JWST ETC Saturation

The instrument mode-specific saturation limits in the JWST Exposure Time Calculator (ETC) are conservative estimates that in most cases are lower than the threshold for non-linearity determined during testing.

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Calculation and warnings

The ETC checks for saturation by first calculating the slope of the integration ramp in units of e-/second, then multiplying by the length of the integration in seconds. If the resulting value exceeds the limit set in the ETC (see Table 1), a warning will be given. There are two types of saturation that may be encountered:

  • Partial saturation: Saturation occurs in the third group or a later group of the integration ramp. These integration ramps are still usable up to when saturation occurs; extending the integration ramp beyond saturation results in unusable data and should be avoided.
  • Full saturation: Saturation occurs in the first or second group of the integration ramp. The warning is raised because at least two groups are needed for the slope calculation. The only way to avoid full saturation is to change to a smaller subarray or use a readout pattern with a shorter group time. Visit the Recommended Observing Strategies and Understanding Exposure Times articles for further understanding and advice on adjusting these parameters. 

ETC saturation limits

The table below presents detector full-well depths and compares them to the saturation limit for specific instrument modes in the ETC.

Table 1. Detector full-well depths and ETC saturation limits.

Instrument/Detector

Full-well depth (e-)

ETC limit (e-)% of full-well depth
MIRI250,000All modes: 193,655~78%


NIRCam/Short-wavelength (SW)


105,750

Coronagraphic Imaging: 100,415

~95%
Imaging: 100,415~95%

Target Acquisition: 100,415

~95%
Time Series: 73,990~70%




NIRCam/Long-wavelength (LW)




83,300

Coronagraphic Imaging: 78,850

~95%
Grism Time Series: 58,100~70%
Imaging: 78,850~95%
Target Acquisition: 78,850~95%
Time Series: 58,100~70%
Wide Field Slitless Spectroscopy: 78,850~95%




NIRISS




95,000

AMI: 30,000~32%

Imaging: 72,000

~76%
SOSS: 72,000~76%
Target Acquisition: 72,000~76%
WFSS: 72,000~76%

NIRSpec
65,000

FULL subarray: 45,000

~70%
77,000All other subarrays: 65,000~84%




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