Webb Flux Calibration Plan Status Karl D. Gordon & Ralph Bohlin 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 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

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

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

Sample

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

NIRSpec Good coverage for bright and faint fluxes

FGS/TFI Need more standards at bright and faint levels

MIRI Imaging Need more standards at bright levels

MIRI Coronagraphy & Spectroscopy Need more standards at bright levels

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

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

Prediction with Spitzer Photometry

Predicted/Observed

Summary “JWST Absolute Flux Calibration I. Proposed Preliminary Calibrators”  Gordon, Bohlin et al. (2009, JWST-STScI ) 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.)

MIRI Sensitivities

Predicted Spectra Most of sample

Predicted/Observed (sigma)

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.

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

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

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 ( 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

Sample

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)

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