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Infrared Radiometer for Millimetre Astronomy

irma. Infrared Radiometer for Millimetre Astronomy. IRMA 20 µm Water Vapour Radiometer Operations in the TMT Site Testing Campaign Richard Querel, David Naylor, Robin Phillips, Regan Dahl, & Brad Gom Astronomical Instrumentation Group, University of Lethbridge, Lethbridge, Alberta, Canada.

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Infrared Radiometer for Millimetre Astronomy

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  1. irma Infrared Radiometer forMillimetre Astronomy IRMA 20µm Water Vapour Radiometer Operations in the TMT Site Testing Campaign Richard Querel, David Naylor, Robin Phillips, Regan Dahl, & Brad Gom Astronomical Instrumentation Group, University of Lethbridge, Lethbridge, Alberta, Canada

  2. IRMA Concept • Measure emission from water vapour lines in the 20μm atmospheric window • Band-pass includes only water vapour transitions • Theoretical atmospheric model supported by FTS measurements from Mauna Kea (Naylor et. al. PASP 96, 167 (1984)) PASP vol. 96, Feb. 1984, p. 167-173 WVR Workshop, Wettzell, Germany, October 10, 2006

  3. Advantages • Operates at 20 μm; near the peak of the Planck function for atmospheric temperatures • Wide bandwidth (~1 μm) …↑signal-to-noise • Photoconductive detectors offer simplicity, high speed, sensitivity and stability • Zero RF interference { 20 µm = 15 THz 183 GHz = 1.6 mm WVR Workshop, Wettzell, Germany, October 10, 2006

  4. IRMA I (1997-1999): • Water vapor sensitivity noise-limit (1 sec integration): • 1.8 mm PWV at 0.5 mm PWV • 3.0 mm PWV at 1.0 mm PWV IRMA II (2000-2001): • Water vapor sensitivity noise-limit (1 sec integration): • 0.26 mm PWV at 0.5 mm PWV • 0.44 mm PWV at 1.0 mm PWV WVR Workshop, Wettzell, Germany, October 10, 2006

  5. Current IRMA Design WVR Workshop, Wettzell, Germany, October 10, 2006

  6. WVR Workshop, Wettzell, Germany, October 10, 2006

  7. WVR Workshop, Wettzell, Germany, October 10, 2006

  8. WVR Workshop, Wettzell, Germany, October 10, 2006

  9. WVR Workshop, Wettzell, Germany, October 10, 2006

  10. WVR Workshop, Wettzell, Germany, October 10, 2006

  11. WVR Workshop, Wettzell, Germany, October 10, 2006

  12. BTRAM Output for Mauna Kea Site 500 cm-1 = 20 μm ~ Peak of Planck Curve WVR Workshop, Wettzell, Germany, October 10, 2006

  13. Voltage → Flux Assume linear detector response Hot & ambient BB readings Need accurate temperature of BB Flux → PWV Atmospheric model Surface T & P Instrument Response AΩ (Throughput) Steps required to convertVoltage → PWV (all are possible sources of error) WVR Workshop, Wettzell, Germany, October 10, 2006

  14. WVR Workshop, Wettzell, Germany, October 10, 2006

  15. Typical Calibration Cycle Voltage (V) WVR Workshop, Wettzell, Germany, October 10, 2006

  16. Typical Calibration Cycle BB TAmbient BB THot Shutter Open & BB Off BB On Voltage (V) Shutter Close Sky Sky WVR Workshop, Wettzell, Germany, October 10, 2006

  17. WVR Workshop, Wettzell, Germany, October 10, 2006

  18. WVR Workshop, Wettzell, Germany, October 10, 2006

  19. For the TMT Site Testing required resolution of 0.1mm @ 1.0mm PWV, we need to know the effective BB T < 0.5K WVR Workshop, Wettzell, Germany, October 10, 2006

  20. Effective Temperature of Blackbody? • Center sensor = 50.6°C; Edge sensor = 45.8°C • Emission calculated for each pixel to determine the total flux emitted from the blackbody. • Determined effective (uniform) surface T = 48.7°C (Data from a 7-14μm Fluke Ti-20 Thermal Imager)

  21. Normal calibration (High T = 305.9 K) WVR Workshop, Wettzell, Germany, October 10, 2006

  22. Modified calibration (High T = 302.4 K; -3.5K) WVR Workshop, Wettzell, Germany, October 10, 2006

  23. 5 days of data using the “Normal” calibration method and the sensor temperatures WVR Workshop, Wettzell, Germany, October 10, 2006

  24. 5 days of data, with a modified (-3.5K) Unit 1 Hot-temperature WVR Workshop, Wettzell, Germany, October 10, 2006

  25. IRMA Cross-Calibration • IDL MPFIT of offset and gain between VISIR data and Gaussian-convolved BTRAM data • Reduced Χ2 = ~0.0959 WVR Workshop, Wettzell, Germany, October 10, 2006

  26. BTRAM WVR Workshop, Wettzell, Germany, October 10, 2006

  27. BTRAM Facts • BlueSky Transmittance & Radiance Atmospheric Model • Built in IDL • Available for Windows & Linux • Line-by-line layer-by-layer Radiative Transfer • Able to simulate: • Atmospheres (7 primary gases) • Laboratory Gas Cells (37 molecules) • Transmission / Emission / Opacity • Batch mode to create data-cubes WVR Workshop, Wettzell, Germany, October 10, 2006

  28. BTRAM Facts • Distributed with HITRAN 2004 Database, any spectral database can be used • Contains 1,734,469 spectral lines for 37 different molecules • 6 built-in FASCODE Atmospheric Profiles • Mid-Latitude Summer (& Winter) • Subarctic Summer (& Winter) • Tropical • US Standard WVR Workshop, Wettzell, Germany, October 10, 2006

  29. BTRAM Facts • Site-specific Atmospheric Profiles included • Antarctic Summer • Chajnantor Winter • Mauna Kea • Customized Atmospheric Profiles can be imported as comma-delimited text (.csv) • Output spectra can be exported as Grams compatible .spc file, or as a text file. WVR Workshop, Wettzell, Germany, October 10, 2006

  30. Difficult to change FASCODE layering BTRAM used FASCODE atmospheric layer parameters in a comparison to ensure its accuracy BTRAM vs. FASCODE WVR Workshop, Wettzell, Germany, October 10, 2006

  31. WVR Workshop, Wettzell, Germany, October 10, 2006

  32. Conclusion & Future work • Calibration depends on our hot blackbody • Uniform “hotter” blackbody is necessary • Atmospheric parameters / model errors? • Lunar spectrophotometer useful calib tool? WVR Workshop, Wettzell, Germany, October 10, 2006

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