NIRCam Imaging Sensitivity
The exposure times vs. flux estimates at signal-to-noise ratio = 10 in NIRCam images presented here have been obtained using the Pandeia JWST Exposure Time Calculator (ETC) Python engine.
The Pandeia Exposure Time Calculator should be used for all observation planning. This article provides ETC results determined using the Python engine to loop through many calculations of signal-to-noise ratio (SNR) for various readout patterns and exposure times, given the 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 Imaging overview 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 background and the assumed photometric aperture sizes as discussed below. Please use the Pandeia Exposure Time Calculator to plan your observations.
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.
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.
Note we do not generally recommend integration times greater than 1000 seconds. After 1000 seconds, the majority of pixels would likely be affected by cosmic rays. See discussion in MIRI Cross-Mode Recommended Strategies.
Table 1. Recommended readout specifications for maximal depth in a given exposure time
|Readout pattern||NGROUPS||NINT||Exposure time (s)||NEXP||Total exposure time (s)|
1Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.