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cso:instruments:heterodyne:fswitch [2012-11-08 05:19]
sradford
cso:instruments:heterodyne:fswitch [2013-04-16 19:18] (current)
sradford
Line 1: Line 1:
 +====== 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.
cso/instruments/heterodyne/fswitch.txt · Last modified: 2013-04-16 19:18 by sradford
 
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