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cso:instruments:heterodyne:pointing [2013-10-31 07:19] sradford |
cso:instruments:heterodyne:pointing [2014-01-22 19:01] (current) sradford |
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+ | ====== Pointing ====== | ||
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+ | Point often for the first 2--3 hours after sunset because the pointing will shift as the temperature of the telescope changes ([[http://www.submm.caltech.edu/%7Ehs/memo/shinnaga-memo04nov.pdf|thermal pointing shifts]]). Repoint no later than an hour after the initial pointing. Repoint every 2.5--3 hours thereafter. | ||
+ | |||
+ | [[chop|Beam switching]] with the wobbling [[cso:telescope:secondary]] may give more consistent results, especially if the weather is marginal. Use a short subscan integration time because there are four subscans at each point. [[onoff|Position switching]] is also possible. | ||
+ | |||
+ | Before pointing, | ||
+ | * Check the correct pointing model is displayed on the antenna status monitor | ||
+ | * Load recent pointing offsets from the pointing logbook \\ ''__UIP__> **fazo //az_offset//**'' \\ ''__UIP__> **fzao //za_offset//**'' | ||
+ | |||
+ | The pointing examples use <sup>12</sup>CO(2-1) and beam switching. | ||
+ | |||
+ | === Line pointing === | ||
+ | |||
+ | There are many more line sources -- typically evolved stars -- than bright continuum sources. But it may be necessary to tune the receiver to a bright line. The example uses <sup>12</sup>CO(2-1) and beam switching. Position switching is also possible. | ||
+ | |||
+ | * Tune the receiver, if necessary, and setup the spectrometer. \\ ''__UIP__> **LO 12CO2-1 /IF 5.28 /SIDE UP**'' \\ ''__UIP__> **SPEC 2 /FFTS1W 5.28 /FFTS2 /RES**'' | ||
+ | * Choose a pointing source with the [[cso:telescope:orrery]]. Then track the source with the telescope. \\ ''__UIP__> **OBSERVE IRC10216** '' | ||
+ | * Start the secondary wobbler. \\ ''__UIP__> **SEC 90 0.534** '' | ||
+ | * Set the baseline windows. These are line free regions used to calculate the spectral baseline.\\ ''__UIP__> **SPEC /BASE 2500 3500 4700 5700** '' | ||
+ | * Do a calibration. \\ ''__UIP__> **CAL**'' | ||
+ | * Measure the pointing. The integration range must include the line. \\ ''__UIP__> **FIVE 20 /FFTS1 /RANGE 4000 4200 ** '' | ||
+ | * Choose a step size somewhat less than the beam size. Here the step size is 20 arc sec because the beam size is about 30 arcsec at the observing frequency. | ||
+ | * Update the pointing corrections. If the results of the pointing fit seem dubious, use the ''FAZO'' and ''FZAO'' commands to update the corrections. | ||
+ | * Repeat pointing measurements until convergence. | ||
+ | * Record the pointing results in the pointing log book to share with other observers. | ||
+ | * After pointing, remove the baseline windows. \\ ''__UIP__> **SPECTROMETER /NOBASELINE **'' | ||
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+ | If the line is not centered in the spectrometer or if there are lines in the baseline windows, take a spectrum prior to pointing to determine the line position and the line free baseline windows. This may be difficult if the pointing is far off. | ||
+ | |||
+ | For standard sources, it is good practice to take a reference spectrum just after pointing as a calibration comparison. | ||
+ | |||
+ | |||
+ | === Continuum pointing === | ||
+ | |||
+ | The bright planets, Mars, Jupiter, and Saturn, are good sources. Uranus and Neptune may be (very) difficult. | ||
+ | |||
+ | * Tune the receiver, if necessary, and setup the spectrometer. \\ ''__UIP__> **LO 12CO2-1 /IF 5.28 /SIDE UP**'' \\ ''__UIP__> **SPEC 2 /FFTS1W 5.28 /FFTS2 /RES**'' | ||
+ | * Choose a pointing source with the orrery. Then track it with the telescope. \\ ''__UIP__> **PLANET MARS** '' | ||
+ | * Start the secondary wobbler. \\ ''__UIP__> **SEC 90 0.534 ** '' | ||
+ | * Do a calibration. \\ ''__UIP__> **CAL**'' | ||
+ | * Measure the pointing. For a continuum source, integrate over the entire spectrometer bandpass. \\ ''__UIP__> **FIVE 20 /FFTS1 **'' | ||
+ | * Choose a step size somewhat less than the beam size. Here the step size is 20 arc sec because the beam size is about 30 arcsec at the observing frequency. | ||
+ | * Update the pointing corrections. If the results of the pointing fit seem dubious, use the ''FAZO'' and ''FZAO'' commands to update the corrections. | ||
+ | * Repeat pointing measurements until convergence. | ||
+ | * Record the pointing results in the pointing log book to share with other observers. | ||
+ | |||
+ | It is good practice to take a reference spectrum of the planet just after pointing to check the beam efficiency. | ||