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
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The report provides a summary of the input parameters and output scalar values associated with the selected calculation. It is broken into sections of related quantities:
- "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".
- "Extracted Flux": Extracted flux corresponding 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).
- "Brightest Pixel Rate": Count rate (e-/sec) of the brightest pixel in the scene.
- "Maximum Fraction of Saturation": Ratio of the "Total Exposure Time" to "Time Between First Reset and Last measurement, per integration" for the brightest pixel. See JWST ETC Time Definitions for additional details of the different times used by the ETC.
- "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.
Instrument and Detector:
- "Instrument Filter/Disperser": Input instrument configuration from the Instrument Setup tab (e.g., filters, grisms).
- "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).
- "Single Exposure Time": "Total Exposure Time" divided by the number of Exposures per specification on the Detector Setup tab.
- "Fraction of Time Spent Collecting Flux": "Time Between First and Last Measurement, per exposure" divided by "Single Exposure Time". See JWST ETC Time Definitions for additional details of the different times used by the ETC.
The "Fraction of Time Spent Collection Flux" (commonly referred to as the "duty cycle") is currently improperly calculated. To match the duty cycle calculated from FITS header quantities produced by the JWST calibration pipeline, the calculation should be the "Time Between First Reset and Last measurement, per integration" multiplied by the number of integrations in an exposure, divided by the the "Single Exposure Time".
- "Time Between First and Last Measurement, per exposure": Measurement time, as defined in JWST ETC Time Definitions.
- "Time Between First Reset and Last measurement, per integration": Saturation time, as defined in JWST ETC Time Definitions.
Extraction Strategy Settings:
- "Radius of Extraction Aperture": The Aperture radius value under the Strategy tab (in arcseconds or pixels).
- "Area of Extraction Aperture": Area within the scene from which flux is extracted (measured in pixels).
- "Wavelength of Interest Used" (spectroscopy) or "Effective Wavelength" (imaging): Corresponds to the desired wavelength specified by the user for spectroscopy modes under the Strategy tab or the effective wavelength of the filter for imaging modes (in microns), respectively. The effective wavelength for a filter is the flux-weighted average and depends on the selected filter and the source spectrum.
- "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.
- "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.
- "Area of Background Measurement": Area of the background region (measured in pixels).
- "Total Sky Background Flux in Background 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.
- "Total Flux in Background 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.
- "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 Ray Events": Rate of cosmic ray events per pixel per group (read).
Coronagraphic Contrast (MIRI Coronagraphic Imaging and NIRCam Coronagraphic Imaging only):
- "Contrast": The unitless companion-to-host flux ratio at the specified radius and azimuthal angle.
- "Azimuth at which Contrast is Measured": Angle in degrees measured clockwise from the positive y-axis where the contrast is measured.
- "Radius at which Contrast is Measured": Distance in arcseconds from the center of the field of view where the contrast is measured.
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 X 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