JWST Calibration Error Budget Jerry Kriss

15 March 20072/14 JWST Flux & Wavelength Calibration Requirements SR-20: JWST shall be capable of achieving data calibration into physical units with absolute accuracies shown in the following table. Table 8-2. Required Calibration Accuracies Absolute Calibration Accuracy Flux (%): Imagery Flux (%): Coronagraphic Imagery Flux (%): Spectroscopy Wavelength (% resolution element): Spectroscopy NIRCam55N/A NIRSpecN/A MIRI FGS-TFI510NA10 There are no requirements on absolute flux calibration for the FGS-Guider.

15 March 20073/14 JWST Astrometric Calibration Requirements Field Distortion Uncertainty MR-120 After calibration, the uncertainty in the Observatory field distortion correction within any SI and the guider shall not exceed arcsec RMS. (There are similar ISIM requirements on knowledge of relative placements of instruments in the focal plane.) These requirements enable the precise target acquisition required for multi-object spectroscopy with NIRSpec and coronagraphy with NIRCam, MIRI, and FGS-TFI.

15 March 20074/14 Required Error Budgets NIRCam  Imaging: Flux  Coronagraphy: Flux NIRSpec  MSA Spectroscopy: Flux and Wavelength  Fixed Slit Spectroscopy: Flux and Wavelength  IFU Spectroscopy: Flux and Wavelength MIRI  Imaging: Flux  Coronagraphy: Flux  Low-resolution Spectroscopy: Flux and Wavelength  Medium-resolution Spectroscopy (IFU): Flux and Wavelength FGS-TFI  Imaging: Flux and Wavelength  Coronagraphy: Flux and Wavelength Astrometric Calibration Both tied to OTE/PSF Stability Flux for both tied to OTE/PSF Stability All tied to OTE/PSF Stability Both tied to Pointing Error Budget All tied to OTE/PSF Stability. MSA & slits tied to Pointing Error Budget.

15 March 20075/14 Generic Error Budget Error in Final Result Stability Errors due to Detector Effects Background Subtraction Errors Routine Calibration Errors Absolute Flux Error Statistical Error Data Processing Errors Pointing Errors OTE/PSF Stability Function of Source Brightness

15 March 20076/14 Elements of the Error Budgets (1) Routine radiometric calibration  Ultimately limited by quality of other calibration observations.  For NIRCam, since the current known standards are too bright, a second tier of calibration standards will be required, which introduces more errors.  Can tie into other error budgets. E.g., pointing errors affect corrections for slit transmission.

Apply Mask Sub Bias/Dark Flat Correct Calculate Phot CR Identify Non-lin Correct NLINFILE BIASDARKFILE PHOTTAB FLATFILE CalNIRCamA raw file from 1 SCA; 1 reset of detector; M MULTIACCUM’s MASKCORR BDCORR FLATCORR PHOTCALC CRID NLINCORR Calibrated Image set Flags set as FITS header keywords Reference files used by pipeline Slope Fit SLOPEFIT Calculate Noise Ref Pixel Correct MASKFILE NOISFILE NOISCALC REFCORR REFCORRTYPE Basic JWST Image Processing

15 March 20078/14 Radiometric Flux Calibration The flux in spectral bin i is Inverse Sensitivity Slit transmission Low-order flat correction Count rate in spectral bin i Width of spectral bin i Corrected counts in pixel j Exposure time Fringe Flat Pixel Flat Non-linearity Raw counts Bias Level Ref. Pix. Corr.

15 March 20079/14 Radiometric Error Propagation Standard propagation of uncorrelated errors then gives the fractional error in the absolute flux as

15 March /14 Elements of the Error Budgets (2) Background subtraction  More important for fainter sources. Fractional errors scale as  Dominated by systematic errors for faint sources  Several potential sources:  “Sky” (includes scattered light from outside the instrument)  Internal scattered light  Overlapping spectra  Overlapping orders  Light leaking through closed shutters (similar to optical ghosts)

15 March /14 Elements of the Error Budgets (3) Detector Effects  Some can be characterized and corrected in routine processing:  Non-linearity  Intrapixel sensitivity  Others that are more unpredictable:  Persistence  Electronic ghosts & EMI pattern noise  Can depend on source brightness:  External signals can swamp source photons  Bright sources cause their own problems in non-linearity and persistence

15 March /14 Elements of the Error Budgets (4) Stability  May affect all aspects of routine calibration:  Flat fields  Flux calibration  Biggest external tie in is to the OTE and PSF Data Processing Errors  These are expected to be small, but we must manage them.  Examples: Approximations, round-off/truncation errors

15 March /14 Example: Error Tree for NIRSpec Flux Calibration

15 March /14 Conclusions JWST calibration requirements, although “lax” by HST standards, will be difficult to meet. The challenges:  For flux calibration, key concerns are:  Current standards are too bright for NIRCam.  PSF stability will have impacts at the few percent level.  Detector effects need to be characterized and quantified better.  Scattered light and backgrounds will be limitations for spectra, long-wavelength MIRI observations, and coronagraphy.  For wavelength calibration, knowledge of target placement is crucial. Relying on target acquisition is not enough.