MIRI photometry is tied to absolute flux calibration based on standard stars, with additional corrections (e.g., aperture and color) needed to derive accurate fluxes. This page summarizes how to apply these elements in practice.

Photometric units and conversions

JWST pipeline-calibrated MIRI data for all observing modes are in surface brightness units, specifically MJy/sr (that is, MegaJanskys per steradian) per average pixel, as noted in the FITS SCI extension header keyword BUNIT in stage 2 & 3 pipeline products: "cal.fits" and "i2d.fits" images. Flat-fielding corrects for variations in responsivity and areal coverage between pixels, justifying the use of the average pixel area in the calculations below.

To measure integrated photometry in MJy, the units in the images must first be converted from MJy/pixel through multiplying by the average area of a pixel in steradian, a constant provided in the FITS header keyword PIXAR_SR. The MIRI imager has a pixel scale of 0.11"/pixel while the calibrated MRS cubes range from 0.13"/pixel to 0.35"/pixel, depending on channel. The measured flux densities in MJy (after any additional necessary corrections, see later sections) can be converted into magnitudes in either the AB or Vega magnitude systems, as described next.

AB magnitudes

The flux density, $//<![CDATA[ \begin{array}{l}F\end{array} //]]>$ (in MJy), can be converted to AB magnitudes using the following equation:

$//<![CDATA[ \begin{array}{l}\mathrm{mag}_{AB} = -6.10 - 2.5\mathrm{log}_{10}(F)\end{array} //]]>$.

The AB magnitude zeropoints, the magnitude that produces 1 count per second, can be calculated as:

$//<![CDATA[ \begin{array}{l}ZP_{AB} = -6.10 - 2.5\mathrm{log}_{10}(\mathtt{PIXAR\_SR})\end{array} //]]>$.

Vega magnitudes

The "Vega" magnitudes reported in the catalog output in stage 3 of the JWST pipeline use Vega-Sirius zeropoints, which are "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, $//<![CDATA[ \begin{array}{l}\mathrm{mag_{Vega-Sirius}} = \mathrm{mag_{Sirius}} - 1.395\end{array} //]]>$ (or $//<![CDATA[ \begin{array}{l}F_{\mathrm{Vega-Sirius}} = F_{\mathrm{Sirius}}/3.6141\end{array} //]]>$). Vega-Sirius magnitudes (referred to as just "Vega" in the equations below) can be obtained from flux densities using the following:

$//<![CDATA[ \begin{array}{l}\mathrm{mag_{Vega}} = -2.5\mathrm{log}_{10}\left(\frac{F}{F_{\mathrm{Vega}}}\right)\end{array} //]]>$.

The Vega magnitude zeropoints can be calculated as:

$//<![CDATA[ \begin{array}{l}ZP_{\mathrm{Vega}}=-2.5\mathrm{log}_{10}\left(\frac{\mathtt{PHOTMJSR}*\mathtt{PIXAR\_SR}}{F_{\mathrm{Vega}}}\right)\end{array} //]]>$.

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 $//<![CDATA[ \begin{array}{l}C_{FD}\end{array} //]]>$ in units Jy / (DN/s) for point sources. The calibration factor for extended sources (surface brightness) PHOTMJSR corresponds to $//<![CDATA[ \begin{array}{l}C_{FD}/A_{\mathrm{pixel}}\end{array} //]]>$, where $//<![CDATA[ \begin{array}{l}A_{\mathrm{pixel}}\end{array} //]]>$ is the average solid angle of a pixel. See Gordon et al. (2022) for a description of the absolute flux calibration plan, with specific information for MIRI in Gordon et al. (2025) and Law et al. (2025).

The JWST stage 2 pipeline stores the PHOTMJSR values in the "jwst_miri_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.

The most recent updates on MIRI absolute flux calibration for each mode can be found in MIRI Calibration Status.

Zero-magnitude flux densities

Table 1. Zero-magnitude flux densities in the Sirius-Vega system (Gordon et al. 2025)

Aperture correction factors

Point spread functions (PSFs) are infinite in extent and thus no aperture can capture all of the flux. Additionally, the background and even the read noise will exceed the target flux at larger distances from the PSF center, requiring an aperture correction factor to fully account for the total flux from a given object. Table 2 provides the circular aperture radius (corresponding to 70% of the PSF encircled energy), inner background annulus radius (80%), outer background annulus radius (85%), and the aperture correction factor for a point source and the pixel scale of the imager (0.11"/pixel). Following background subtraction, the summed flux within the aperture should be multiplied by the aperture correction factor to adjust the photometry for an infinite aperture.

Table 2. Apertures, background annuli, and aperture correction factors (Gordon et al. 2025)

Color corrections

A color correction can be applied to MIRI filter photometry to account for spectral differences between the calibration stars and the science target:

Where λ_pivot is the pivot wavelength for a given filter, F(λ) is the flux density of the science target, R(λ) is the filter response function, and K is the color correction factor:

The pivot wavelength for each imaging and coronagraphic imaging filter, as well as color correction factors for different spectral shapes, are provided in Table 3. The filter response functions can be found in the MIRI Technical Library.

As shown in the table below, the color corrections are typically <2% but increase for cooler targets (e.g., the 200 K blackbody, see rightmost column). Users should determine if a color correction is necessary based on the spectral shape of their target. Table 3 provides color correction factors for SEDs that scale with various powers of the wavelength (λ², λ, λ^-1, and λ^-2) and various blackbody temperatures from 10,000 K down to 200 K.

Table 3. Color correction for different spectral shapes

References

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

Gordon, K. D. et al. 2025, AJ, 169, 6
The James Webb Space Telescope Absolute Flux Calibration. II. Mid-Infrared Instrument Imaging and Coronagraphy

Law, D. R. et al. 2025, AJ, 169, 67
The James Webb Space Telescope Absolute Flux Calibration. III. Mid-infrared Instrument Medium Resolution Integral Field Unit Spectrometer

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