The prime objective of the JWST flux calibration plan is to enable high accuracy relative and absolute flux calibration for all science instruments, by converting science data from instrumental to physical units. The overall plan is described, including the list of the stellar flux calibration sources. 

See also: JWST Data Absolute Wavelength Calibration, JWST Data Calibration Considerations, JWST Science Calibration Pipeline

JWST has a suite of 4 scientific instruments covering near- and mid-IR wavelengths. From about 0.6 to 5.3 μm, NIRCam, NIRISS, and NIRSpec provide a variety of imaging, coronagraphic, and spectroscopic modes. From 5 to 28.5 μm, MIRI offers the same array of observing capabilities.

For all these instruments/modes, the nominal requirement is 2% absolute flux prediction accuracy of standard stars, with the goal of improving it as much as possible. This is the required prediction accuracy to achieve the overall flux calibration requirements as these include other terms beyond the flux prediction accuracy (aperture correction accuracy, stability, etc.). To achieve this, a sample of stars with well known absolute flux and spectral shape across the 0.6 to 28.5 μm range will be used. Having this unified program will not only enable cross-instrument calibration of the JWST science instruments, but also with HST, Spitzer and other ground-based telescopes. Details on how the JWST Science Calibration Pipeline applies the photometric calibration are given here.

JWST science instruments calibration goals

To achieve the 2% goal (and potentially push below 1% for the predicted fluxes), multiple calibrator stars of the same spectral class are needed to account for the differences between the actual stars and the adopted stellar model atmospheres. Calibrators of different spectral types are required to control for systematic uncertainties in the stellar atmospheres modelling.

The JWST flux calibration sample is composed by hot stars (white dwarfs and OB stars; see Table 1), A dwarfs (Table 2), and solar analogs (late F and early G dwarfs; Table 3). These spectral types can be modeled to high accuracy. Previous works on Hubble calibrators (Bohlin & Cohen 2008, Bohlin 2010) have shown that there is random modeling noise on the order of 2% for an individual calibration star. Observing 4 stars reduces the random uncertainty to 1% for the average JWST flux calibrations in each spectral bin. The sample covers the sensitivity range for all instruments, and at least 5 of them are observable with each filter/grating (Gordon et al. 2009; Gordon & Bohlin 2012). Gordon et al. (2022) covers the current plan for absolute flux calibration for JWST in detail and provides further information.

Table 2. JWST calibrators: A dwarfs

Table 3. JWST calibrators: Solar analogs

Table 4. Stars no longer considered as calibrators

SED details

Pure hydrogen white dwarfs (WDs) are straightforward to model. GD153, GD71, and G191B2B are hydrogen WDs and have temperatures and gravities derived from fitting the models to their observed Balmer lines. Their models are normalized to precision Landolt (1992) V band photometry and are the primary absolute flux standards for all of the HST flux calibrations (Bohlin et al. 1995). For the cases of the pure helium WD LDS749B, the A stars, and the G stars, their spectral distributions below 2.5 μm are measured from calibrated STIS and NICMOS spectrophotometry; then, the best fitting model is used to estimate fluxes longward of 2.5 μm. However, NICMOS is no longer available and STIS covers only wavelengths below 1 μm. Because the stellar models are most uncertain in regions of heavy line blanketing, broadband averages are used to find the best models, which match the observed fluxes to an rms scatter in the broad bands of less than 1% for all of our WD, A, and G standard stars. Thus, the continuum regions of the model extensions above 2.5 μm should be good to the same 1%–2% that is the quoted precision for STIS+NICMOS. 

Instrument sensitivities

Information about the JWST science instruments' sensitivities are provided in the individual instrument pages for NIRCam, NIRISS, NIRSpec, and MIRI. The proposed list of calibrators has been defined to optimally cover these sensitivity ranges for the different instrument modes, based on the following sensitivity levels:

Bohlin, R. C. et al. 1995, AJ, 110, 1316
White Dwarf Standard Stars: G191-B2B, GD 71, GD 153, HZ 43

Bohlin, R. C. & Cohen, M. 2008, AJ, 136, 1171 
NICMOS Spectrophotometry and Models for A Stars

Bohlin, R. C. 2010, AJ, 139, 1515 
Hubble Space Telescope Spectrophotometry and Models for Solar Analogs

Gordon, K. et al. 2009, JWST-STScI-001855 (PDF)
JWST Absolute Flux Calibration I: Proposed Primary Calibrators, 

Gordon, K. & Bohlin, R. C. 2012, JWST-STScI-002540 (PDF)
JWST Absolute Flux Calibration II: Expanded Sample of Primary Calibrators

Gordon, K.D., et al. 2022, AJ, 163, 267
The James Webb Space Telescope Absolute Flux Calibration.  I.  Program Design and Calibrator Stars

Landolt, A. U. 1992, AJ, 104, 340 
UBVRI Photometric Standard Stars in the Magnitude Range 11.5 < V < 16.0 Around the Celestial Equator

Mullally, S.E., et al. 2022, AJ, 163, 136
Searching for TESS Photometric Variability of Possible JWST Spectrophotometric Standard Stars