====== Frequency Switched Observations ======

Frequency switching at the CSO is implemented by switching the first LO. 

==== Observing ====

The basic UIP command is:

''FSWITCH <Separation(MHz)> [<Cycles>] ''

The two line images will be separated by twice the ''Separation'', symmetrically offset from 0. For example, to use 3 s integrations, ±10 MHz frequency separation, and 5 repetitions: 

''__UIP__> uip\spectrometer 3 '' \\
''__UIP__> uip\fswitch 10 5 '' 

=== Caveats ===

Although the shortest possible integration time for the AOS5 and FFTS1 spectrometers is 0.5 s, it has been reported that short integration times, 1-2 s, cause the FFTS1 to crash. Integration times of 3 s and longer seem to work without problems.

The spectra may display resonant artifacts at unpredictable frequencies. To avoid these artifacts it may be necessary to retune the LO, changing the IF offset of the astronomical line.

=== Telluric lines ===

Telluric (atmospheric) CO or other lines may be seen. These lines will show up at the apparent LSR velocity of the telescope, which is not zero because of the Earth's orbital motion, etc. The telluric line strength increases with airmass, i. e., secant (zenith angle). You may need to adjust the frequency separation (or observe at a different time of year) to avoid these overlapping with the astronomical lines. 

To predict the apparent telescope velocity for future observations, use ASTRO, part of the Gildas software suite. For example:

<code>
ASTRO> cat "/opt/uip/cat/default_catalog.cat"         ! load catalog
ASTRO> observatory cso                                ! choose observatory
I-OBSERVATORY,  Selected CSO observatory
I-OBSERVATORY,  Time needs to be reset
ASTRO> time 0:0 16-apr-2013                           ! set observation time, UT
ASTRO> source irc10216                                ! select source
IRC10216    Azimuth   74.73333    Elevation  -3.18819
IRC10216    V(S/OBS) =     6.513 [S/LSR=-26.000,LSR/G=  7.122,G/OBS= 25.391]
ASTRO> say 'astro%source%v_sou_lsr-astro%source%v_sou_obs'    ! apparent LSR telescope velocity
-32.513272262193 
</code>

For past data, the apparent telescope velocity can be obtained in CLASS by:

''LAS> say '299792.5*R%HEAD%SPE%DOPPLER+R%HEAD%SPE%VOFF' ''

==== Data reduction ====

Frequency switched data are stored as regular, position switched, scans in the data file. Before folding them, the headers must be modified. In CLASS, set the SWITCH_MODE to FREQUENCY and specify the offset frequency in MHz, the integration time in seconds, and the weight of two phases:

''LAS\MODIFY SWITCH_MODE FREQUENCY 2 <Offset_1(MHz)> <Time_1(s)> 1 <Offset_2(MHz)> <Time_2(s)> -1''

Normally, the switching is symmetric so ''Offset_2'' = - ''Offset_1'' and the weights are equal and opposite, ±1. For the above example, with ±10 MHz offset and 5×3 s integration per phase, use:

''LAS> las\modify switch_mode frequency 2 10 15 1 -10 15 -1 ''\\

It is also possible to get the integration time from the headers:

''LAS> modify switch_mode frequency 2 10 r%head%gen%time 1 -10 r%head%gen%time -1 ''

Finally, fold the spectra:

''LAS> las\fold''

It may be necessary to remove baselines before or after folding data.