IRAC High Precision Photometry
Observers that use long staring mode observations, including exoplanet observations and brown dwarf monitoring observations, should be aware that the skydark may changed in the middle of the observation. This change is possible due to the dynamic nature of the skydark application which chooses the best dark which is nearest in time to a given BCD (single image). For observations that do not require high relative precision, the current calibration assures the best absolute photometric calibration.
The change in skydark will cause a small jump in the stellar flux and background levels, when inspected as a function of time. To check whether this happened, please check a BCD file FITS header taken near the beginning of your observation, and look for a header keyword SDRKEPID, which is found under the "DATA FLOW KEYWORDS" section of the header. Compare the value of this keyword to the same keyword value in a BCD taken near the end of the observation. If the SDRKEPID is different, the applied skydark has changed during the observation. Please notify the SSC so that the data will be reprocessed and archived with the same skydark applied throughout the observation. The SSC is taking pre-emptive measures to prevent this from happening in the future for high relative precision observations. All such cases during the cryogenic mission have already been reprocessed and archived.