Our group is leading a collaboration with the Herzberg Institute of Astrophysics (HIA) to develop IRMA, an Infrared Radiometer for Millimetre Astronomy. IRMA monitors a spectral emission line of water vapour just as a 183 GHz radiometers does, but at a wavelength of ~20 microns. The potential advantage of working at this wavelength is that the water lines are very strong and the technology is less complicated, which allows good signal-to-noise to be achieved on very short time scales and for less cost. Graeme Smith, a previous graduate student, built a prototype system for his Masters thesis which was tested at the JCMT in December 1999. The results from the prototype device were very promising, in that the correlation of the water vapour derived from it and from the JCMT 183 GHz radio device was very good. In August 2000, an upgraded IRMA device was installed at the JCMT and was run for an extended test period of about one year, with measurements timed to coincide with radiosonde launches from Hilo which measure the structure of the atmosphere, and with SCUBA sky dips which are currently used for baseline water vapour measurements. Data from the second prototype system has being analyzed, and was about an order of magnitude better than the first prototype, which already met the ALMA specs!
The prototype system in place at the JCMT was configured to do sky dips whenever SCUBA did a sky dip (in fact, IRMA does many sky dips in the time it takes SCUBA to do one). The IRMA data has been shown to correlated with the SCUBA estimate of precipitable water vapour in both the 850 and 450 windows. Analysis shows that the 20 micron emission is tracking the submillimetre transparency. This is a significant result since it shows that we can say what is happening at submillimetre wavelengths from 20 micron measurements.
A revised system is nearing completion which will be fully self contained and feature an onboard computer and cryogen-free Stirling cycle cooler. Two of these new IRMA devices will be tested in Chile to help answer a number of uncertainties and questions that need to be resolved before IRMA could be adopted as the baseline design for a water vapour radiometer for ALMA. Some of these questions are: The sensitivity of IRMA to cirrus and water clouds, the effect of changes in the pressure and temperature in the atmosphere and the different cone of the atmosphere sampled by IRMA compared to a radio-frequency device.
For more information go to the IRMA project site