## MANUAL mode

The last (and worst) alternative is to specify all the atmospheric parameters. This is known as the MANUAL mode (specified by OBSSET CAL MANUAL command). The sky emission is then computed from 4 parameters which are the atmospheric temperatures ( and ) and zenith opacities ( and ) in signal and image band respectively

(13)
(14)
Outside temperature, pressure and humidity are not used for the calibration, although they are still necessary for the telescope pointing, and the water vapor content is no longer needed (you replace four independent parameters by a set of four other independent ones). and are computed from Eqs. 6 and 11, so that the forward efficiency and gain image ratios are still required. For the calibration to be correct, it is however up to you to ensure that the values supplied are realistic at the frequency of your observations, and with the weather conditions you have...

This mode is just an “educated guess” for the atmospheric calibration, but it still corrects properly the relative gains of all the backend channels.