NIRCam's flux calibration is based on observations of standard stars. Flux calibration and zeropoints are periodically updated in the JWST pipeline to reflect new observations and/or analysis.

Calibrated image units and conversions

JWST pipeline-calibrated NIRCam images are in surface brightness units, specifically MJy/sr (that is, MegaJanskys / steradian) per average pixel, as noted in the FITS "SCI" extension header keyword BUNIT in stage 2 pipeline products: "cal.fits" and "i2d.fits" images. It is appropriate to use average pixels since flat fielding corrects for variations in responsivity and areal coverage between pixels.

To measure photometry in MJy, convert the image to units of MJy/pixel by multiplying by the average area of a pixel in sr, a constant provided in the FITS header keyword PIXAR_SR. Note this value varies slightly (by a few percent) among the 8 short wavelength detectors and between the 2 long wavelength detectors. They correspond approximately to the nominal pixel sizes: 0.031" and 0.063", for the short and long wavelength channels, respectively.

AB magnitudes

Flux densities in MJy can be converted to AB magnitudes using Astropy units. Explicitly:

mag_AB = –6.10 – 2.5 log10( flux[MJy/sr]*PIXAR_SR[sr/pix] ) = ZP_AB – 2.5 log10(flux[MJy/sr]),

where ZP_AB is the zeropoint:  ZP_AB = –6.10 – 2.5 log10(PIXAR_SR[sr/pix]). For example, ZP_AB = 28.0 for PIXAR_SR = 2.29e-14 (corresponding to pixel size 0.0312").

Vega magnitudes

STScI delivers "Vega" zeropoints that use Sirius as the color reference (Rieke et al. 2022), scaled to Vega. These are computed using the CALSPEC model "sirius_stis_005.fits", which defines magnitude -1.395 at all wavelengths. Therefore, mag_Vega-Sirius = mag_Sirius - 1.395 (or flux_Vega-Sirius = flux_Sirius / 3.6141).  The "Vega" magnitudes reported in the catalog output in stage 3 of the JWST pipeline uses these Vega-Sirius zeropoints. A link to the table of Vega-Sirius zeropoints is provided below. Vega-Sirius magnitudes (referred to as just "Vega" in the equations below) can be obtained from flux densities in MJy/sr using the following:

mag_Vega = -2.5 * log10(flux[MJy/sr] * PIXAR_SR[sr/pix] / flux_Vega[MJy]).

NIRCam images in units of MJy/sr are derived from data rate images (rate.fits), in units of counts/s  (DN/s). The conversion factor is stored in the FITS header under the PHOTMJSR, keyword, which has units of (MJy/sr) / (DN/s).  Therefore,

flux(DN/s) = flux[MJy/sr] / PHOTMJSR.

One can then define the instrumental magnitude mag_inst = -2.5 * log(flux[DN/s])

and obtain again mag_Vega as

mag_Vega = mag_inst + ZP_Vega

where ZP_Vega is the Vega zeropoint:

ZP_Vega = -2.5 * log10(PHOTMJSR*PIXAR_SR / flux_Vega[MJy])

Absolute flux calibration

PHOTMJSR is derived by observing standard stars and comparing the measured DN/s to an appropriate CALSPEC model. This provides the correct conversion factor C_FD in units Jy/(DN/s) for point sources. The calibration factor for extended sources (surface brightness) PHOTMJSR corresponds to C_FD/A_pix, where  A_pix is the average solid angle of a pixel. See Gordon et al. (2022) for a description of the absolute flux calibration plan.

The JWST stage 2 pipeline stores the PHOTMJSR values in the "jwst_nircam_photom_####.fits" reference files, where #### indicates a file number. These reference files can be viewed in CRDS. The CRDS context history describes updates to reference files, and the context used to process a given file is stored in the CRDS_CTX FITS header keyword. Observers can subscribe to updates following the instructions at JWST Science Calibration Pipeline.

Calibration updates

Calibration is an ongoing process that improves as more data are obtained. During commissioning, the flux calibration was derived by placing standard stars on NIRCam detectors B1 (in the shortwave channel) and B5 (in the longwave channel). The standard stars used in commissioning were G-type stars P330-E and P177-D. Observations of stars in the LMC calibration field were used to translate the flux calibration to the other 8 NIRCam detectors. That initial calibration was delivered in CRDS context "jwst_0942.pmap". The observations used for that update can be found in JWST Commissioning Program ID 1074 (to access this program, go to the Program Information page and enter "1074" in the ID Number field). 

Early in Cycle 1, the calibration was improved by including observations of additional standard stars, each imaged directly on all 10 NIRCam detectors. This update was delivered via "jwst_0995.pmap", and included a subset of the data described in Table 1. The latest update for imaging and coronagraphy was delivered via "jwst_1490.pmap" in March 2026, and includes all standard stars and observing programs listed in Table 1 (Boyer et al. 2026). The latest grism calibration update was delivered via "jwst_1478.pmap" in February 2026, and is based on the solar analog standard star P330-E (Pirzkal et al. 2026). 

Note that all absolute flux calibration updates use the filter curves posted on the NIRCam Filters page. 

Table 1. Standard Stars & program IDs included in the flux calibration delivered in CRDS context "jwst_1490.pmap"

Zeropoints

Since the total system throughput is slightly different for every detector, the PHOTMJSR values and zeropoints are also slightly different for every detector. There are 136 different filter+detector combinations, and the zeropoints for each are available in the files below.  Note that fluxes measured in subarrays can be slightly offset from those measured with the Full detectors, up to about 1% (see Boyer et al. 2026). Subarray-dependent PHOTMJSR values were therefore delivered starting with the March 2026 delivery (pmap 1490). 

References

Boyer, M., et al. 2026 AJ, submitted
The James Webb Space Telescope Absolute Flux Calibration VI: Near-Infrared Camera Imaging and Coronagraphy 

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

Pirzkal, N. et al. 2026 AJ, 171, 232
The James Webb Space Telescope Absolute Flux Calibration V. Near-Infrared Camera Wide Field Slitless Spectroscopy

Rieke, G., et al. 2022 AJ, 163, 45
Infrared Absolute Calibration. I. Comparison of Sirius with Fainter Calibration Stars

Rigby, J., et al. 2023, PASP, 135, 048001
Characterization of JWST science performance from commissioning