In frequency switching mode the telescope remains at a fixed position and data is accumulated while the center of the receiver passband is switched between signal and offset frequencies separated by FOFF (MHz), at a switch rate of CPER (seconds). Data accumulated at the offset frequency is subtracted from the signal frequency data before storage as a frequency switched scan; the SPA data reduction command 'FOLD' can be then used to rectify and shift the offset to lie on top of the signal passband.
The advantage of frequency switched over position switched observations are that signal photons are collected during the entire integration leading to a typical 2 gain in S/N for a fixed scan integration time, and telescope switching overhead is eliminated. The disadvantage of frequency switching is its relatively limited range ( 30 MHz). For SEQUOIA, it is recommended to switch less than 10 MHz due to gain instabilities within the frontend system at different frequencies. Also, due to an impedance mismatch between the SEQUOIA dewar and IF processor, one can expect a very large baseline ripple (27 MHz) corresponding to the length of the cable wrap. This does not compromise spectra with narrow ( 8 km/s but may require a higher order baseline subtraction in the processing of the data.
The current state of the frequency switching routine parameters is displayed by typing FS , and the frequency switching hardware is initiated by typing FSON. See Table 4.4 for keywords and typical parameter values. As in other data-taking routines, the observation will not proceed unless all error conditions are absent. Please leave frequency switching under computer control. The programs will temporarily stop the frequency switching when data is stored on disk, and then automatically turn it back on. Do not stop an FS observation while it is storing data.
When exiting FS observing mode you must type FSOFF to stop the frequency switching hardware, else signals sent to the computer when switching is on will interfere with further observations.