Star trace recipe

On a regular basis, the observatory takes data that can be used to determine the tilt of spectra as a function of position along the slit and the relationship between the position of an object along the slit and the corresponding location of the spectrum on the array.

The data consists of n images of a bright star stepped along the slit and spectra of the star in the MR and LR gratings at each of these positions. In total, n images and 2*n spectra are obtained.

The eclipse recipe that processes this data is called isaacp lw_startrace. The input file consists of an ASCII file containing the data described above. The input frames can be corrected for slit curvature at the very beginning via a command line option. In this case, the user has to provide the ARC table containing the model of the slit curvature.

The recipe produces several files.

1.

Positions table. The location of the stellar images and spectra are found and stored in a table named [outname]_positions.tfits.

2.

Positions correspondence table. The relationship between the position of the star on the slit and the location of the corresponding spectra are fitted with a polynomial. There is one polynomial for LR and one for MR and the co-efficients of both are stored in the table called [outname]_corresp.tfits.

3.

Extracted spectra. The spectra are traced, extracted, wavelength calibrated and stored in a table named [outname]_extracted.tfits. The spectra are traced with a 3rd order polynomial and the results are stored in [outname]_shapes.tfits.

4.

A table with 2d polynomial modelling the slit tilt. For each grating, the spectral tilt is modelled with a 2d polynomial, which is written to a table named [outname]_poly2D_XR.tfits. This table is the one used by spjitter to correct the spectral tilt.

An example of what the SW version of the recipe produces is given in figure  and a flowchart of what the SW version of the recipe does is given in figure . The LW version is similar.