Welcome to the JWST Exposure Time Calculator


Welcome to version 3.0 of the JWST ETC!

This release features accuracy updates based on inflight measurements, updates to IFU strategies, and new instrument-specific options for Cycle 3: see the Release Notes for details, and be sure to review the Known Issues for this release.

When you log in, workbooks from ETC 2.0 and earlier will be marked with a previous version number in the "Ver" column:

  • 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  • If you copy an out-of-date workbook, and load the copy, all its calculations will be automatically updated for you with the current version of the software.
  • For more information, see ETC Releases and Out-of-Date Workbooks.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, work anonymously, or enter Quick Start mode.
  • The JWST ETC allows you to have multiple folders, with multiple workbooks within each folder. 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.
  • Folders and 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 (but not the folders). Anonymous users are limited to 10 workbooks.
  • 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, Chrome, or Safari.
  • 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 are longer because the underlying computations are more complex.

The use of highly sampled spectra may result in significantly longer run times for spectroscopic modes, with the longest run times for higher resolving powers.

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 plan observations for General Observer (GO) programs. The ETC approximates our current best knowledge and understanding of the performance of the JWST instruments, based on inflight measurements and calibrations. It has been validated against inflight observations analyzed by the instrument teams. 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 accounted for, such as field distortion.