Welcome to the JWST Exposure Time Calculator


Welcome to version 1.1.1 of the JWST ETC!

This version features faster performance, new target acquisition modes, improved timing models, and more: see the Release Notes for details.

This is a patch release that corrects an error in the MIRI SLOW mode and updates the sets of available options in the UI: see the Release Notes for details.

When you log in to the 1.1.1 ETC, your old workbooks will be marked "Out of Date":

  • When you load them, they will open in Read-Only mode: this ensures that your previous results are not overwritten and remain available to you for reference.
  • If you copy an out of date workbook, and load the copy, all its calculations will be automatically updated for you with the current version of the software.
  • For more information, see ETC Releases and Out-of-Date Workbooks.

Be sure to review the Known Issues with this release.


- Log in with MyST, or work anonymously

- The JWST ETC allows you to have multiple workbooks. Each workbook allows you to define sources, place them in scenes, and use the scenes in calculations. Sources may be used in multiple scenes, and scenes may be used by multiple calculations, which will be automatically recalculated to reflect any changes made. Reasonable default values are provided for all fields.

- Workbooks for logged-in users are automatically saved and will be available in your workbook list upon return. If you are working anonymously, you can log in with MyST at any time, and your workbooks will be transferred to your user account.

- For ETC Documentation, see Help->User Guide. For help or to provide feedback, contact the JWST Help Desk (Help->Help Desk).

- We recommend the use of Firefox, or Chrome. Safari has been observed to occasionally cause problems.

- Be sure to read the Known Issues page for important information about system accuracy, limitations, and workarounds.

System Performance

Calculation run times for certain instrument modes (NIRISS WFSS, SOSS; NIRSpec IFU; coronagraphy) are longer because the underlying computations are more complex.

The use of highly sampled spectra, or sources with extremely narrow lines, may result in significantly longer run times for spectroscopic modes.

Occasionally the UI may appear to be unresponsive or stalled. Try reloading the page; this often clears the issue.

System performance depends on usage/load; if the system appears unresponsive, wait 1-2 minutes. Please avoid repeatedly clicking, because this will make the problem worse.If the system remains unresponsive, contact the JWST Help Desk.

Accuracy of ETC Calculations

This version of the ETC is intended to help users to propose and plan observations for the Director's Discretionary Early Release Science Program (DD-ERS) and for the Cycle 1 Guaranteed Time (GTO) and General Observer (GO) programs. The ETC approximates our current best knowledge and understanding of the performance of the JWST instruments, based on ground measurements and calibrations. It has been validated against independent instrument models provided by the instrument teams. It is important to note that there are remaining uncertainties associated with system throughputs, detector noise properties, etc., which will not decrease until the observatory is in orbit. Users should exercise appropriate caution when interpreting results from the ETC. A number of known issues remain, which may affect predicted sensitivities. See Known Issues for additional details.

The ETC is not intended to be a complete observation simulator, and some higher-order effects are not taken into account, such as field distortion.