NIRCam's flux calibration is based on observations of standard stars taken during commissioning and during normal operations. 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

Vega magnitudes 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])

A link to the table of Vega zeropoints is provided below.

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 references 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. Additionally, observers can subscribe to updates following the instructions on the JWST Data Calibration Reference Files page.

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 P330E and P177D. 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 (PID) 1074. 

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 the data described in Table 1. Note that all Cycle 1 absolute flux calibration updates use the post-commissioning filter curves, posted on the NIRCam Filters page. 

Table 1. Absolute flux data included in CRDS context jwst_0995.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 file below.  Note that the zeropoints in this file use the mean pixel size for a given detector (included in the table), and the version 5 (Nov 2022) throughputs. The Vega spectrum is from CALSPEC (alpha_lyr_stis_010.fits).

References

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

Rigby, J., et al. 2022 arXiv:2207.05632
Characterization of JWST science performance from commissioning