IRMA 20IRMA 20µm Water Vapour Radiometer µm Water Vapour Radiometer Operations in the TMT Site Testing CampaignOperations in the TMT Site Testing Campaign

Richard Querel, David Naylor, Robin Phillips,Richard Querel, David Naylor, Robin Phillips,

Regan Dahl, & Brad GomRegan Dahl, & Brad Gom

Astronomical Instrumentation Group,Astronomical Instrumentation Group,

University of Lethbridge,University of Lethbridge,

Lethbridge, Alberta, CanadaLethbridge, Alberta, Canada

Infrared Radiometer forInfrared Radiometer forMillimetre AstronomyMillimetre Astronomyirma

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IRMA ConceptIRMA Concept• Measure emission from water vapour Measure emission from water vapour

lines in the 20lines in the 20μm atmospheric windowμm atmospheric window

• Band-pass includes only water Band-pass includes only water vapourvapour transitionstransitions

• Theoretical atmospheric model Theoretical atmospheric model supported by FTS measurements from supported by FTS measurements from Mauna Kea (Naylor Mauna Kea (Naylor et. al.et. al. PASP 96, PASP 96, 167 (1984))167 (1984))

PASP vol. 96, Feb. 1984, p. 167-173

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AdvantagesAdvantages

• Operates at 20 Operates at 20 μμm; near the peak of the m; near the peak of the Planck function for atmospheric temperaturesPlanck function for atmospheric temperatures

• Wide bandwidth (~1 Wide bandwidth (~1 μμm) …m) … ↑↑ signal-to-noisesignal-to-noise

• Photoconductive detectors offer simplicity, Photoconductive detectors offer simplicity, high speed, sensitivity and stabilityhigh speed, sensitivity and stability

• Zero RF interferenceZero RF interference { 20 µm = 15 THz183 GHz = 1.6 mm

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IRMA I (1997-1999):IRMA I (1997-1999):

• Water vapor sensitivity noise-limit Water vapor sensitivity noise-limit (1 sec integration): (1 sec integration):

• 1.8 1.8 m PWV at 0.5 mm PWVm PWV at 0.5 mm PWV

• 3.0 3.0 m PWV at 1.0 mm PWVm PWV at 1.0 mm PWV

IRMA II (2000-2001): IRMA II (2000-2001):

• Water vapor sensitivity noise-limit Water vapor sensitivity noise-limit (1 sec integration): (1 sec integration):

• 0.26 0.26 m PWV at 0.5 mm PWVm PWV at 0.5 mm PWV

• 0.44 0.44 m PWV at 1.0 mm PWVm PWV at 1.0 mm PWV

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Current IRMA Design

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BTRAM Output for Mauna Kea SiteBTRAM Output for Mauna Kea Site

500 cm500 cm-1-1 = 20 = 20 μμm m ~ Peak of Planck Curve~ Peak of Planck Curve

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Steps required to convertSteps required to convertVoltage Voltage →→ PWV PWV

((allall are possible sources of error) are possible sources of error)

Voltage Voltage →→ Flux Flux• Assume linear Assume linear

detector responsedetector response• Hot & ambient Hot & ambient

BB readingsBB readings• Need accurate Need accurate

temperature of BBtemperature of BB

Flux Flux →→ PWV PWV• Atmospheric modelAtmospheric model• Surface T & PSurface T & P• Instrument ResponseInstrument Response• AAΩΩ (Throughput) (Throughput)

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Typical Calibration CycleTypical Calibration CycleV

olta

ge (

V)

Vol

tage

(V

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BB TBB TAmbientAmbientBB TBB THotHot

Shutter CloseShutter Close

SkySky SkySky

Shutter Open Shutter Open & BB Off & BB Off

Typical Calibration CycleTypical Calibration Cycle

BB OnBB On

Vol

tage

(V

)V

olta

ge (

V)

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For the TMT Site Testing required resolution of 0.1mm @ 1.0mm PWV, we need to know

the effective BB T < 0.5K

2020

Effective Temperature of Blackbody?Effective Temperature of Blackbody?

• Center sensor = 50.6°C; Edge sensor = 45.8°CCenter sensor = 50.6°C; Edge sensor = 45.8°C

• Emission calculated for each pixel to determine the Emission calculated for each pixel to determine the total flux emitted from the blackbody.total flux emitted from the blackbody.

• Determined effective (uniform) surface T = 48.7°CDetermined effective (uniform) surface T = 48.7°C

(Data from a 7-14(Data from a 7-14μμm Fluke Ti-20 Thermal Imager)m Fluke Ti-20 Thermal Imager)

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Normal calibration (High T = 305.9 K)Normal calibration (High T = 305.9 K)

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Modified calibration (High T = 302.4 K; -3.5K)Modified calibration (High T = 302.4 K; -3.5K)

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5 days of data using 5 days of data using the “Normal” the “Normal” calibration method calibration method and the sensor and the sensor temperaturestemperatures

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5 days of data,5 days of data,

with a modifiedwith a modified

(-3.5K) Unit 1(-3.5K) Unit 1

Hot-temperatureHot-temperature

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IRMA Cross-CalibrationIRMA Cross-Calibration

• IDL MPFIT of IDL MPFIT of offsetoffset and and gaingain between VISIR between VISIR data and Gaussian-convolved BTRAM datadata and Gaussian-convolved BTRAM data

• Reduced Reduced ΧΧ22

= ~0.0959= ~0.0959

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BTRAM

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BTRAM FactsBTRAM Facts

• BBlueSky lueSky TTransmittance & ransmittance & RRadiance adiance AAtmospheric tmospheric MModelodel

• Built in IDLBuilt in IDL• Available for Windows & LinuxAvailable for Windows & Linux• Line-by-line layer-by-layer Radiative TransferLine-by-line layer-by-layer Radiative Transfer• Able to simulate:Able to simulate:

– Atmospheres (7 primary gases)Atmospheres (7 primary gases)– Laboratory Gas Cells (37 molecules)Laboratory Gas Cells (37 molecules)– Transmission / Emission / OpacityTransmission / Emission / Opacity– Batch mode to create data-cubesBatch mode to create data-cubes

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BTRAM FactsBTRAM Facts

• Distributed with HITRAN 2004 Database, Distributed with HITRAN 2004 Database, any spectral database can be usedany spectral database can be used

• Contains 1,734,469 spectral lines for 37 Contains 1,734,469 spectral lines for 37 different moleculesdifferent molecules

• 6 built-in FASCODE Atmospheric Profiles6 built-in FASCODE Atmospheric Profiles– Mid-Latitude Summer (& Winter)Mid-Latitude Summer (& Winter)– Subarctic Summer (& Winter)Subarctic Summer (& Winter)– TropicalTropical– US StandardUS Standard

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BTRAM FactsBTRAM Facts

• Site-specific Atmospheric Profiles includedSite-specific Atmospheric Profiles included– Antarctic SummerAntarctic Summer– Chajnantor WinterChajnantor Winter– Mauna KeaMauna Kea

• Customized Atmospheric Profiles can be Customized Atmospheric Profiles can be imported as comma-delimited text (.csv)imported as comma-delimited text (.csv)

• Output spectra can be exported as Grams Output spectra can be exported as Grams compatible .spc file, or as a text file.compatible .spc file, or as a text file.

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BTRAM vs. FASCODEBTRAM vs. FASCODE

• Difficult to change Difficult to change FASCODE layeringFASCODE layering

• BTRAM used BTRAM used FASCODE FASCODE atmospheric layer atmospheric layer parameters in a parameters in a comparison to comparison to ensure its accuracyensure its accuracy

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Conclusion & Future workConclusion & Future work

• Calibration depends on our hot blackbodyCalibration depends on our hot blackbody

• Uniform “hotter” blackbody is necessaryUniform “hotter” blackbody is necessary

• Atmospheric parameters / model errors?Atmospheric parameters / model errors?

• Lunar spectrophotometer useful calib tool?Lunar spectrophotometer useful calib tool?