SW_SPJITTER

The pipeline recipe that reduces SW spectroscopic data is called isaacp sw_spjitter . The recipe uses an initialisation file, which contains parameters that define how the recipe runs. To generate one, type isaacp sw_spjitter -g spjitter.ini at the prompt and this will create an initialisation file called spjitter.ini. This file can be edited and modified by the user to setup some parameters.

The initialisation file contains parameters which indicate

• which table to use to correct for slit curvature
• which table to use for wavelength calibration
• which table to use to correct for spectral tilt (the one produced by the startrace recipe) and
• which flat field to use

The input file should be an ASCII file listing the raw FITS files.

The recipe starts by classifying the input images according to the cumulative offsets in the headers. The classic way of taking IR spectroscopic data is to observe the target along two positions along the slit, which we will call A and B. An example, may be the sequence AAABBBBBBAAA. After flat fielding all the data, the recipe will take the first three A frames and average them, take the first three B frames and average them, etc. The recipe then subtracts one average from the other, corrects for slit curvature, spectral tilt and wavelength calibrates.

If the tables for the wavelength calibration and the correction for slit curvature are missing, the recipe will use the OH night sky lines. If there are too few of these, the recipe will use a model to do the wavelength calibration and will skip the correction for slit curvature.

If the flat field is missing, the recipe skips the flat fielding step.

The subtracted frames will contain positive and negative spectra. The two spectra are combined by multiplying the image by -1 and adding it to the original after a suitable shift. The resulting frames are then added together to give the final result.

At the end, a spectrum can be extracted. Either the user specifies the position of the spectrum they want to extract in the initialisation file, or the spectrum of the brightest object is extracted.

Figure  shows the individual steps of the recipe.

  

Figure: sw_spjitter algorithm

The output consists of the reduced FITS image, an ascii file which logs the reduction steps (and informations on how they were performed) and the extracted spectrum (TFITS), if it was found.