FOS Filter-Grating Wheel Repeatabillty:
Dependence on Motor Selection
George Ha.rtig
Space Telescope Science Institute

Instrument Science Report CAL/FOS060
May 1989


Abstract


Tlie repeata.bility of the FOS Filter-Gra.ting Wheel (FGW) was
measured in a series of special tests a.t LMSC on 11-12 Ma.y 1989,
aimed at evalustixig the effects of motor selection on tlie FGW
performance. Mea.surements were made of the repea.ta.bility in botb X
sud Y at the caxnera mirror position. on both red sud blue sides, with
the standard motor selections, a.nd in X for 7 dispersers on ea.ch
uide, with ea.cb motor. The FGW repea.ta.bility wa.s fouud to be
siguifica.ntly degraded, by about a fa.ctor of 2, when the A motor ii
used, a.s compa.red to mea.surements made with the B motor selected,
on both sides of the spectrograph. With the B motor, the FGW
repeata.bility rema.ins somewhat uusa.tisfa.ctory, but no evidence for
performan.ce degradation with time or usage is present. The B motor
sbould always be selected, regardless of which detector is in use,
contrary to current procedure. The cause of the reduced repeatability
with the A motor, sud other effects seen prominently in the data.,
remain uxiexpla.ined.