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Gas cell observations of methanol from 0.6 to 1.9 THz using the Herschel space
observatory HIFI instrument
Ronan D. Higgins, NUI Maynooth, Co. Kildare, IrelandJohn C. Pearson, Jet Propulsion Laboratory, Pasadena, CA, USASteven D. Lord, NASA Herschel Science Center, Caltech, Pasadena, CA, USADavid Teyssier, Herschel Science Centre, ESAC, Madrid, Spain
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 2Gascell calibration of the HIFI spectrometer
Talk outline
• The Herschel HIFI instrument• Gas cell calibration• Removing instruments effects• Extracting methanol line information• Outlook and Conclusions
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 3Gascell calibration of the HIFI spectrometer
Herschel Space Observatory
• Largest telescope in space – twice the area of the Hubble
• Cryo-cooled detectors . E.g. to 235 mK absolute.
• Two cameras/medium resolution spectrometers (PACS and SPIRE) and a very high resolution hetero dyne spectrometer (HIFI)
• Resides 1.6 million km outward from Earth co-rotating in a gravity well (L2), Ideal for studying the cold molecular universe
• 3.5-5 year mission (depending on helium consumption) 1.6 billion euro cost
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 4Gascell calibration of the HIFI spectrometer
The HIFI Spectrometer• Heterodyne Instrument for the Far Infrared (HIFI)• HIFI has a frequency range of 480 to 1250 GHz and 1410-1910 GHz• What are Heterodyne techniques?
• Using Heterodyne techniques gives HIFI very high spectral resolution, R=lambda/delta_lambda = 106
Double side band spectra
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 5Gascell calibration of the HIFI spectrometer
Calibrating HIFI• HIFI produces a double side band (DSB) spectra, both the
upper and lower side band are folded into the final spectra. • By taking spectra at multiple LO frequencies and analyzing
the relative shift in spectra one can deconvolve the DSB spectra into single side band spectra (SSB) spectra, however prior knowledge of instrument behavior is needed
SpectraLSB + USB
Standing waves Side band ratio
Double SideBand Spectra
Deconvolution(χ2 minimization, MEM)
Single Side bandSpectra
Spurious response
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 6Gascell calibration of the HIFI spectrometer
HIFI calibration using a Gas cell• Using HIFI as a laboratory spectrometer attached to a gas cell was
used to calibrate the instrument
• As molecular lines are constants of nature and given knowledge of the pressure, temperature and column density coupled with the line frequency and intensity (taken from the molecular model) and self broadening parameters one can probe the instrument response
• Comparing modeled to observed spectra returns information on the instrument line width, standing waves, side band ratio, spurious response (multiple LO signals)
• Additionally the gas cell spectra can be used to develop data pipelines and analysis scripts to be used on future astronomy data
• Finally as this is the first look at a number of spectra in this range, the gascell campaign can be used to perform some hot/cold load calibrated spectroscopy
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 7Gascell calibration of the HIFI spectrometer
HIFI calibration using a Gas cell
Re-imager Gas cell
HIFI incryostat
Hot/cold source
Pumps
Temperaturesensors
Gas cell was designed and built by the LERMA group in Paris and operated at SRON, Groningen
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 8Gascell calibration of the HIFI spectrometer
Calibration gas selection• Calibration gas selection is driven by a number of criteria such as
– safety (toxic?), effect on gascell, will it stick to cell walls?
– number of spectral lines present, cost/availability
– molecule complexity, can molecule be accurately modeled?
Over 6000 spectra taken (5000 CH3OH)
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 9Gascell calibration of the HIFI spectrometer
Example spectra, the good
Example CH3OH spectra, the good
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 10Gascell calibration of the HIFI spectrometer
Example spectra, the good
Example CH3OH spectra, the good
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 11Gascell calibration of the HIFI spectrometer
Example CH3OH spectra, the standing waves
Optical standing waves
Electrical standing waves, described in more detail in the paper, “Electrical standing waves in the HIFI HEB mixer amplifier chain” SPIE, Vol. 7215
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 12Gascell calibration of the HIFI spectrometer
Example CH3OH spectra, the spurious
Spur completes saturates spectrometer CCD
Second LO signal down mixing sky signal
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 13Gascell calibration of the HIFI spectrometer
Side band ratio• Side band ratio is a measure of the imbalance between
the upper and lower side band (USB, LSB).
• Side band ratio is an inherent mixer property which tend to extremes towards the mixer band edges
• Mapping the side band ratio was one of the key goals of the gas cell campaign
• By taking spectra at multiple LO frequencies one can measure the side band ratio using the line intensity
USBLSBLSBLSB
The same line shown at different IF frequencies
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 14Gascell calibration of the HIFI spectrometer
Side band ratio, diplexer band
USBLSB
Large line intensity variation with IF frequency due to diplexer misalignment
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 15Gascell calibration of the HIFI spectrometer
Side band ratio
Beamsplitter bands Diplexer bands
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 16Gascell calibration of the HIFI spectrometer
Side band ratio
• A number of test observations are proposed for the In-flight phase of HIFI to characterize the side band ratio
• It is proposed to take spectra at much higher LO granularity to track the variation across the upper and lower side bands
• Using this data in conjunction with an optical model of the diplexer it possible to remove the diplexer effects and analyze the inherent mixer side band ratio
64th International Molecular Spectroscopy Symposium - June 22-26, 2009
Slide 17Gascell calibration of the HIFI spectrometer
Conclusions
• The gas cell campaign was largely successful from an instrument calibration point of view and gave the calibration team a head start in solving various instrument issues
• The gas cell campaign generated a large amount of quality methanol data between 0.6 and 2.0 THz
Future Work• Using In-Flight data and gas cell measurements develop optical
model to predict distorted line intensities for band 3,4,6,7• Fit line profiles to bands 1 & 2 and extract self broadening
parameters. Eventually progressing to the other bands, up to 2.0 THz
• Investigate the line intensities of the higher torsional states