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James Webb Space Telescope
JWST Sensitivity

This website 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 for a point source

Plotted is the faintest flux 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:
  • -Solid squares show JWST requirements for NIRSpec (R=2700) and MIRI in medium resolution mode (R=2400 to 1200).
  • -Curves show the predicted NIRSpec sensitivities at R=2700 (upper set of curves) and R=1000 (lower set of curves).
  • -Solid circles show the expected performance of the NIRCam grisms at R=1000.
  • -Down-pointing triangles show the predicted performance of SOFIA FLITECAM at R=2000.
  • -Open circles show IRS on Spitzer, with the SH and LH gratings at R~600.
  • -Open square shows NIRSPEC on Keck at R=1000.
  • -Diamonds show NIRI on Gemini at R=1000.
  • -Up-pointing triangles show ISAAC on VLT at R=3000.

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 squares show the requirements for NIRSpec and MIRI.
  • -A curve shows the expected performance of NIRSpec.
  • -Open circles show the performance of the HST ACS and WFC3 grisms.
  • -Open squares show 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.

Data tables

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