How to Use the Background Model

Input parameters:

  • Single location: Enter an object name or coordinates. Enter the wavelength in microns (0.5-1000 microns), year, and input day (1-366) OR "In Viewing Zone". The estimates for zodiacal brightness depend on the date. Selecting "In Viewing Zone" will give the median value for times of the year that the object is in a typical spacecraft viewing zone. Currently this is set to solar elongations between 85 and 120 degrees.

    For the Earth-Sun Lagrangian "L2" location, the ephemeris file (nominal JWST ephemeris) spans the range 2018 Oct 1 to 2029 Apr 30, i.e. Day 274, 2018 to Day 120, 2029. For Earth, any day in 2000-2099 is acceptable. The results are not very different between L2 and Earth.

  • Table upload: This mode allows the user to specify multiple locations on the sky. Provide the name of a table (which must conform to one of the styles listed here). Here is an example table. The limit is 1 million rows.

    A background "spectrum" can be obtained by submitting a table with the wavelength column set to each point of the desired spectrum.


  • Single location mode

    In Single Location mode, an HTML table gives the backgrounds (in MJy/sr) for the various components and the total.

  • Table upload mode

    Both HTML and downloadable ASCII tables are provided if there are less than 50 rows. For the downloadable table, click on "Download table". If the "In Viewing Zone" option was selected ("View Zone" = 1 on the web page, ido_view = 1 in table), then the zodiacal value will be the median in the viewing zone (see above) and the day and solar elongation will be given as -999.

 Technical Notes

 The background model consists of:

  • For Version 1, a zodiacal light model which uses a combination of the Kelsall et al. (1998) and Reach et al. (1997) parameters for the zodiacal dust cloud and the asteroid bands. Both the scattered component and thermal component are treated in a self-consistent way. The albedo of the zodiacal dust at wavelengths shortward of 0.5 microns is not as well determined, and so is assumed to be the same as the albedo at 0.5 microns.

  • NOTE: Version 4 vs. Version 1: In Version 4, a newer zodiacal dust model based on the Wright (1998) model has been implemented. As in the case of the Kelsall model, the Wright model has been fit to the COBE/DIRBE data. The parameters for the Wright (1998) model are adopted from Gorjian, Wright & Chary (2000) and are different from the parameters provided in Wright (1998) itself. The model outputs higher intensity values than the Kelsall model due to a different set of assumptions that go into fitting the DIRBE data. Specifically, Wright (1998) adopted the 25 micron strong no-zody condition that states that the minimum 25 micron residual at high Galactic latitude after subtraction of a zodiacal light model from the DIRBE observations has to be zero. Kelsall et al. do not enforce this condition and thereby obtain lower values for the zodiacal light intensity. While the condition applies only at 25 microns, the contribution of the emission at all wavelengths is affected. The zodiacal dust intensities in Version 4 are in the rough range of 1.3-2 higher at 25 microns.

    Note that due to spacecraft pointing constraints, the Earth trailing blob which is visible in the zodiacal dust model would never be observed by a pointing-constrained spacecraft at L2.

  • A model for the emission from the diffuse interstellar medium of our galaxy, which uses a combination of the analyses presented in Arendt et al. (1998) and the Schlegel et al. (1998) maps. Also included is an estimate of diffuse scattered starlight down to 0.5 microns based on the Zubko et al. (2004) model integrated with observations of Brandt & Draine (2012).

  • A model for the extragalactic background light (EBL) which uses a combination of the Mazin & Raue (2007) model in the optical/MIR and Chary & Pope (2010) models in the FIR. The EBL model combines upper limits from TeV gamma-rays as well as from integrated galaxy counts in the Spitzer/Herschel era. See Dwek & Krennrich (2013) for a full set of references.

  • For Version 1, a component for galactic light from stars based on the Mathis, Mezger & Panagia (1983) model for the interstellar radiation field in our galaxy modified by Chary (1999) to fit the DIRBE data. It accounts for the contribution of both young and old stars, but has a limited model of the galaxy. (The contribution of starlight should not be factored into the background used by an exposure time calculator for telescopes which will resolve out most of the stars, but it should be included for any straylight model since off-axis Galactic emission is likely the second most dominant source of astrophysical straylight, after the zodiacal light.)

  • NOTE: Version 4 vs. Version 1: In Version 4, the stellar brightnesses are based on the galactic model of Wainscoat et (1992). In this case, the program output includes the model starcounts.

    Final note: Since the components are listed separately in the output, it is possible to mix and match the results of the Version 1 and Version 4 zodiacal models (i.e. "Kelsall/Reach" vs. "Wright") and stellar models (i.e. "Mathis" vs. "Wainscoat").


  • Arendt, R.G. et al., 1998, ApJ 508, 74
  • Brandt, T. & Draine, B. 2012, ApJ 744, 129
  • Chary, R. & Pope, A. 2010, astro-ph/1003.1731
  • Chary, R., 1999, PhD Thesis, UCLA
  • Dwek, E. & Krennrich, F., 2013, Astroparticle Physics, 43, 112
  • Gorjian, V., Wright, E., & Chary, R. 2000, ApJ 536, 550.
  • Kelsall, T., et al., 1998, ApJ, 508, 44
  • Mathis,J., Mezger, P., & Panagia, N., 1983, A&A, 128, 212
  • Mazin, D., & Raue, M., 2007, A&A 471, 439
  • Reach, W., et al., 1997, Icarus, 127, 461
  • Schlegel, D.J., Finkbeiner, D.P. Davis, M. 1998, ApJ 500, 525.
  • Wainscoat, R.J., et al., 1992, ApJS 83, 111.
  • Wright, E. 1998, ApJ 496, 1.
  • Zubko, V., Dwek, E.. & Arendt, R. 2004, ApJS 152, 211.

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