Heterodyne Calibration

Observations with RxA and HARP are calibrated on a T_A^* scale (in Kelvin), in which the spectra are corrected for atmospheric attenuation and scattering and spillover seeing ambient temperature.
This is done by combining measurements of hot and cold loads and knowledge of the sky and ambient temperatures.

The temperature scale is checked by observing a number of standard sources at certain standard frequencies (depending on the observing program for the night). Also the bandwidth chosen depends on the observing program. Some of these source/frequency observations have been observed very frequently (e.g. CO in IRC+10216, CRL2688 and CRL618) and can be used to monitor the calibration of the telescope. Other combinations have been observed only once.

Results are given separately for RxA3 and HARP.

In addition regular observations are made of Mars, Uranus, and Jupiter in order to monitor the main beam efficiency η_mb (and the aperture efficiency η_a) of the JCMT. Also some observations of the full Moon have been made. Most observations are made for RxA3 at 230.538 GHz (CO 2-1) and for HARP at 345.796 GHz (CO 3-2), both for a bandwidth of 1000 MHz.

Results are given separately for RxA3 and HARP.

For point sources the conversion from T_A^* in Kelvin to flux density in Jansky for the JCMT is
S(Jy) = 15.6 T_A^*(K) / η_a.

In general, our rule of thumb is that night time spectral standard observations should be within 10% of the ‘canonical’ value. For convenience, we have analysed observations (considered as ‘good’, and not including very anomalous results or poor baselines) towards some of our standard sources at CO 3-2 (HARP) and CO 2-1 (RxA3). The mean peak and integrated value, along with the standard deviation and percentage error are shown in the table below for these sources. Please note that in general the distribution may not be Gaussian (due to pointing errors, which will tend to skew the distribution below the ‘true’ value).  This does not include any RxA3m observations, and excludes day time observations and observations when RxA3 is believed to have been misaligned.