APEX 345 GHz RECEIVER

The APEX 345 GHz heterodyne receiver (delivered by GARD at Onsala Space Observatory, Chalmers, Sweden) was commisioned during the summer of 2005. The first regular observations were taken place during the fall of 2005. It is operating in double sideband (DSB) mode with an intermediate frequency (IF) of 6 GHz and its tuning range is 279 GHz to 381 GHz. The DSB receiver temperature (about 60-70 K) is fairly constant over the entire tuning range, see Fig. 1 below. The telescope beam size at 345 GHz (image band at 357 GHz) is determined to 17.5”. The tuning of the receiver is done remotely

FAST FOURIER TRANSFORM SPECTROMETER

The APEX backend is a fast Fourier transform spectrometer (FFTS) delivered in March 2006 by MPIfR in Bonn. It has a total bandwidth of 1 GHz which can be configured into 1-16384 channels in powers of 2, i.e. N = 2 n. The FFTS noise bandwidth, B, and the effective channel resolution is yet not known. They are assumed to be equal to the channel separation B = 10 6/N kHz.

OBSERVING TIME ESTIMATE

In this section we will show how to estimate the observing times with the APEX 345 GHz receiver.

The equivalent system temperature outside the atmosphere is calculated from the expression:

(1)

Table 1: Typical zenith opacities at Chajnantor during good conditions (pwv = 0.5 mm )

The rms noise in the spectrum is calculated from:

(2)

where C_obs is a factor depending on the observing method used, B is the noise bandwidth per channel of the spectrometer and t_int the integration time ON source. C is equal to for beam 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 t_int in Eq. (2), we get

(3)

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 T_sys = 106 K which corresponds to a SSB value of 212 K. For beam and position switching C_obs = and we want to obtain a ΔT of 0.1 K which gives t_int = 1.2 min. For beam and position switching this value has to be multiplied by a factor depending on the time spent in the OFF position and an overhead time which includes telescope movements and calibrations. We will assume a value of 3.2 for this factor. The total time for the observation is thus 3.2 min.

The estimated rms noise values should of course be compared with the expected signal intensity. The main beam and moon efficiencies of APEX at 345 GHz are expected to be 0.85 and 1.0, respectively. Thus, a CO(3-2) line with a brightness temperature of 10 K would give an intensity of 10 * 0.85 = 8.5 K, if the source just fills the main beam, or 10 * 0.9=9 K, if the source is extended.

APEX Heterodyne receivers observing time calculator

FUTURE HETERODYNE RECEIVERS

In the future APEX will equipped with SIS receivers covering the bands at 275 - 370 GHz, 375 - 500 GHz as well as the submillimetre and THz bands.