gas cell observations of methanol from 0.6 to 1.9 thz using the herschel space observatory hifi...

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



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



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



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



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



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



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)



Example spectra, the good

Example CH3OH spectra, the good



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



Example CH3OH spectra, the spurious

Spur completes saturates spectrometer CCD

Second LO signal down mixing sky signal



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



Side band ratio, diplexer band

USBLSB

Large line intensity variation with IF frequency due to diplexer misalignment



Side band ratio

Beamsplitter bands Diplexer bands



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



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

gas cell observations of methanol from 0.6 to 1.9 thz using the herschel space observatory hifi...

Documents

gas cell spectra

band spectra ssb spectra

gas cellusing hifi

band dsb spectra

final spectra

gas cellgas cell

conclusionsgascell calibration

bandspectragascell calibration