Description of Infrared Water Vapour Monitor (IWVM)

Figure 2 is an image of the unit in operation at the JCMT. The computer and electronics are seen in the foreground, the IWVM in the background. The IWVM uses a plane steering mirror which directs light onto a fast off-axis parabolic mirror at whose focus is an infrared photoconductor detector. A cold bandpass filter located immediately ahead of the detector defines the spectral bandwidth. A cold aperture in front of the filter defines the detector field of view. A reflective chopper produces a 200Hz modulated signal of the atmosphere which is calibrated by pointing the steering mirror to 2 blackbody sources. The detector and parabolic mirror define the field of view on the sky which is chosen such that it samples a patch of atmosphere of similar size as a submm telescope at the scale height altitude of water vapour ($\sim$10 m @ 1 km).
In the current design a standard, compact, LN₂ dewar cools the detector. For installation and long term operation on a submm antenna, a closed cycle cooler would be used (eg. Cryotiger) and the IWVM mounted on the outer rim of the antenna (much like a finding scope). The cold space required is rather small ($\sim$ 18 cm³). The Moon provides an ample signal to align the IWVM (in the Dec 99 run the moon was about first quarter and was easily detected). Once the submm telescope is centred on the Moon it is expected that the IWVM can be aligned to an accuracy of a few arcminutes.
The IWVM can operate in sky-dip or stare modes. In the sky-dip mode the system steps in 0.18$^{\circ}$ increments from the zenith to $\sim$70$^{\circ}$ (corresponding to a range of 1 - 3 airmasses). In the stare mode the system can be fixed at a given zenith angle.
Key components of the IWVM are:

• Infrared detector - 20$\mu$m represents the practical limit for MCT detectors operating at 77K. (Si:As detectors would offer a significant gain in sensitivity (several orders of magnitude) but require cooling to $\sim$12K. When I was approached by NRC to study an infrared solution to phase correction of submm interferometry one of the boundary conditions was a temperature no lower than 77K.) I am discussing with an infrared detector manufacturer ways of improving the performance of the MCT detectors for this application. I believe that the detectors will not be a critical issue.
• Low noise preamplifiers - our group has extensive experience in infrared technology and routinely constructs preamplifiers that out perform those supplied by detector manufacturers. Other aspects of the electronics (eg. chopper controller, lock-in amplifier, analog-to-digital conversion) are standard. No critical items.
• Infrared filter - this is the most critical item. Unfortunately there are no manufacturers currently building filters for this wavelength region. Furthermore, and naturally, any existing 20$\mu$m filters were designed to avoid the water vapour lines. Prof. Peter Ade (QMW), with whom I have a long standing collaboration, is currently trying to extend his filter fabrication technology from the 200 to the 20$\mu$m range. While the initial attempts have yielded promising results more work needs to be done in this area, in particular the production of smaller scale electron beam lithography masks.
• Optics - the fast off-axis parabolic mirror and the electric discharge machined (EDM) reflective chopper. No critical items.