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James Webb Space Telescope
JWST Sensitivity

This page compares the sensitivity of JWST’s instruments to those of current observatories. The purpose is to help astronomers visualize JWST’s capabilities, and how JWST will advance their own science.

There are many ways to measure performance: limiting point source flux, limiting line flux; time to survey a given area of the sky or number of spectroscopic targets; and so on. We select illustrative simple cases for fairly short (10,000s) integrations. For other scenarios, users can scale from the data tables below, or use the JWST Exposure Time Calculators.

Where the numbers come from: JWST sensitivities used here are requirements: the minimum performance required of each instrument. This is a conservative approach; the on-orbit performance may be better.

Sensitivities for other observatories are taken from instrument handbooks, online calculators, and published papers, as documented in the data tables. If a user believes they have discovered an error in these sensitivities, they are requested to contact Jane.R.Rigby@nasa.gov.

Photometric performance

Plotted is the faintest flux for a point source that can be detected at 10 sigma in a 104 s integration. Fluxes are given in Janskies as well as AB magnitudes.

chart chart

Symbols are as follows:
Telescopes are labeled and color coded.

  • -Red points show required sensitivities for the JWST instruments NIRCam (0.6-5 micron) and MIRI (5-28.3 micron)
  • -"Hubble" points are for the WFC3, ACS, and (reddest point) NICMOS instruments on HST.
  • -"Gemini" points are for the GMOS and NIRI instruments.
  • -"Spitzer" points are for the IRAC and MIPS instruments on Spitzer.
  • -"SOFIA" points are the predicted performance of the FLITECAM and FORCAST instruments.
  • -"Herschel" points are for PACS and SPIRE.
  • -ALMA points are shown at two configurations: Cycle 0 array (top curve), and final (lower curve.)

The data table lists the value and origin of each number plotted.

Spectroscopic performance (R=600-3000)

Plotted is the faintest line flux that can be detected at 10 sigma in a point source in a 104 s integration.

Symbols are as follows:

  • -Red squares show the required performance of NIRSpec at R=2700, the required performance of MIRI in medium resolution mode (R=2400 to 1200), and the expected performance of the NIRCam grisms at R=1000..
  • -Red curves show the predicted performance of NIRSpec at R=2700 (upper set of curves) and R=1000 (lower set of curves).
  • -Black squares show IRS on Spitzer, with the SH and LH gratings at R~600.
  • -Ground-based observatories are plotted as follows:
    • -green squares show NIRI on Gemini at R=1000.
    • -the pink square shows NIRSPEC on Keck at R=1000.
    • -blue squares show ISAAC on VLT at R=2000 to 3000.
  • -Cyan squares show FLITECAM on SOFIA at R=1700.

The data table lists the value and origin of each number plotted.

Low-resolution (R~100) spectroscopic performance

Plotted is the faintest continuum that can be detected at 10 sigma for a point source in 104 s. Continuum fluxes are in units of Janskies and AB magnitudes.

Symbols are as follows:
  • -Solid red squares show the REQUIRED performance of the JWST instruments.
  • -Red curves show the expected performance of the JWST instruments at other wavelengths.
  • -Blue squares show the performance of the HST ACS and WFC3 grisms.
  • -Black squares show the performance of the low-resolution (SL and LL) gratings of IRS/Spitzer at R~60-120).

The data table lists the value and origin of each number plotted. Estimated NIRISS sensitivities are tabulated here.

Data tables

The tables below give the numbers used in the above plots, with references.