An Infrared Radiometer for Millimetre Astronomy
IRMA data from Gemini/LCO/TMT(password protected)
Latest news for the IRMA project (25 Nov 2005).
Latest webcam images of IRMA units at:
Gemini South (Chile) , Las Campanas Observatories (Chile).
M.Sc./Ph.D. position for analysing IRMA data, for more details contact David Naylor.
|Developed as a collaboration between the University of Lethbridge and the Herzberg Institute of Astrophysics (HIA), IRMA is a compact, light weight and relatively low cost design for determining atmospheric water vapour at telescope sites around the world. It uses an infrared MCT detector to measure the emmission from water vapour rotation transitions in a band from ~19.5 - 20.5um. The total power detected in this band is converted to a PWV value using an atmospheric model (ULTRAM). The IRMA instrument consists of a 35x22x19cm box weighing approximately 12kg. Inside this box the detector is placed in a vacuum vessel that is cooled using a compact, low power consumption Stirling cycle cooler. A 5 segment chopper blade provides a 455Hz chopped signal to the electronics which is controlled by a small PC104 microcomputer. The sky is viewed via a 10cm f1 90 degree off axis parabaloid through an opening in the top of the instrument. The opening can be sealed during bad weather by a lid mechanism that includes an attached black body for instrument calibration. The IRMA box can be either attached directly to a telescope and aligned with the main telescope beam, or it can be mounted in a small alt/az mount which contains its own small micro and can be commanded to point the IRMA box in any direction (thus enabling skydips or sky maps to be performed).|
Proof of concept Prototypes
The IRMA concept has been tried once before by ... at Kitt Peak in the 1970s.
However, this experiment was not a success, primarily due to the lack of a sufficiently sensitive
detector. In order to show that advances in IR detector and filter technology have improved
sufficiently over the following 2 decades, we first built a prototype system that 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 radiometer was very good.
Based on the experience of this first run an upgraded IRMA device was tested at the JCMT in August
2000 and was run for an extended test period of about four months. Measurements were timed to
coincide with radiosonde launches from Hilo which measure the structure of the atmosphere, and with
SCUBA sky dips which were used for baseline water vapour measurements. Data from the second
prototype system showed that it was about an order of magnitude better than the first prototype and
was easily meeting the sensitivity requirements for interfermoteric phase correction. The prototype
system in place at the JCMT was configured to perform a skydip whenever SCUBA skydips were run
(generally several times per night). The IRMA data was then 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
confirms in practice, the theory that 20 micron measurements can be used to predict submillimetre
IRMA was originally conceived as a phase correction tool for ALMA.The Atacama Large Millimetre Array (ALMA) is currently the largest project in ground-based astronomy, and will provide unprecedented resolution for millimetre-wavelength observations. The scientific goals for the ALMA project range from cosmology to astrochemistry to planetary science. Critical to ALMA's success will be the correction of atmosphere-induced distortion of the astronomical signal, particularly that due to water vapour. Since each telescope looks through a different column of the atmosphere, inhomogeneous distribution of water vapour in space and time will create different phase delays in the astronomical signal for each telescope. One method of phase correction is to measure the amount of water vapour in the path between each telescope and the source on short time scales (1 second or less), the measured signals can be corrected for the path change due to the water vapour, effectively 'sharpening' the resultant image.
The baseline plan for phase correction for ALMA is to use 183 GHz radiometers, development versions of prototypes used at the JCMT, CSO and SMA on Mauna Kea, Hawaii. The ESO 183 GHz site testing page is here and the main project page is here.
Measuring PWV at 20um rather than at 183GHz has certain advantages due primarily to the much stronger emmission lines at these frequencies - 20um coincides closely with the peak of the atmospheric Planck curve. This, coupled with the wide bandwidth of uncontaminated water vapour lines at these frequencies allows an excellent signal-to-noise ratio to be achieved on very short time scales and, critically, allows simple detector hardware to be used as no complex RF detection equiment is required (for the radio teelscope application this also means there is no danger of RF interference being generated). This also results in a reduced cost for the overall system. The main disadvantage is that the IR detector needs to be cooled to at least ~150K (and preferably to 70K as in the current system) which requires the expensive Stirling cooler that makes up more than half the cost of the IRMA system components.
IRMA at the SMA
Following completion of the first three IRMA units we took them to Mauna Kea for initial testing. At first, we had two units on stands outside the JCMT where we were able to test and debug the systems without interfering with an operational telescope. Once we were happy with their operation we attached all three units to three different SMA antennas and performed four months of testing.
IRMA as a PWV and 20 um opacity monitor
IRMA as a site testing tool
IRMA papers and presentations
Technical descriptions of IRMA and some of the results obtained have been published in the following
papers or conference presentations:
For a (rather out of date) overview of the project, please watch the IRMA video
To see what is happening right now,
we (sometimes) have a WebCam that looks over the IRMA lab.
Last modified on 2007-12-20
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