Possible skydark change in long staring mode observations in warm IRAC mission
27 July 2012
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