This page contains details on the medium resolution (R ~ 200) visiblity amplitude mode for KI. This mode is achieved by use of a grism in place of the usual prism inside the fringe tracking dewar. This grism produces 42 channels across the K' band, spaced at roughly 10 nm intervals and ~15 nm FWHM. The K' filter is used because the band-edge performance is significantly better.
The grism mode observations are conducted in the same way and with the same efficiency as low resolution observations. The differences are the sensitivities (see the sensitivity page) and the calibration requirements. Due to structure within the passband, the calibrator K band magnitudes MUST bracket (with higher and lower magnitudes) the source K band magnitude and be within 0.5 mags. We also recommend using at least 3 calibrators in order to make comparisons amoung the calibrators. When the target is appropriately calibrated, the relative (channel-to-channel) calibration is 0.02 average deviation from the mean in visibility amplitude squared. This has been determined from the spectral response of a calibrator star with no spectral features in the K band (i.e. a flat spectral response after calibration).
Currently it is NOT possible to observe in both medium and low resolution in a single night.
Last updated: Aug 8, 2006