JWST ETC Downloads

The JWST Exposure Time Calculator provides an option to download the final and intermediate output products, along with input data used for an ETC calculation. The data products are available for download as FITS files.

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        Video Tutorial:  ETC General Overview

The JWST ETC outputs 2-D images and line plots as well as scalar values for every ETC calculation that is performed using the scene and sources, background spectrum, instrument configuration, observing setup, and extraction strategy selected by the user.

All the final output products and intermediate products from the ETC calculations are available for download using the link provided on the Download pane. The tarball contains FITS files of the input 3-D model scene, 3-D data cube for the IFU calculation, and 2-D images that may be used for more detailed and specific analysis as desired by the ETC user. The extracted flux, backgrounds, contamination, and SNR used for the line plots are available as FITS tables.

The tarballs are named for the workbook number, calculation number, and time and date that the calculation was performed. The folder the tarball contains will be named similarly, except in the case that the calculation is an unmodified default calculation. In that case, the workbook number will be given as "wb2", and the rest of the information will reflect the date that version of the JWST ETC was released.

Image directories


"backgrounds.fits" contains background flux for the field of view at the time of the observation requested. Background components are assumed to be uniform across the field, dependent on wavelength. The "backgrounds.fits" file has an array in its second header (hdu 1) which has 5 records: the first column is wavelength (μm), the second column is the total combined background (MJy/sr), and the last 3 columns are the thermal, stray light, and infield components (MJy/sr) of the total combined background, respectively.


  • cube/cube_flux.fits
  • cube/cube_flux_plus_background.fits

These contain the scene "observed" by the ETC, with sources convolved by instrument PSFs. It is the scene cube incident on the detector, in units of mJy. The difference between the 2 files is that "cube_flux_plus_background.fits" contains additive background noise (scaled to the wavelength-dependent fluxes in "backgrounds.fits").

First header (hdu 0) contains a 3-D array whose dimensions are wavelength, then x, then y. Pulling out a single slice of the array will provide the scene at a particular wavelength. The values of CRVAL1 and CDELT1, CRVAL2 and CDELT2 are referring to the spatial position of the (0, 0) pixel and scale (in arcseconds) of each of those slices.

Second header (hdu 1) has 2 fields: wavelength (in μm), and index number. Those index numbers map the wavelengths to the slices of the 3-D array in the first header.

If you have used an IFU mode (MIRI MRS, MIRI MRS time series, NIRSpec IFU), you will have different 3-D output files:

  • "cube/cube_reconstructed.fits" - a cube of the signal plus Gaussian noise, in units of e/s
  • "cube/cube_reconstructed_signal.fits" - a cube of the signal, in e/s
  • "cube/cube_reconstructed_noise.fits" - a cube of the noise (including all noise sources), in e/s
  • "cube/cube_reconstructed_snr.fits" - the signal cube divided by the noise cube
  • "cube/cube_reconstructed_saturation.fits" - 3-D saturation map, where 1 = partially saturated, and 2 = fully saturated

These files represent the scene as reconstructed from the "observation." All files have a single header containing a 3-D array with the axes in order wavelength, x, and y. All information for reconstituting the coordinates and wavelengths is given in the single header. Note that the header defines, per limitations of the FITS standard, a linear wavelength scale. For NIRSpec IFU in particular, the actual wavelength scale is not linear; see "lineplot/lineplots_wave_pix.fits" for the actual wavelength scale in this case.

There will also be files in the "model" directory, which will be empty except for IFU modes.

  • cube/model/cube_flux_n.fits
  • cube/model/cube_flux_plus_background_n.fits

These files are equivalent in format and content to the "cube/cube_flux.fits" and "cube/cube_flux_plus_background.fits" files obtained from the other modes and described above, with the difference that each of the files is a y-axis slice through the input scene cube. Within each file, only 2 y-axis locations in the array are populated for all values of x and wavelength, corresponding to one IFU element. The rest of the cube contains zeroes.


  • "image/image_detector.fits" - the signal, in e/s
  • "image/image_saturation.fits"- the SNR
  • "image/image_snr.fits" - the saturation, where 1 = partially saturated and 2 = fully saturated
  • "image/image_ngroups_map.fits" - a map giving the number of groups before a given pixel will saturate

These are the 2-D images displayed by the ETC. First header (hdu 0) contains a 2-D array of all of the values. As with the scene cubes, the header contains the spatial information for the scene.

There will be "unrotated" versions of each of the above images for all slit and slitless modes. These files are identical to the files listed above except for the spatial orientation of the trace.

For coronagraphic imaging, these files correspond to the chosen PSF subtraction strategy.

Plot directories


All of these files have 2 FITS headers, with an array of data (and explanatory headers) in the second header (hdu 1):

  • "wave_calc" - 1-D wavelengths (in μm), corresponding to the spectra (whether input from the user or from the ETC library) input into the calculation.
  • "wave_pix" - 1-D wavelength (in μm), corresponding to the output of the calculation. Unless a spectroscopic mode is specified, this file will have a single record with a single entry, which is the wavelength of the requested observation.

Some of these correspond to the line plots displayed in the ETC, but many do not.

These files contain inputs to the calculated observation, and have 2 records, wavelength (equivalent to "wave_calc") and the specified value, in their second header (hdu 1):

  • "target" - combined flux of all the targets in the scene re-sampled onto the calculation's input wavelength grid, in mJy (as a function of wavelength)
  • "fp" - the combined flux in e/s incident on the detector focal plane
  • "bg" - the (assumed spatially uniform) background flux input into the calculation, in MJy/sr
  • "bg_rate" - the background flux input into the calculation in e/s

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

The following files are extracted from the calculated observation. The data are stored in the second header (hdu 1), and consist of 2 records: wavelength (equivalent to "wave_pix") and value. Unless a spectroscopic mode is used, the records will only have one entry—for the wavelength of the observation—in each. The values correspond to the scalar values displayed in the Reports pane of the ETC. For spectroscopic modes, full arrays are defined in the records, and the scalar value reported in the Reports pane is the value at the reference wavelength.

  • "total_flux" - the total flux onto the detector extracted from the calculation, in e/s
  • "extracted_flux_plus_bg" - the flux of the target extracted from the calculation, in e/s
  • "extracted_flux" - the flux of the target extracted from the calculation, with background subtracted, in e/s
  • "extracted_noise" - the extracted noise (incorporating all instrumental noise sources), in e/s
  • "sn" - the extracted SNR (calculated as extracted_flux/extracted_noise)
  • "extracted_bg_only" - the background flux extracted from the output of the calculation, in e/s
  • "extracted_bg_total" - the background plus contamination from sources in the scene, in e/s
  • "extracted_contamination" - just the background contamination, in e/s
  • "n_partial_saturated" - number of partially saturated pixels in the output, as a function of wavelength
  • "n_full_saturated" - number of fully saturated pixels in the output, as a function of wavelength
  • "contrast" - (coronagraphic imaging only) companion-to-host flux ratio, as a function of wavelength

Input data

"input.json" - a JSON-formatted text file containing a complete record of the inputs to the ETC. This can be used as input to the ETC when run as a scriptable standalone Python program.

Latest updates
    Updated for ETC 3.0.

    Updated for ETC 2.0.

    Updated for ETC 1.5. Video link added.

  • Updated for ETC 1.3. Added description of coronagraphy-only lineplot.
Originally published