The programs UFC and USC interpolate in tables of ADU encoder values corresponding to increments of 1 mm in the mechanisms without any known start value. To convert this table of ADU's to something useful, one needs a start value, i.e. the ADU value at a known mm position. This position must be redetermined if certain mechanical changes are made to the spectrograph.
Position USC and UFC at the 4246 Å positions indicated on the mechanisms,
driving the appropriate mechanisms in manual mode. These correspond to 0 mm in
USC and 0 mm in UFC. The distance between the echelle and cross--disperser
gratings should be 3532 mm, and the air distance between the secondary
collimator and cross--disperser should be 382.4 mm. Read the encoder values
for USC and UFC LVDTs
(in ADUs) and determine the mm offset by consulting the
table of encoder values. For example, if at 4246 Å = 0 mm USC reads
24400 ADU, and USCLVDT.DAT has entries 49 = 24140 and 50 = 24470, 24400
corresponds to position 49.7879, which is then the offset.
The programs UFC and USC must be edited to change the offset value they use.