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Measuring Phase Variations at the SMA Using the IRMA Water Vapour Monitor

This study explores the use of the IRMA WaterVapour Monitor to measure phase variations at the SMA, providing accurate water vapor resolution. The advantages and features of IRMA are discussed, along with current and future work.

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Measuring Phase Variations at the SMA Using the IRMA Water Vapour Monitor

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  1. Measuring Phase Variations at the SMA Using the IRMA WaterVapour Monitor Robin Phillips James DiFrancesco Tyler Bourke David Naylor

  2. IRMA II: Test measurements: Dec 2000 – Mar 2001 Water vapor resolution (1 sec integration): 0.26 mm pwv at 0.5 mm pwv 0.44 mm pwv at 1.0 mm pwv IRMA I & II • Developed in Lethbridge • (student project) • Tested on JCMT • Situated outside membrane • Mainly used in skydip mode IRMA I: • Development: 1997 – 1999 • Test measurements: Dec 1999 • Water vapor resolution (1 sec integration): • 1.8 mm at 0.5 mm pwv • 3.0 mm at 1.0 mm pwv

  3. IRMA I & II

  4. IRMA I Results • Data collected in December, 1999 • Water vapor resolution (1 sec integration time): • 1.8 mm at 0.5 mm pwv • 3.0 mm at 1.0 mm pwv • Stared with 183 GHz WVM • High degree of correlation with 183 GHz measurements

  5. IRMA Advantages • Operates at 20 μm; near the peak of the Planck function for atmospheric temperatures • Wide bandwidth => better signal-to-noise • Photoconductive detectors offer simplicity, high speed, sensitivity and stability • Small size and mass, low maintenance • Low complexity => high reliability, low cost • Zero RF interference 20 µm = 15 THz 183 GHz = 1.6 mm

  6. IRMA III Features • Compact design to maximise possible mounting options • Cryogen-free Stirling-cycle refrigerator • Minimisation of moving parts for mechanical longevity • Sealed modular construction to resist harsh environmental conditions • Ethernet-based onboard computer to simplify communications • Fully remote operation • Performance target: • 10 m beam width at 1 km • Resolution better than 1 µm PWV in 1 s integration @1mm • Minimum 10 Hz sample rate

  7. Hawaii tests • ~3 weeks initial testing outside JCMT • Moved to SMA • Operated for 4 months

  8. Current and future work • Contracted by Gemini to upgrade system and operate loaner for 3-6 months. • TMT project has purchased 3 units for site testing in Chile, Mexico (and maybe Hawaii). • Las Campanas observatories have purchased one unit (with a loaner until we can build one). • Collaboration with AASTINO project to site test Dome C.

  9. Data reduction steps • Detector signal goes to ADC which provides onboard Rabbit micro with ‘counts’ representing voltage. • Voltage needs converting to spectral power using periodic black body measurements • Conversion varies with: • Cooler base temperature • Internal box temperature • Sepectral power needs converting to pwv using radiative transfer model of atmosphere. • Requires atmospheric profile • Varies with ambient temperature and pressure

  10. Voltage during calibration sequence

  11. Black body temperature during calibration

  12. SMA data: Peak-to-peak ~200deg phase shifts (obs freq 230GHz) IRMA data: 0.1mm pwv => 0.6mm path => 155deg phase shifts

  13. SMA data: Peak-to-peak ~200deg phase shifts (obs freq 230GHz) IRMA data: 0.1mm pwv => 0.6mm path => 155deg phase shifts

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