The focus excursions measured by the autofocus system are a measure of the seeing: standard atmospheric theory predicts that the seeing-induced image diameter is linearly related to the RMS atmospheric-induced focus fluctuations. In this mode the fast guider is acting as a form of Differential Image Motion Monitor (DIMM), the standard site-appraisal seeing monitoring systems. Zrms should therefore be a robust and invaluable measure of the seeing (as opposed to the delivered image quality!) at UKIRT.
We are in the process of exploring the possibility of using Zrms to derive reliable routine measures of the delivered image quality, which is an important parameter for flexible scheduling purposes, and are currently attempting to calibrate the system. An approximate calibration, which is reasonably accurate when the UFTI image FWHM ~0.”6, but certainly pessimistic when it is <0.”4, is currently in use for rough image-size estimates, e.g. when the spectrometer is in use.