eslavichjgbot
commented
5 years ago Comment by Howard Bushouse on JIRA:
It's possible for dedicated background exposures to be obtained in conjunction with nodded LRS fixed-slit exposures. For example, you may have a group of 4 exposures where exposures 1 and 2 are the nodded exposures containing the source and exposures 3 and 4 are nodded exposures on background. For the calspec2 background step, the Cal WG has indicated that the 2 background exposures be combined into a 2-D background image and subtracted from each of the 2 nodded source exposures. For the case of the master background subtraction step, we should use the 1-D extracted spectra (x1d products) from calspec2 processing of the dedicated background exposures to create a 1-D master background spectrum, which is then subtracted from each of the nodded source exposures. So in this case, the master background handling degenerates to the normal handling of dedicated backgrounds, rather than the more complicated handling of nodded exposures.
Issue JP-523 was created on JIRA by Howard Bushouse:
Master spectroscopic background subtraction can be accomplished for MIRI LRS fixed-slit exposures that nod a point source within the slit (dither type "ALONG_SLIT_NOD") by extracting background signal from each of the nodded exposures and using that data to create a master background spectrum, which is then subtracted from each of 2-D nodded exposures. The "ALONG_SLIT_NOD" option puts the source at 2 different positions in the slit: one to the left of center and the other to the right of center. According to JDox:
"The along slit nod option places the target alternately at ++8.25 px and -8.25 px from slit centre position (+/- ~0.9 arcsec)"+
Because each exposure contains the source somewhere within the field, the extracted spectral products (x1d) from calspec2 processing are likely not useful in creating the master background. Instead, special extractions will need to be performed in areas of the field known to contain only background.