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Calibration-Diagnostics

This document discusses various calibration and diagnostic methods essential for spectral and polarimetric observations in astrophysics. It details the use of empirical and hardware calibrators, emphasizing their applications in the NIRSPC/G setups for disk and corona modes. The accuracy requirements for photometric, gain, and polarimetric calibration are highlighted, particularly the challenges associated with achieving the highest precision levels (10^-4). The importance of using solar data and advanced algorithms for calibration and diagnostics is emphasized, along with the significance of minimizing scattered light in IR disk and coronal polarimetry.

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Calibration-Diagnostics

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  1. Calibration-Diagnostics • Spectral • Photometric/Gain • Polarimetric • Empirical or hardware calibrators • NIRSPC or G • Disk or Corona

  2. Empirical or special hardware • Spectral • Sun is best source • Most accurate • Not always available • Th-Ar and laser sources should be available • From prime focus they make good system+instrument diagnostics • Mostly solar reference, but lamp and laser reference from prime focus will be used by NIRSPC/G for setup and diagnostics in all disk and coronal modes

  3. Calibration • Photometric/Gain • No absolute photometric gain calibration requirement (throughput requirement ) • Flat-field • Spatial, use prime focus Halogen (1% accuracy) • Solar data for 10^-3 accuracy (KLL algorithm) • Spectral, use prime focus Halogen, (KLL TBD) • In general highest accuracy flat-fields result from data algorithms. Prime focus halogen flat-field screen useful for diagnostic and spectral calibration

  4. Calibration • Polarimetric • Highest accuracy calibration requirement: 10^-4 • External calibrator has little chance to achieve this (note wavelength, field, and time dependence of ATST Mueller matrix) – although empirical techniques haven’t yet met this goal either • Important cross terms in Mueller are different for disk and coronal observations, empirical calibration schemes will be different

  5. Empirical IR Disk Polarimetry Calibration • V often dominates raw spectropolarimetry signal • Fully resolved spectropolarimetric IR line profiles necessary (e.g. from sunspot umbra) • Assumes no pi component in V line profile • Algorithm tested and used to approximately “few” x 10^-3 accuracy • Sol. Phys. 153, 143 (1994) • Limitations: low scattered light …?

  6. IR disk polarimetry calibration

  7. IR Coronal polarimetry calibration • Q,U often dominates (by a large factor) raw V polarimetric signature • Spectropolarimetric line profiles not resolved, weak-field regime only, although disk calibration technique might be used and extrapolated to needed IR wavelengths

  8. Echelle Grating Camera Lens Collimator NICMOS3 IR camera Fiber Bundle SOLARC Lessons Secondary mirror Prime focus inverse occulter/field stop Re-imaging lens LCVR Polarimeter Input array of fiber optics bundle Primary mirror

  9. May 6 2004 Observations • Full Stokes vector observations were obtained on April 6, 2004 on active region NOAA 0581 during its west limb transit. • Stokes I, Q, U, & V Observation: • 20arcsec/pixel resolution • 70 minutes integration on V • 15 minutes integration on Q & U • Stokes Q & U Scan: • RV = 0.25 R • From PAG 250° to 270° • Five 5° steps Fe X 171Å image of the solar corona at approximately the time of SOLARC/OFIS observation from EIT/SOHO. The rectangle marks the target region of the coronal magnetic field (Stokes V) observation.

  10. IR Coronal Polarimetry

  11. Spectral Profiles and Mueller Crosstalk V I FeXIII U Q V profile shift: few x 0.1 pixel

  12. Recovered V profile:<0.0001 crosstalk correction B=4.6G

  13. Measuring Crosstalk v,vfit I/10 v q u

  14. Results: Coronal Magnetograms B=4,2,0,-2 G

  15. Conclusions • Empirical IR disk crosstalk calibration possible at few x 10^-4 • Low scattered light critical, umbral observation • Empirical IR coronal crosstalk calibration likely at few x 10^-5 • Wavelength stability critical

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