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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After you have done this for all sky frames, click "Done" to save the structure and return to the main window.
2. Set the Extraction Preferences
Now you are ready to extract the spectrum. But first, you must tell the extraction code that you wish to use blank sky frames rather than the default of deriving a sky model from the science frame. In the Extraction tab, click on "Preferences" to bring up the selection window.
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Likewise, it may be more straightforward to employ boxcar extraction, or set the weighting function for optimal extraction by hand.
3. Associate object-sky pairs
Once you have saved your extraction preferences, click an object and start the extraction. A primitive GUI will appear with a list of your science targets. For each object in the list, highlight its line and click "set sky." This will bring up a list of the files you have identified with the SKY tag above. Click the one that goes with each science frame. When you click "Done" the software will begin extracting your spectrum. If you have specified user-selected apertures you will be prompted to work with the aperture GUI.
4. Finishing touches
Once extraction is complete, telluric correction and order combining is performed as for standard reductions.
5. What the code is doing
The blank sky code is considerably more sophisticated than just a straight subtraction of the science-sky frames. We have found that the OH lines are so sharp and bright that the science and sky frames must be registered to ~0.01 pixel accuracy to avoid the appearance of overwhelming P-cygni-like residuals. Since FIRE flexes at the several tenths of a pixel level, this must be corrected out.
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