISC table real estate, pico mirrors for IR QPDs (M4 and M14) are mostly degenerate, i.e. if you rotate one mirror the beam position on both of the QPDs change by similar amount in the same direction.
Using nominal QPD sled parameters and as-built TMSY parameters (that is not that different from TMSX),
QPDA position shift = [3.2105, 3.1256] * [M4 rotation, M14 rotation]'
QPDB position shift = [4.8555, 4.9264] * [M4 rotation, M14 rotation]'
where [A B]' means a column vector. As you can see this is mostly degenerate.
For example, to keep the beam position on QPDA fixed while moving QPDB position down by 1mm, you PIT down M14 by about 5.0 mrad and then PIT up M4 by about 4.9mrad:
| To bring QPDB down by 1mm... | QPDA pos | QPDB pos | Beam position on M14 | Beam position on HPBD | Beam position on viewport |
| M4 up by 4.883 mrad | 15.7mm up | 23.7mm up | 5.2mm up | 7.4mm up | 20mm up |
| M14 down by 5.016 mrad | 15.7mm down | 24.7mm down | Not affected | Not affected | Not affected |
| Total | no change | 1mm down | 5.2mm up | 7.4mm up | 20mm up |
QPD diameter is 3mm, so as far as we see the beam on one QPD when the other is centered and the beam is not close to the edge of the clear aperture for viewport-TMS path, it should be possible to center both without losing the beam on ISCT.
I think that's the case for Y, but it's not clear if that was the case for X at some stage.
I used the following parameters from T1000247:
Lens1, focal length 333mm, z=0, Lens2, focal length -111mm, z=240mm.
QPDA, z=650mm, QPDB, z=950mm
I eyeballed the following parameters from D1000484:
M4 to M14: about 21"
M14 to Lens1: about 2"
M4 to high power beam dump: about 30"
M4 to M13 (the last steering mirror that stees the beam to the ISCTEX): about 23"
I used the following parameters from D0902168:
M13 to the viewport, horizontal distance 1.371m, declination angle about 20 degrees.