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.

Figure 1. Representative spectra
 

Click on image for a larger view.

Spectra of a representative sample of the proposed primary calibration sources: white dwarfs (blue, dashed), A-stars (green, dotted),and G-stars (red, solid). The multipliers, W4 and W2, on the ordinates are in units of μm (Gordon & Bohlin 2012)


Table 2 JWST primary calibrators: hot stars

Name

RA (J2000)

Dec (J2000)Spec. TypeVKAlias
lam Lep05 19 34.52-13 10 36.4B0.5 IV4.295.09HD 34816
10 Lac22 39 15.68+39 03 01.009 V4.885.50HD 214680
mu Col05 45 59.90-32 18 23.209.5 V5.185.99HD 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

NameRA (J2000)Dec (J2000)Spec. TypeVKAlias
del UMi17 32 13.00+86 35 11.3A1 Van4.344.26HD 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 22.6A0 V5.695.75HD 42525
HR 701818 37 33.52+62 31 35.7A0 V5.745.75HD 172728
HR 546714 38 15.22+54 01 24.0A1 V5.835.76HD 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 10145211 40 13.65-39 08 47.7A9 V8.206.82
HD 281100 31 18.50-43 36 23.0A3 V7.507.04

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 5567707 14 31.29+13 51 36.8A2 V9.419.16

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

NameRA (J2000)Dec (J2000)Spec. TypeVKAlias
18 Sco16 15 37.27-08 22 10.0G2 Va5.503.99HD 146233
16 Cyg B19 41 51.97+50 31 03.1G3 V6.204.66HD 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 14233115 54 19.79-08 34 49.4G3 V8.757.13
HD 16706018 17 44.14-61 42 31.6G3 V8.927.43
HD 11516913 15 47.39-29 30 21.2G3 V9.207.71

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 

14.492MASS J16194609+5534178

C26202 

03 32 32.84

-27 51 48.6

F7 V 

16.64 

14.822MASS J03323287-2751483

SF 1615+001A 

16 18 14.24

+00 00 08.6

G1 V 

16.75 

15.312MASS J16181422+0000086
NGC 2506 G3108 00 14.21-10 47 29.5G1 V
16.25Gaia EDR3 3038045185547143936


Table 5. Stars no longer considered as calibrators

NameRA (J2000)Dec (J2000)Spec. TypeVKReason
ksi2 Cet02 28 09.56+08 27 36.2B9 III SB:4.304.39possible binary
HD 6075307 33 27.32-50 35 03.3B3 IV6.686.83possible 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 2783604 24 12.47+14 45 29.6G1 V7.586.01binary
HD 20945822 03 10.77+18 53 03.5F9 V7.636.31variable

HD 38949 

05 48 20.06 

-24 27 49.9 

G1 V 

7.80

6.44 

variable
GSPC P041-C14 51 57.98+71 43 17.4G012.1610.53

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

CycleA dwarfsHot starsSolar analogsRepeatability
11536153715381539
24496449744984499


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 StatusNIRCam Calibration StatusNIRISS 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




Notable updates


Originally published