On Improving the Calibration of Millimetre and Submillimetre Photometry at the James Clerk Maxwell Telescope / JCMT

A method is described for deriving the atmospheric extinction coefficients corresponding to the passband filters in the James Clerk Maxwell Telescope (JCMT) common-user photometer. This method takes advantage of the extinction determined by skydips performed at 225 0Hz at the adjacent Caltech Submillimeter Observatory (CSO). Scaling factors, by which the CSO extinction must be multiplied in order to give the required extinction, can be calculated by monitoring standard sources over a range in airmass on stable nights. Values are presented for most of the JCMT filters, and for the 800- m filter it is shown that the factor is not constant, but is correlated with the CSO extinction. An empirical relationship is derived between the 800- m extinction and the CSO extinction.