JWST Absolute Flux Calibration
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. The goal of the JWST absolute flux calibration Program is to achieve a consistent absolute flux calibration across all instruments and modes.
This article covers the overall structure of the absolute flux calibration program, and links where necessary to instrument mode-specific articles on the practical implementation and results of the program.
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Goals and requirements
Table 1 lists the overall mission requirements for absolute flux calibration, which vary by instrument and observation mode. Since many factors can affect this calibration (e.g., aperture correction accuracy, stability, etc) the nominal requirement on the absolute flux prediction accuracy of standard stars is 2%, with a goal of improving it as much as possible. 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 enable it for HST, Spitzer and other ground-based telescopes.
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 modeling.
Table 1. Absolute flux requirements
Instrument | Imaging | Coronagraphy | Spectroscopy† |
---|---|---|---|
NIRCam | 5% | 5% | 10%‡ |
NIRSpec | N/A | N/A | 10% |
NIRISS | 5% | N/A | 10%‡ |
MIRI | 5% | 15% | 15% |
† For sources observed through slits, the value is for well-centered observations.
‡ Not a formal requirement, by analogy with NIRSpec spectroscopy.
Sample of JWST primary standards
The JWST flux calibration sample is composed of hot stars (white dwarfs and OB stars; see Table 2), A dwarfs (Table 3), and solar analogs (late F and early G dwarfs; Table 4) based on the list provided by Gordon et al. (2022). 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.
This sample contains sources faint enough to be observed with NIRCam, as well as calibrators suitable to characterize the longer MIRI wavelengths. Stars with existing or planned HST/Spitzer observations were favored. Hubble STIS spectroscopy obtained over the last few years provides the basis for fitting the model atmospheres to give predicted fluxes at JWST wavelengths. Ground-based observations of each calibration star in the optical and near- infrared will also be acquired, to provide an independent prediction of the stellar atmosphere parameters (e.g., T(eff) & log(g)). This sample should provide sufficient high quality calibration stars to meet the required absolute flux calibration JWST error budget for the imagers (NIRCam and MIRI ). The predicted spectra for a representative set of the primary calibrators are shown in Figure 1 from 0.8 to 28 μm. The predicted spectra for all the calibrator stars are available from CALSPEC.
Note that only a subset of this sample will be observed in each cycle; Table 4 lists stars that have been included in previous lists of potential JWST standards but are now excluded for the reason given. Most of the variables in Table 4 were identified by TESS (Mullally et al. 2022).
In practice, even the brightest of these calibration standard stars are faint enough at 28 μm that the MIRI MRS has augmented them with observations of bright asteroids and extraplanetary disks; see MIRI MRS Calibration Status for details.
Table 2 JWST primary calibrators: hot stars
Name | RA (J2000) | Dec (J2000) | Spec. Type | V | K | Alias |
---|---|---|---|---|---|---|
lam Lep | 05 19 34.52 | -13 10 36.4 | B0.5 IV | 4.29 | 5.09 | HD 34816 |
10 Lac | 22 39 15.68 | +39 03 01.0 | 09 V | 4.88 | 5.50 | HD 214680 |
mu Col | 05 45 59.90 | -32 18 23.2 | 09.5 V | 5.18 | 5.99 | HD 38666 |
G191-B2B | 05 05 30.62 | +52 49 51.9 | DA0.8 | 11.78 | 12.76 | |
GD 71 | 05 52 27.62 | +15 53 13.2 | DA1.5 | 13.03 | 14.12 | |
GD 153 | 12 57 02.33 | +22 01 52.6 | DA1.2 | 13.35 | 14.31 | |
LDS 749B | 21 32 16.23 | +00 15 14.4 | DBQ4 | 14.73 | 15.22 | |
WD 1057+719 | 11 00 34.24 | +71 38 02.9 | DA1.2 | 14.68 | 15.47 | |
WD 1657+343 | 16 58 51.11 | +34 18 53.3 | DA0.9 | 16.1 | 17.4 |
Table 3. JWST calibrators: A dwarfs
Name | RA (J2000) | Dec (J2000) | Spec. Type | V | K | Alias |
---|---|---|---|---|---|---|
del UMi | 17 32 13.00 | +86 35 11.3 | A1 Van | 4.34 | 4.26 | HD 166205 |
HR 701 | 02 22 54.67 | -51 05 31.7 | A5 V | 5.91 | 5.44 | HD 14943 |
eta1 Dor | 06 06 09.38 | -66 02 22.6 | A0 V | 5.69 | 5.75 | HD 42525 |
HR 7018 | 18 37 33.52 | +62 31 35.7 | A0 V | 5.74 | 5.75 | HD 172728 |
HR 5467 | 14 38 15.22 | +54 01 24.0 | A1 V | 5.83 | 5.76 | HD 128998 |
HR 6514 | 17 26 04.84 | +58 39 06.8 | A4 V | 6.50 | 6.14 | HD 158485 |
HD 163466 | 17 52 25.37 | +60 23 46.9 | A7 Vm | 6.86 | 6.34 | |
HD 101452 | 11 40 13.65 | -39 08 47.7 | A9 V | 8.20 | 6.82 | |
HD 2811 | 00 31 18.50 | -43 36 23.0 | A3 V | 7.50 | 7.04 | |
HD 37725 | 05 41 54.37 | +29 17 50.9 | A3 V | 8.35 | 7.90 | |
HD 116405 | 13 22 45.12 | +44 42 53.9 | A0 V | 8.34 | 8.48 | |
HD 180609 | 19 12 47.20 | +64 10 37.2 | A3 V | 9.41 | 9.12 | |
HD 55677 | 07 14 31.29 | +13 51 36.8 | A2 V | 9.41 | 9.16 | |
BD+60 1753 | 17 24 52.27 | +60 25 50.8 | A1 V | 9.67 | 9.64 | |
J1757132 | 17 57 13.23 | +67 03 40.8 | A8 Vm | 12.0 | 11.16 | 2MASS J17571324+6703409 |
J1802271 | 18 02 27.16 | +60 43 35.5 | A2 V | 11.99 | 11.83 | 2MASS J18022716+6043356 |
J1805292 | 18 05 29.3 | +64 27 52.1 | A3 V | 12.28 | 12.01 | 2MASS J18052927+6527520 |
J1743045 | 17 43 04.49 | +66 55 01.7 | A8 V | 13.5 | 12.77 | 2MASS J17430448+6655015 |
Table 4. JWST calibrators: Solar analogs
Name | RA (J2000) | Dec (J2000) | Spec. Type | V | K | Alias |
---|---|---|---|---|---|---|
18 Sco | 16 15 37.27 | -08 22 10.0 | G2 Va | 5.50 | 3.99 | HD 146233 |
16 Cyg B | 19 41 51.97 | +50 31 03.1 | G3 V | 6.20 | 4.66 | HD 186427 |
HR 6538 | 17 32 00.99 | +34 16 16.1 | G1 V | 6.56 | 5.05 | HD 159222 |
HD 205905 | 21 39 10.15 | -27 18 23.7 | G1.5 IV-V | 6.74 | 5.32 | |
HD 106252 | 12 13 29.51 | +10 02 29.9 | G1 V | 7.36 | 5.93 | |
HD 37962 | 05 40 51.97 | -31 21 04.0 | G2 V | 7.85 | 6.27 | |
HD 142331 | 15 54 19.79 | -08 34 49.4 | G3 V | 8.75 | 7.13 | |
HD 167060 | 18 17 44.14 | -61 42 31.6 | G3 V | 8.92 | 7.43 | |
HD 115169 | 13 15 47.39 | -29 30 21.2 | G3 V | 9.20 | 7.71 | |
GSPC P330-E | 16 31 33.81 | +30 08 46.4 | G0 V | 13.01 | 11.42 | 2MASS J16313382+3008465 |
GSPC P177-D | 15 59 13.58 | +47 36 41.9 | G0 V | 13.48 | 11.86 | 2MASS J15591357+4736419 |
SNAP-2 | 16 19 46.10 | +55 34 17.9 | G2 V | 16.2 | 14.49 | 2MASS J16194609+5534178 |
C26202 | 03 32 32.84 | -27 51 48.6 | F7 V | 16.64 | 14.82 | 2MASS J03323287-2751483 |
SF 1615+001A | 16 18 14.24 | +00 00 08.6 | G1 V | 16.75 | 15.31 | 2MASS J16181422+0000086 |
NGC 2506 G31 | 08 00 14.21 | -10 47 29.5 | G1 V | 16.25 | Gaia EDR3 3038045185547143936 |
Table 5. Stars no longer considered as calibrators
Name | RA (J2000) | Dec (J2000) | Spec. Type | V | K | Reason |
---|---|---|---|---|---|---|
ksi2 Cet | 02 28 09.56 | +08 27 36.2 | B9 III SB: | 4.30 | 4.39 | possible binary |
HD 60753 | 07 33 27.32 | -50 35 03.3 | B3 IV | 6.68 | 6.83 | possible binary |
J1808347 | 18 08 34.74 | +69 27 28.7 | A3 V | 11.69 | 11.53 | variable |
J1812095 | 18 12 09.57 | +63 29 42.3 | A3 V | 12.01 | 11.29 | variable |
J1732526 | 17 32 52.63 | +71 04 43.1 | A4 V | 12.21 | 12.25 | variable |
HD 27836 | 04 24 12.47 | +14 45 29.6 | G1 V | 7.58 | 6.01 | binary |
HD 209458 | 22 03 10.77 | +18 53 03.5 | F9 V | 7.63 | 6.31 | variable |
HD 38949 | 05 48 20.06 | -24 27 49.9 | G1 V | 7.80 | 6.44 | variable |
GSPC P041-C | 14 51 57.98 | +71 43 17.4 | G0 | 12.16 | 10.53 | binary |
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:
MAX observable flux = flux that can be observed in the normal observing modes (full frame and subarrays) without reaching saturation
MIN observable flux = flux that can be observed with a S/N of 200/50 in 3,600 s for imaging/spectroscopy.
Observing programs
Users can learn more about the observing programs by clicking the links in the table below and/or retrieving the programs in APT.
Table 6. Absolute flux calibration observing programs
Cycle 3 plans are still a work in progress. Program IDs should be assigned in April.
Results
The flux calibration accuracy of the 4 JWST science instruments are described by the MIRI Calibration Status, NIRCam Calibration Status, NIRISS Calibration Status, and NIRSpec Calibration Status pages respectively.
Further documentation
The descriptions of the Cycle 1 calibration observing programs have more information on the planned observations for absolute flux calibration. (The link is to a webpage maintained by STScI outside of JDox.)
References
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