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The original specification for ISIS was to have flexure no more that 5 microns/hour along and perpendicular to the slit during telescope tracking. However measurements of the flexure caused by movements in elevation, and movements of the instrument rotator, suggest that flexure during tracking could be up to 15 microns/hour. The cause of the extra flexure is not yet known, and until it is corrected it is recommended that observers requiring accurate radial velocities should take a calibration lamp exposure every 15 minutes.
Measurements of radial velocity standard stars taken during commissioning show that if care is taken with the calibration exposures it is possible to measure radial velocities to an r.m.s. accuracy in the range 1-2 km/s with the highest dispersion gratings (H2400B and R1200R) with either arm of ISIS. The systematic offset between ISIS radial velocities and the IAU velocity system is measured to be:-
from blue arm observations of radial velocity standard stars and nebulae.
The stability of FOS-II has been measured to be better than 1 micron/hour when tracking an object through the zenith.