NIRCam Absolute Flux Calibration and Zeropoints
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.
Flux densities in MJy can be converted to AB magnitudes using Astropy units. Explicitly:
magAB = –6.10 – 2.5 log10( flux[MJy/sr]*
PIXAR_SR[sr/pix] ) = ZPAB – 2.5 log10(flux[MJy/sr]),
where ZPAB is the zeropoint: ZPAB = –6.10 – 2.5 log10(
PIXAR_SR[sr/pix]). For example, ZPAB = 28.0 for
PIXAR_SR = 2.29e-14 (corresponding to pixel size 0.0312").
Vega magnitudes can be obtained from flux densities in MJy/sr using the following:
magVega = -2.5 * log10(flux[MJy/sr] *
PIXAR_SR[sr/pix] / fluxVega[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] /
One can then define the instrumental magnitude maginst = -2.5 * log(flux[DN/s])
and obtain again magVega as
magVega = maginst + ZPVega
where ZPVega is the Vega zeropoint:
ZPVega = -2.5 * log10(
PHOTMJSR*PIXAR_SR / fluxVega[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 CFD in units Jy/(DN/s) for point sources. The calibration factor for extended sources (surface brightness)
PHOTMJSR corresponds to CFD/Apix, where Apix 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 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 a subset of the data described in Table 1. The latest update was delivered via "jwst_1126.pmap" in September 2023, and includes all data described in Table 1. Note that all post-commissioning 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
|PID||Obs||Module||Target||Subarray||Stellar type||CALSPEC model|
|1537||014||B||GD 71||SUB160||White Dwarf|
|1537||015||A||GD 71||SUB160||White Dwarf|
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 post-Commissioning throughputs.
Note that STScI is delivering "Vega" zeropoints that use Sirius as the color reference (Rieke et al. 2022). These are labeled "zp_vega-sirius" in the file mentioned below, and use the "sirius_stis_005.fits" spectrum in CALSPEC. The catalog output in stage 3 of the JWST pipeline uses the Vega-Sirius zeropoints. The traditional Vega zeropoints (CALSPEC model "alpha_lyr_stis_011.fits") are also provided to facilitate a direct comparison to the zeropoints delivered with the "jwst_0995.pmap" context delivered early in Cycle 1 (October 2022).
Gordon, K., et al. 2022 AJ, 163, 267
The James Webb Space Telescope Absolute Flux Calibration. I. Program Design and Calibrator Stars
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