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cso:instruments:heterodyne:obstime [2014-01-31 20:24] sradford |
cso:instruments:heterodyne:obstime [2014-04-15 00:05] (current) sradford |
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+ | ====== Observing Time ====== | ||
+ | |||
+ | For heterodyne observations, estimate the sensitivity with the radiometer equation: | ||
+ | * //T//<sub>rms</sub> = //K// //T//<sub>sys</sub> (//t ∆f //)<sup>-1/2</sup>, | ||
+ | where //T//<sub>sys</sub> is the system temperature, //t// is the integration time, //∆f// is the channel width, and //K// ≈ 2, depending on the switching scheme and observing strategy. Invert this equation to estimate the necessary observing time. Do not neglect to add overhead for setup, pointing, calibration, etc. | ||
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+ | ===== System Temperature ===== | ||
+ | |||
+ | |||
+ | Estimate system temperature on the //T//<sub>A</sub><sup>*</sup> scale with: | ||
+ | |||
+ | * //T//<sub>sys</sub> [SSB, //T//<sub>A</sub><sup>*</sup>] = [exp(//τA// ) / //α//] × 2 × (//T//<sub>rx</sub> [DSB] + //α// [1 - exp(-//τA// )] //T//<sub>atm</sub> + (1 - //α// ) //T//<sub>spill</sub> ) , | ||
+ | |||
+ | where | ||
+ | * //τ// is the atmospheric optical depth at the observing frequency, see [[http://cso.caltech.edu/atm|model atmospheric spectra]], | ||
+ | * //A// is the airmass = secant (zenith angle), | ||
+ | * //α// is the hot spillover efficiency = 1 - (fraction of power falling on ground, etc.), ≈ 93%, | ||
+ | * //T//<sub>atm</sub> is the atmospheric temperature, ≈ 280 K, | ||
+ | * //T//<sub>spill</sub> is the warm spillover temperature, ≈ 280 K, and | ||
+ | * the Cosmic (Microwave) Background Radiation has been ignored. | ||
+ | |||
+ | For the above parameters and //T//<sub>Rx</sub> = 40 K [DSB], the approximate SSB system temperature at 230 GHz is:\\ | ||
+ | {{tsys-230.png?600}} | ||
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+ | |||
+ | If you're observing small sources, further adjust for the main beam efficiency. | ||
+ | |||
+ | If the atmospheric optical depth in the two sidebands is different, please review the discussion of [[calibration#sideband correction]]. |