Radiometry and Gas Cell measurements

1. Radiometry and Gas Cell measurements D. Teyssier, ESAC On behalf of the AIV/ILT team

2. Radiometry Objectives • Over the full HIFI RF range (every 4 GHz in B1-5, 2.4GHz in B6-7) and for two chopper positions on M3: • Measure the receiver conversion gain and sensitivity (noise temperature Tsys) • Measure the beam coupling coefficients to the internal Hot (h) and Cold (c) loads. These parameters contribute to the intensity calibration uncertainty • Contribution to error budget • 1×hat any frequency • 0.2×c at 500 GHz 0.5×c at 1900 GHz Radiometry and Gas cell measurements

3. Radiometry Plans and test set-up • Use of an pair of external (reference) hot and cold black bodies to calibrate the internal load measurements and derive beam coupling factors. • Both external BB can accommodate all 7 beams + chop • Measure receiver sensitivity towards both pairs of loads Radiometry and Gas cell measurements

4. Radiometry Measurement strategy • The basic measurement sequence consists in taking total power spectra towards each of the 4 loads (order ~ 4 sec. long) Radiometry and Gas cell measurements

5. Radiometry Measurement strategy • The basic measurement sequence consists in taking total power spectra towards each of the 4 loads (order ~ 4 sec. long) • There are several limitations to this simple approach: • The total power receiver stability in bands 6-7 is expected to be shorter than that (order ~ 1 sec in 1 MHz channel) In those bands, we will use pairs of fast-chopped spectra (chopper phase of 1 sec) co-added over the required 4 (or more) sec. • Fast-chopped internal cold-internal hot loads • Fast-chopper internal cold-external hot • Fast-chopped internal cold-external cold As a consequence, redundancy of the measurement on the internal cold load Radiometry and Gas cell measurements

6. Radiometry Measurement strategy • The basic measurement sequence consists in taking total power spectra towards each of the 4 loads (order ~ 4 sec. long) • There are several limitations to this simple approach: • The rotation of the external cold flap and (to lesser extent) of the internal chopper induces baseline instabilities at IF level, that are not cancelled out when computing the load coupling coefficients Radiometry and Gas cell measurements

7. Radiometry Measurement strategy • The basic measurement sequence consists in taking total power spectra towards each of the 4 loads (order ~ 4 sec. long) • There are several limitations to this simple approach: • The rotation of the external cold flap and (to lesser extent) of the internal chopper induces baseline instabilities at IF level, that are not cancelled out when computing the load coupling coefficients A possible cure (TBC by tests) could consist in repeating the internal load measurements with both positions of the cold flap (closed and open). It adds 2 extra measurements in total power mode (B1-5), and one extra pair of fast-chop measurement in B6-7 (negligible overhead in regards of total measurement time). Radiometry and Gas cell measurements

8. Radiometry Measurement strategy • In agreement with the stability analysis team, we added a short (100-200 sec) bonus stability measurement at the end of each frequency step • This should provide a picture of the stability dependence over the RF range, at reasonable extra measurement time cost • Measurements will be grouped per LO sub-band (i.e. 1 sub-band per day or ½ day) to limit dead-times required for stabilization Analysis tools • The noise temperature spectra can be checked almost on-line using the QLA application of both HRS and WBS • The off-line analysis tool (IA environment) is ready and was already successfully used during the DM and QM. Refinements are needed to handle fast-chopped data, as well as the possible use of additional readouts to treat the baseline spur problem • Analysis team: E. Caux, F. Helmich, B. Larsson, M. Pérault, D. Teyssier, support from T. Marston for IA tool updates on short time scale Radiometry and Gas cell measurements

9. Gas cell Objectives • Measurement of the side-band gain ratio (Gssb) over all HIFI mixer band at as many LO frequencies as possible • After the error on planet model this is the next most critical calibration parameter • Contribution to error budget • 2×Gssb at any frequency • Measurement of an unbiased methanol spectral survey over the complete available HIFI tunable frequency range • Exercise spectral survey observing mode (incl. data processing) • Ultimate validation of HIFI as a high-resolution spectrometer in the 0.5-1.9 THz range Radiometry and Gas cell measurements

10. Gas cell Plans and test set-up • Use of dedicated gas cell combined to an external black body pair and a re-imager coupling the beam to the FPU • Complete operation under vacuum • Gaussian beams kept within 4 criterion • Windows designed to avoid standing waves • Gas cycle in and out the gas cell is automated, allowing fairly reproducible measurements. Automation was checked without cell. Representative cycle TBD. • Gas physical conditions (pressure, temperature, etc) are accurately monitored (of interest for a posteriori gas modelling) Radiometry and Gas cell measurements

11. Gas cell Measurement strategy • The basic measurement sequence consists in taking total power spectra towards each of the 2 loads (hot and cold loads) through the cell cavity with and without gas (4 spectra total) Radiometry and Gas cell measurements

12. Gas cell Gas cell Measurement strategy • The basic measurement sequence consists in taking total power spectra towards each of the 2 loads (hot and cold loads) through the cell cavity with and without gas (4 spectra total) • Limitations to this approach: • The LO-mixer cavity creates LO-power modulation translating into side-band ratio modulation: a measurement at a single LO setting can sit anywhere on the sinusoidal pattern We plan to repeat the measurement at 5-10 frequency steps (no re-tuning) over the expected ripple period (~200 MHz) to average out the spurious modulation Each measurement can remain short (2-4 sec.) S/N not expected to be a problem in final spectrum Radiometry and Gas cell measurements

13. Gas cell Gas cell Gas cell Measurement strategy • The basic measurement sequence consists in taking total power spectra towards each of the 2 loads (hot and cold loads) through the cell cavity with and without gas (4 spectra total) • Limitations to this approach: • As for the radiometry, drift on short time scale will affect the measurements in B6-7 • Unlike the radiometry, we cannot fast-chop in the gas cell setup because the moving optics are controlled by a TEI (not synchronized with backend readout either) elementary readout time for one total power integration can be at most decreased to 1 sec. if HRS is not used (WBS # of channel is already reduced by essence in B6-7) Radiometry and Gas cell measurements

14. Gas cell Measurement strategy: gas in use • The baseline consists in using saturated lines • OCS is the main species up to 900 GHz (above this, unsaturated) • CO and 13CO will be used wherever it offers line up to Band 7 • Methanol believed to offer very strong lines above 1.5 THz • SO2 is currently considered as a backup for high frequency bands • H2CO (formaldehyde) could be used as a backup using a copy of the KOSMA gas cell. However it implies a totally different operational sequence (gas cycling not synchronized with backend readout) • However, even at frequencies when some species will not saturate (typically above 1 THz) side-band ratio measurements will be performed for every available transition • A posteriori modelling of the line opacity believed to be possible with good (better than 1%) accuracy provided physical parameters of the gas (and gas path) well controlled Radiometry and Gas cell measurements

15. Gas cell Measurement strategy: targeted frequencies • Whenever possible, side-band ratio measurements will focus on frequencies close to those tunings expected to be heavily demanded by the science • For the methanol survey, redundancy will be above 4, and LO tuning irregularly spaced (Comito & Schilke 2002) • Many measurements of the methanol survey are potential side-band ratio measurements. Gas pressure will be optimized for side-band ratio issue Radiometry and Gas cell measurements

16. Gas cell Measurement strategy: targeted frequencies • As for the radiometry campaign, measurements will be grouped per LO sub-band (i.e. 1 sub-band/day) to limit dead-times required for stabilization Analysis tools • The off-line analysis tool (IA environment) is ready although it could never be tested on real data (no sample in the database yet). • Refinements are needed to handle the combination of side-band ratio measurements taken at various LO steps (rebinning, etc). • Side-band deconvolution tool is available under IA (IPAC contribution). Expert is in the team. • Line modelling could be done a posteriori using in-house tools from the team members, or CASSIS (treatment of absorption lines still need to be updated). • Analysis team: E. Caux,C. Comito, E. Dartois, J. Pearson, M. Pérault, D. Teyssier, support from T. Marston for IA tool updates on short time scale Radiometry and Gas cell measurements