Post-COSTAR FOS aperture wheel repeatability measurements



 Michael Dahlem & Anuradha Koratkar

Space Telescope Science Institute



 FOS Instrument Science Report CAL/FOS-131

1994 December





 ABSTRACT



Repeated exposures in an uninterrupted sequence through different science 
apertures (A-3, A-4, and B-2), while leaving other observing parameters unchanged, 
were used to test the repeatability of the aperture wheel positioning. The spectra were 
taken with the red side detector, using the G400H grating in the Image mode. 

The standard deviations of the positions of spectra in single exposures with 
respect to each other are found to be independent of the chosen aperture. The two-
dimensional cross correlation shows that the rms deviations of the positions of the 
spectra in the x- and y-direction are of order 0".02-0".04. These values are about 10 
times higher than those derived from the pre-launch vacuum tests (CAL/FOS-008). 
Thus, it is unlikely that the deviations arise solely from varying aperture wheel 
positions. Independent measurements indicate that variations of this order could be 
attributed to residual image motions caused by the uncertainty of the magnetic fields in 
the Digicons after on-board GIM correction, especially on the red side. The variations 
occur on timescales down to 10 minutes, indicating that the quality of the velocity 
calibration of FOS spectra taken with the red side detector is good to about 1/10 of a 
diode over these timescales.