1. 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, Ireland John C. Pearson, Jet Propulsion Laboratory, Pasadena, CA, USA Steven D. Lord, NASA Herschel Science Center, Caltech, Pasadena, CA, USA David Teyssier, Herschel Science Centre, ESAC, Madrid, Spain

2. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 2 Gascell calibration of the HIFI spectrometer Talk outline The Herschel HIFI instrument Gas cell calibration Removing instruments effects Extracting methanol line information Outlook and Conclusions

3. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 3 Gascell 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

4. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 4 Gascell 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 = 10 6 Double side band spectra

5. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 5 Gascell 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 Spectra LSB + USB Standing waves Side band ratio Double Side Band Spectra Deconvolution (χ2 minimization, MEM) Single Side band Spectra Spurious response

6. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 6 Gascell 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

7. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 7 Gascell calibration of the HIFI spectrometer HIFI calibration using a Gas cell Re-imager Gas cell HIFI in cryostat Hot/cold source Pumps Temperature sensors Gas cell was designed and built by the LERMA group in Paris and operated at SRON, Groningen

8. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 8 Gascell 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)

9. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 9 Gascell calibration of the HIFI spectrometer Example spectra, the good Example CH3OH spectra, the good

10. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 10 Gascell calibration of the HIFI spectrometer Example spectra, the good Example CH3OH spectra, the good

11. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 11 Gascell 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

12. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 12 Gascell calibration of the HIFI spectrometer Example CH3OH spectra, the spurious Spur completes saturates spectrometer CCD Second LO signal down mixing sky signal

13. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 13 Gascell 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 USBLSB The same line shown at different IF frequencies

14. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 14 Gascell calibration of the HIFI spectrometer Side band ratio, diplexer band USBLSB Large line intensity variation with IF frequency due to diplexer misalignment

15. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 15 Gascell calibration of the HIFI spectrometer Side band ratio Beamsplitter bands Diplexer bands

16. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 16 Gascell 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

17. 64th International Molecular Spectroscopy Symposium - June 22-26, 2009 Slide 17 Gascell 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