In some cases one cannot see flux from a science target unless many exposures are added together. Nevertheless you might want to sly subtract the data and/or use FIREHOSE to solve for the 2D wavelength map and rectify the image. In this case one can choose "reference star" mode in the Object/Aperture Definition tab of the extraction preferences menu.
FIREHOSE does not by default associate science frames with reference stars since this reference star mode is still a relatively new feature. To get it to work properly, you will need to modify the fire structure to tell it which frames go together. The easiest way to do this is by using the "Run Script (and Save)" button on the "Structure" tab in firehose. The script is used to add keywords to the fire structure by hand. The keywords you need to check are
- POSSLIT - this can be either "A" or "B" and denotes the position of the object in the slit
- OBJTRACEFILE - this is the filename of the reference star exposure to be associated with your science frame.
You will need to set posslit for all of your reference and science frames (the pipeline makes sure A's and B's are matched between ref and science). You only need to set objtracefile for the science frames, since reference and calibration frames do not need this information.
The script file is a simple is a simple text file, which MUST be named firestrct_script.txt and MUST be located in your reduction directory (i.e. where you run firehose).
The snippet below shows an example of what your firestrct_scipt.txt file should look like:
---------------- Start of snippet -------------
### object obj_id posslit objtracefile
0061 RXJ2248 3 A fire_0059.fits
0062 RXJ2248 3 B fire_0060.fits
0063 RXJ2248 3 A fire_0059.fits
%
0149 RXJ2248 3 A fire_0154.fits
0150 RXJ2248 3 B fire_0155.fits
0151 RXJ2248 3 B fire_0155.fits
0152 RXJ2248 3 A fire_0154.fits
---------------- end of snippet ---------------
The line beginning with ### is critical, as it tells firehose the format of the subsequent lines of text. The "###" field denotes an exposure number, then the next 4 fields are keywords to be edited in the fire structure.