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It is necessary to intersperse sky observations with comparison lamp observations for wavelength calibration. Because of the flexure of ISIS it is necessary to do this rather more often than with, for instance, the IDS on the INT. For observers who need high wavelength accuracy, for instance for absolute radial velocity measurements of stars, it is advisable to take comparison lamp exposures about every 15 minutes. For observers who wish to measure less precise radial velocities (for instance in most extragalactic systems), or line ratios, then comparison lamp exposures every hour should suffice. Separate comparison lamp exposures are definitely required for objects separated by more than 5 degrees in elevation, and for observations at different mount position angles. In particular observations of extended objects at different position angles must be calibrated separately.
The procedure for taking calibration lamp exposures is described in section 9.1.2. It may be possible to autoguide through a wavelength calibration exposure; this depends upon the brightness of the guide star and the probe co-ordinates. If not it will be necessary to switch the autoguider off during the calibration exposure; and re-acquire the guide star afterwards.