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Webb Flux Calibration Plan Status

Karl D. Gordon & Ralph Bohlin. Webb Flux Calibration Plan Status. WIT Team Meeting STScI 17 Nov 2009. Goals. Enable the highest accuracy possible flux calibration for all Webb instruments Need a sample of stars with well known fluxes from 0.8-28 microns

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Webb Flux Calibration Plan Status

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  1. Karl D. Gordon & Ralph Bohlin Webb Flux Calibration Plan Status WIT Team Meeting STScI 17 Nov 2009

  2. Goals • Enable the highest accuracy possible flux calibration for all Webb instruments • Need a sample of stars with well known fluxes from 0.8-28 microns • Nominal requirement is 2% absolute flux accuracy • Push for a uniform Hubble/Spitzer/Webb calibration • Allows for comparison of results between telescopes • Multi-wavelength astronomy! • Interface with Instrument Team efforts • Mainly MIRI & NIRCam

  3. Flux Calibration Basics • Absolute Flux Calibration = knowledge of conversion between DN/sec and physical units for the range of possible fluxes • Need: • Source with known flux at the wavelengths of interest • Measure the DN/sec with your instrument • Divide the known flux by DN/sec → calibration factor • Sources with known fluxes (reference standards) • Referenced to laboratory calibrated black bodies • Vega – good for V, K, & 10 micron only • MSX 8 micron measurements • Secondary sources • Established via relative measurements to above reference standards • Use models to interpolate/extrapolate to wavelengths of interest

  4. JWST Absolute Flux Calibration I.Proposed Primary Calibrators • Set of 14 stars with high quality Hubble/Spitzer data • 4 white dwarfs (including 3 primary calibrators for Hubble) • 6 A-stars (primary Spitzer/IRAC calibrators) • 4 solar analogs (including NICMOS primary calibrator) • Use different types to control systematic uncertainties • How do these stars map to the Webb instrument sensitivities? • Maximum flux: saturation in the smallest subarray • Minimum flux (imaging): S/N=200 in 1 hour • Minimum flux (spectroscopy): S/N=50 in 1 hour (resolution element) • Gordon, Bohlin, Fullerton, Beck, & Robberto • JWST-STScI-001855

  5. Sample

  6. NIRCam • Good coverage for bright fluxes • Need more standards at faint levels to check for flux nonlinearities

  7. NIRSpec • Good coverage for bright and faint fluxes

  8. FGS/TFI • Need more standards at bright and faint levels

  9. MIRIImaging • Need more standards at bright levels

  10. MIRICoronagraphy&Spectroscopy • Need more standards at bright levels

  11. JWST Absolute Flux Calibration II.Expanded List of Calibrators • Metric on what is needed per instrument capability • How many stars? • How many of different types? • Range of brightnesses? • Add more bright/faint stars • MIRI George Rieke's work • NIRCam solar analogs in clusters NICMOS work • Suggestions for other targets? • Write report • Nominal due date is Jul 2010

  12. Webb/Spitzer/Hubble Cross-calibration • Program started by Ralph & Jerry K. for Webb/Hubble • Continued in cycle 17 with more STIS observations • Spitzer observations added to complete the set • Cycle 5 DDT program • Was accepted as part of the regular Spitzer calibration program • Team: Gordon, K. D., Bohlin R., Rieke G.,Carey, S., Armus, L., Ardila, D., Noriega-Crespo, A., Deustua, S., Engelbracht, C., Meixner, M., Flanagan, K. • Stated plan is to write a single paper establishing a common Spitzer/Hubble calibration (Bohlin et al.) • Assuming both Spitzer and Hubble calibrations need to be slightly adjusted • Status • Spitzer photometry done (some cleanup needed) • Hubble/STIS cycle 17 spectroscopy in progress

  13. Prediction with Spitzer Photometry

  14. Predicted/Observed

  15. Summary • “JWST Absolute Flux Calibration I. Proposed Preliminary Calibrators” • Gordon, Bohlin et al. (2009, JWST-STScI-001855) • Hubble/Spitzer cross-calibration program • Spitzer cycle 5 DDT time + archive data • Hubble cycle 17 time + archive data • Plan is for a refereed paper (Bohlin, Rieke, Gordon, et al.) • cal.stsci.edu • Twiki site for Webb calibration efforts • Goal is to have a place for internal and external collaboration • WebbFluxCal: Assembling the set of calibration stars (reports I & II) • AbsLevel: Setting the absolute flux level • HubbleSpitzerCal: Hubble/Spitzer cross calibration • Others? (astrometry, wavelength, etc.)

  16. MIRI Sensitivities

  17. Predicted Spectra • Most of sample

  18. Predicted/Observed (sigma)

  19. Calibration Challenges • How does one derive the absolute calibration for an instrument? • How does this differ between photometry and spectroscopy? • How does this differ between point and extended sources? • What is a good absolute calibration goal (5%, 1%, or 0.1%) ? • Think science instead of requirements. • (But also good to make sure we reach our requirements.) • How many objects is a good number to avoid systematic biases? • Think Vega = pole-on rapid rotator with a disk! • What is the difference between calibration and characterization? • How do Hubble and Spitzer differ in their calibration strategies? • Hubble = UV/Opt community, Spitzer = IR community • What has already been done for JWST (by instrument)? • JWST will necessarily use indirect calibrators.

  20. Calibration versus Characterization • Calibration is to get the conversion between instrument and physical (Jy) units • Characterization is the process to make sure this calibration applies to the range of observations that can be taken • Range of fluxes (bright to faint) • Different exposure times/readout patterns • Subarrays • Targets of different colors (blue/red - asteroids, QSOs, etc.) • Extended sources

  21. Calibration in the IR • IRAS • Stars and asteroids • Indirect calibration • ISO • Stars and asteroids • Indirect calibration • MSX • Large calibration effort = NIST blackbodies in space! • Direct calibration • Spitzer • Stars and asteroids (MIPS 160um only – blue leak) • Indirect calibration

  22. Absolute Physical Calibration in the Infrared • Rieke, G. et al. 2008, AJ, 135, 2245 • Consistent calibration of A dwarf and solar analogs (1.5%) • Based on direct calibrations in the infrared • Ground-based at 2.2 and 10um • Space-based at 4-21um (MSX) • Used to be done by extrapolating optical direct calibrations • Fluxes quoted in the “Vega” system • Mythical star with Kurucz 1993 model spectrum of an A0 star (Kurucz 2005) with Teff = 9550, log g = 3.95, log z = -0.5 • Normalized to corrected (for debris disk) direct Vega flux measurements • Solar analogs confirm “Vega” results • Solar spectrum (0.2-2.5 um) from Thuillier et al. (2003, satellite) • Sparser measurements and models for longer wavelengths • Solar analog star color scatter checked with 2MASS/IRAC/MIPS • Large sample of A dwarf and solar analog stars measured by IRAC/MIPS used to generate the zero points for IRAC/MIPS

  23. Sample

  24. Next Steps • Can all the Webb instruments observe the set of primary calibrations defined by the Webb/Spitzer/Hubble program? • What are the existing instrument team plans for calibration? • Are there supporting observations needed prior to Webb? • Are there modeling efforts needed prior to Webb? • How do we bootstrap the calibration to fainter Webb levels? • And we shouldn't forget • Astrometric calibration (Jay) • Commissioning (Carl)

  25. Karl D. Gordon Space Telescope Science Institute Status of Webb Calibration Plans WIT Calibration Kickoff STScI 17 Dec 2008

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