For sources that are extended and may cover a large fraction of the slit length, it is sometimes advisable to chop between the science pointing and a nearby region of blank sky. The blank sky frame, which should have the same exposure time as the science frame, is used to estimate the sky background. FIREHOSE is not nominally set up to do this, but at the request of several users we have added in such a functionality, in alpha testing/shared risk mode. This page describes how it should be used.
1. Set up the fire structure for blank-sky frames
Unlike other calibration frames, FIREHOSE cannot automatically determine if a particular frame contains a science object or blank sky. So, users must specify explicitly in the fire structure which frames are to be used in this way. The association of specific sky frames with their corresponding object happens downstream, at extraction.
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