NIRCam Imaging Sensitivity

Exposure times vs. flux estimates at signal-to-noise ratio = 10 in NIRCam images can be obtained using the Pandeia JWST Exposure Time Calculator (ETC) Python engine.

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The Pandeia Exposure Time Calculator Old should be used for all observation planning. This article provides ETC results using the Python engine to loop through many calculations of signal-to-noise ratio (SNR) for various readout specifications and resulting exposure times, given assumptions detailed below. These calculations are then interpolated to determine depth (SNR = 10) vs. exposure time. The script is available on Github.

Depth vs. exposure time

The NIRCam Imagingoverview and NIRCam Sensitivity articles show SNR = 10 sensitivity estimates for imaging in all NIRCam filters given a total exposure time of 10 ks (166.7 minutes = 2.78 hours). Figure 1 shows similar estimates for a range of exposure times in seven wide filters and one medium filter.

Sensitivity estimates can vary significantly depending on the backgrounds and the assumed photometric aperture sizes as discussed below. Please use the Pandeia Exposure Time Calculator Old to plan your observations.

Figure 1. Depth vs. exposure time


Depth (10-sigma) vs. total exposure time for four exposures of NIRCam imaging in seven wide filters and one medium-band filter, assuming the "1.2 × MinZodi" background defined below. Values are for point sources with photometric apertures of 0.08" (0.16") diameters with 0.6"–0.99" (1.2"–1.98") background sky annuli for the short (long) wavelength channel. Depths are interpolated from results obtained with the Pandeia JWST ETC Python engine. Dashed line extrapolations assume depth in units of flux goes as sqrt(t), or in magnitudes: depth = depth0 + 1.25 * log(t / t0). Note 5-sigma depth estimates are 0.75 mag fainter than the 10-sigma estimates shown here.


Background

JWST's background model varies with the target coordinates (RA, Dec) and time of year. These calculations assume the fiducial "1.2 × MinZodi" (1.2 times the minimum zodiacal light) background at RA = 17:26:44, Dec = -73:19:56 on June 19, 2019, as used in the NIRCam Imaging article. The background model for these observations must be generated using the online ETC GUI and then imported into the Python ETC engine.

Figure 2. Background vs. wavelength

Background vs. wavelength assumed in these calculations, as generated by the online ETC GUI for "1.2 × MinZodi" at (RA = 17:26:44, Dec = -73:19:56) on June 19, 2019.


Exposure time

Recommended readout patterns and exposure times used for the calculations on this page are described below. They are based on ETC calculations that show these to yield optimal signal-to-noise ratios for a given exposure time. RAPID 1, BRIGHT2, SHALLOW4, MEDIUM8, and DEEP8 yield high signal-to-noise ratio most efficiently and are preferred to maximize depth. The other readout patterns (BRIGHT1, SHALLOW2, MEDIUM2, DEEP2) may be preferred in some cases, for example, to provide a greater dynamic range (with a shorter first group) to sample bright stars before saturation.

Figure 3. Signal-to-noise ratio vs. exposure time for various readout patterns

Comparison of signal-to-noise ratio for various readout specifications for F200W imaging of an AB mag 29 point source. Each point shows the estimate given four exposures, each comprising a single integration that consists of multiple groups (between two and 10) of a given readout pattern. This analysis shows that RAPID, BRIGHT2, SHALLOW4, MEDIUM8, and DEEP8 yield higher signal-to-noise ratio than other patterns for a given exposure time. These are plotted as filled circles; the other patterns are plotted as stars. Note BRIGHT2 is restricted to a maximum of four groups when reading out the full detectors in both modules.
When reading out the full detectors in both modules, RAPID and BRIGHT2 are limited to four groups, DEEP8 is limited to 20 groups, and all other patterns are limited to 10 groups. The table below gives the full range of recommended readout specifications for exposure times between 42.9 and 4176.6 s.  The final two columns assume four such exposures.

Table 1. Recommended readout specifications for maximal depth in a given exposure time

Readout patternNGROUPSNINTExposure time (s)NEXPTotal exposure time (s)
RAPID2132.24128.8
RAPID3142.94171.8
RAPID4153.74214.7
BRIGHT24196.64386.5
BRIGHT151107.44429.5
BRIGHT161128.84515.4
BRIGHT171150.34601.3
BRIGHT181171.84687.2
BRIGHT191193.34773.0
BRIGHT1101214.74858.9
SHALLOW451268.441073.7
SHALLOW461322.141288.4
SHALLOW471375.841503.1
SHALLOW481429.541717.9
SHALLOW491483.241932.6
SHALLOW4101536.842147.4
MEDIUM861633.542533.9
MEDIUM871740.842963.3
MEDIUM881848.243392.8
MEDIUM891955.643822.3
MEDIUM81011062.944251.8
DEEP8611170.344681.2
DEEP8711385.045540.2
DEEP8811599.846399.1
DEEP8911814.547258.0
DEEP81012029.248117.0
DEEP81112244.048975.9
DEEP81212458.749834.9
DEEP81312673.5410693.8
DEEP81412888.2411552.8
DEEP81513102.9412411.7
DEEP81613317.7413270.6
DEEP81713532.4414129.6
DEEP81813747.1414988.5
DEEP81913961.9415847.5
DEEP82014176.6416706.4

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    Corrected RAPID exposure times in Table 1