Absolute Flux Calibration

The prime objective of the JWST flux calibration plan is to enable high accuracy relative and absolute flux calibration for all science instruments, by converting science data from instrumental to physical units.  The overall plan is described, including the list of the stellar flux calibration sources.

On this page

The JWST has a suite of four 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. For all these instruments/modes, the nominal requirement is 2% absolute flux prediction accuracy of standard stars, with the goal of improving it as much as possible. This is the required prediction accuracy to achieve the overall flux calibration requirements as these include other terms beyond the flux prediction accuracy (aperture correction accuracy, stability, etc.). To achieve this, a sample of stars will well known absolute flux and spectral shape across the 0.6 to 28.5 microns range will be used. Having this unified program will not only enable the cross-instrument calibration of the JWST science instruments, but also with HST, Spitzer and other ground-based telescopes. Details on how the JWST pipeline applies the photometric calibration are given here.



JWST science instruments calibration goals

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 modelling.  

The JWST flux calibration sample is composed by hot stars (white dwarfs and OB stars, see Table 1), A-stars (Table 2), and G-stars (solar-like, see Table 3). 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 is observable with each filter/grating (Gordon et al. 2009; Gordon & Bohlin 2012).



Sample of JWST primary standards

The sample of standard is listed in Tables 1 to 3 and these tables are updated versions of those given by Gordon & Bohlin 2012. The 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 during 2010-2013 time frame will provide the basis for fitting the model atmospheres that will predict fluxes at JWST wavelengths. Ground-based observations of each calibration star in the optical and near- infrared will be also 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 & 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 calibrators stars are available from CALSPEC.

Note the current sample will be enhanced to provide sources for the calibration characterization for Cycle 1, not just the base flux calibration.  


Table 1. JWST primary calibrators: hot stars

Name

RA (J2000)

Dec (J2000)Spec. TypeVK

ksi02 Cet

02 28 09.54 

+08 27 36.2 

B9III 

4.28 

4.39 

lam Lep

05 19 34.52 

-13 10 36.4 

B0.5IV 

4.28 

5.00 

10 Lac

22 39 15.68 

+39 03 01.0 

O9V 

4.88 

5.50 

mu Col

05 45 59.89 

-32 18 23.2 

O9.5V 

5.17 

5.99 

G191B2B 

05 05 30.62 

+52 49 54.0 

DA0 

11.781 

12.764 

GD71 

05 52 27.51 

+15 53 16.6 

DA1 

13.032 

14.115 

GD153 

12 57 02.37 

+22 01 56.0 

DA1 

13.346 

14.308 

LDS749B 

21 32 16.01 

+00 15 14.3 

DBQ4 

14.73 

15.217 

WD1057+719 

11 00 34.31 

+71 38 03.3 

DA1.2 

14.8 


WD1657+343 

16 58 51.10 

+34 18 54.3 

DA1 

16.1 



Table 2. JWST primary calibrators: A stars

NameRA (J2000)Dec (J2000)SpTypeVK

HD 158485 

17 26 04.84 

+58 39 06.8 

A4V 

6.50 

6.14 

HD 14943 

02 22 54.67 

-51 05 31.7 

A5V 

5.91 

5.44 

HD 37725 

05 41 54.37 

+29 17 50.9 

A3V 

8.35 

7.90 

HD 163466 

17 52 25.37 

+60 23 46.9 

A2 

6.86 

6.34 

HD 116405 

13 22 45.12 

+44 42 53.9 

A0V 

8.34 

8.48 

HD 180609 

19 12 47.20 

+64 10 37.2 

A0V 

9.41 

9.12 

BD+60 1753 

17 24 52.27 

+60 25 50.7 

A1V 

9.67 

9.64 

1757132 

17 57 13.25 

+67 03 40.9 

A3V 

12.0 

11.16 

1812095 

18 12 9.56 

+63 29 42.3 

A2V 

11.736 

11.286 

1808347 

18 08 34.75 

+69 27 28.7 

A3V 

11.9 

11.53 

1802271 

18 02 27.17 

+60 43 35.6 

A3V 

11.985 

11.832 

1805292 

18 05 29.3 

+64 27 52.1 

A1V 

12.278 

12.005 

1732526 

17 32 52.64 

+71 04 43.1 

A3V 

12.530 

12.254 

1743045 

17 43 04.48 

+66 55 01.6 

A5V 

13.5 

12.772 


Table 3. JWST Primary calibrators: G stars

NameRA (J2000)Dec (J2000)SpTypeVK

HD 159222 

17 32 00.99 

+34 16 16.1 

G1V 

6.56 

5.00 

HD 205905 

21 39 10.15 

-27 18 23.7 

G2V 

6.74 

5.32 

HD 106252 

12 13 29.51 

+10 02 29.9 

G0 

7.36 

5.93 

HD 37962 

05 40 51.97 

-31 21 04.0 

G2V 

7.85 

6.27 

HD 38949 

05 48 20.06 

-24 27 49.9 

G1V 

8.0 

6.44 

P330E 

16 31 33.85 

+30 08 47.1 

G0V 

13.01 

11.379 

P177D 

15 59 13.59 

+47 36 41.8 

G0V 

13.48 

11.857 

C26202 

03 32 32.88 

-27 51 48.0 

G0-5 

16.64 


SF1615+001A 

16 18 14.23 

+00 00 08.4 

G0-5 

16.75 


SNAP-2 

16 19 46.13 

+55 34 17.7 

G0-5 

16.2 



Figure 1. Representative spectra

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)


SED details

Pure hydrogen 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 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; and then the best fitting model is used to estimate the 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 is provided in the following 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 3600 s for imaging/spectroscopy. 



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




Published

 

Latest updates
  •  
    Fixed typo in the coordinates of 1812095 (+63, not +64 for declination)
  •  
    Table 1, 3 updated
    Replaced Figure 1
    Updated references