JWST ETC Reports
The JWST Exposure Time Calculator (ETC) Reports pane provides a summary of the calculation that is selected in the calculation table. The Reports pane provides a report on the scalar output values, warnings, error messages, and a link to the downloads associated with the highlighted calculation.
Video Tutorial: ETC General Overview
Words in bold italics are also buttons
or parameters in GUI tools. Bold
style represents GUI menus/
panels & data software packages.
The report provides a summary of the input parameters and output scalar values associated with the selected calculation. This includes:
- "Instrument Filter/Disperser": Input instrument configuration from the Instrument Setup tab (e.g., filters, grisms).
- "Extraction Aperture Position": The (x, y) coordinates (in arcseconds) of the center of the extraction aperture specified by the user with either the Centered on source or Specify position in scene option under the Strategy tab.
- "Wavelength of Interest used to Calculate Scalar Values": Corresponds to the desired wavelength specified by the user for spectroscopy modes under the Strategy tab or the effective wavelength of the filter for the imaging modes (in microns). The effective wavelength for a filter is the flux-weighted average and depends on the selected filter and the source spectrum.
- "Size of Extraction Aperture": The Aperture radius value under the Strategy tab (in arcseconds).
- "Total Time Required for Strategy": Total time for the full observation (in seconds), calculated based on the parameters specified by the user under the Detector Setup and Strategy tabs.
- "Total Exposure Time": Total time spent on-target (in seconds), calculated based on the parameters specified by the user under the Detector Setup and Strategy tabs. It accounts only for time spent on-target (i.e., this value will be half the value of "Total Time Required for Strategy" for the Nod Off Scene strategy).
- "Maximum Fraction of Saturation": Ratio of the "Total Exposure Time" to the saturation time for the brightest pixel.
- "Maximum Number of Groups Before Saturation": Largest number of groups that prevents saturation in the brightest pixel. If the value is 10,000 groups or larger, ">9999" is reported.
- "Extracted Flux": Extracted flux corresponds to the measured count rate (e–/sec) within the extraction aperture for the source after background subtraction.
- "Standard Deviation in Extracted Flux": Variance includes various noise terms as described in Pontoppidan et al. (2016). The square root of the variance is reported here (in e–/sec).
- "Extracted Signal-to-Noise ratio": The signal-to-noise ratio is computed directly as the ratio of "Extracted Flux" and "Standard Deviation in Extracted Flux".
- "Input Background Surface Brightness": Surface brightness of the background (in MJy/steradian) depending on the Position, Background configuration, and Date specified by the user on the Backgrounds tab.
- "Total Background Flux in Extraction Aperture": If a background region was selected, the total background flux corresponds to the count-rate (e–/s) from the sky background and contamination from any signal that contributes counts to the background extraction aperture (e.g., from overlapping sources or extended wings of the source). If no background region is specified, a noiseless sky background is reported.
- "Total Sky Background Flux in Extraction Aperture": Count rate from the total sky background (for the configuration selected by the user). If no background region is specified in the Strategy tab, the contamination from within the scene is not accounted for and the total background is equal to the total sky background.
- "Fraction of Total Background due to Signal from Scene": Fraction of the total background contributed by any other signal from within the scene that is contaminating the total background flux. If no background region is specified, this fraction is zero.
- "Average Number of Cosmic Rays per Ramp": Computed by multiplying the assumed cosmic ray flux (in events/sec/pixel) by the ramp time.
For coronagraphic imaging, the report also includes:
- "Radius at which Contrast is Measured": Distance in arcseconds from the center of the field of view where the contrast is measured.
- "Azimuth at which Contrast is Measured": Angle in degrees measured clockwise from the positive y-axis where the contrast is measured.
- "Contrast": The unitless companion-to-host flux ratio at the specified radius and azimuthal angle.
If there are warnings, this is indicated in the calculations table by the orange exclamation point in the status column. An explanation for the warnings issued are provided under the Warnings tab of the reports pane. The warnings imply that there are problems that the user should be aware of that do not prevent the calculation from running. Some of the common warnings include information about partial or full saturation, background aperture definition which either overlaps the photometry aperture or extends outside the image, or a message that the SNR is less than 20 for target acquisition (TA) calculations.
An error is indicated by the red cross in the status column of the calculation table. The errors indicate that the calculation did not complete and may be due to problems with the input parameters. The message in the Errors tab will provide additional information.
All the output products and intermediate products from the ETC calculations are available for download using the link under the Download tab. See JWST ETC Downloads for a full listing and descriptions of the files included in the download.
Pontoppidan, K. M., Pickering, T. E., Laidler, V. G. et al. 2016, Proc. SPIE 9910, Observatory Operations: Strategies, Processes, and Systems VI, 991016
Pandeia: a multi-mission exposure time calculator for JWST and WFIRST