Atacama Pathfinder EXperiment
APEX
Observing Time Estimation
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Atacama Pathfinder EXperiment
APEX
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Observing Time Calculation for APEX-2AThe equivalent system temperature outside the atmosphere is calculated from the expression:
where Trec is the receiver temperature, Tatm is the integrated physical atmospheric temperature (normally about 270 K), τo is the atmospheric zenith opacity and El the elevation of the source. Table 1 lists typical values of the atmospheric zenith opacity at Chajnantor. Eq. (1) provides only a rough estimate (the atmospheric transmission page provides more accurate values ), but good enough for most applications. The fact that the telescope also picks up some signal from the ground is not taken into account.
Table 1: Typical zenith opacities at Chajnantor during good conditions (pwv = 0.5 mm ) The rms noise in the spectrum is calculated from:
where Cobs is a factor depending on the observing method used, B is the noise bandwidth per channel of the spectrometer and tint the integration time ON source. C is equal to
As an example we will now calculate the integration time necessary in order to obtain an r.m.s. noise of 0.1 K with the FFTS bandwidth of 1 GHz at 345 GHz using the various observing methods. The elevation of the source will be assumed to be 60° and the zenith opacity at 345 GHz is about 0.09 (Table 1). The receiver temperature as a function of frequency is shown in Fig. 1 and is about 70 K at 345 GHz. This gives Tsys = 109 K which corresponds to a SSB value of 218 K. For beam and position switching Cobs =
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Last update : March 1, 2006
for switching and position switching (because noise is added when the OFF spectrum is subtracted from the ON spectrum) and 1 for frequency switching. Solving for tint in Eq. (2), we get 