Centering the direct beam on the detector

There is a CBASS command-line instruction that will calculate the motions to make to the upstream and downstream detector-table motions to put the direct beam at the center of the detector for all positions of the detector on the lift table:

bc_calcs v1 h1 D1 v2 h2 D2

Here v and h are the vertical and horizontal distances in mm, to the right and upwards, respectively, of the direct beam from the physical beam centers. D is the specimen-to-detector distance in mm. Measurements should be made at two different distances along the detector table. The result will be written to the CBASS log file [what's the name, John?]. The result will include the increment to move the TZU (upstream vertical table motors tzo and tzi), TZD (the single downstream vert. motor), TYU and TYD (up- and downstream Y motors). If this proves to be reliable, we'll simply bump the motors automatically from CBASS, maybe after a query to the user and confirmation.

The equations that provide this correction are these:

a = (h2-h1) / (D2-D1) b = h1 – a D1

c = (v2-v1) / (D2-D1) d = v1 – c D1

delTYU = - 930 a -b

delTYD = -1570 a – b

delTZU = 630 c + d

delTZD = 1730 c + d