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Space Telescope Imaging Spectrograph
2-D Scattered Light Correction Algorithm for STIS Echelles

Two-Dimensional Scattered Light Correction Algorithm for STIS Echelles

STIS has four echelle modes (E140H, E140M, E230H, E230M) all of which are subject to scattered light. For echelle gratings, scattered light is typically modeled by one-dimensional interpolation of the apparent background between echelle orders. This is the algorithm currently implemented in the "x1d" task in the "Calstis" package. See the STIS Data Handbook for a general description of the Calstis package. Currently, the one-dimensional algorithm is also used for all archive requests, which are processed with on-the-fly-reprocessing (OTFR)

As illustrated in the figure a standard one-dimensional algorithm does not adequately correct for the effects of scattered light. Don Lindler (Advanced Computer Concepts) and Chuck Bowers (Goddard), who helped build STIS, developed a two-dimensional iterative deconvolution method that models and removes scattered light in STIS echelle spectra. The algorithm is quite robust and reduces errors due to scattered light by at least a factor of ten.

We are in the process of writing a detailed instrument science report that will quantify the magnitude of the error due to residual scattered light, using both algorithms. Using the one-dimensional algorithm, the error in the cores of saturated interstellar lines is about 9% of the continuum at 1200 Angstroms (E140H and E140M), 3% at 1670 Angstroms (E230H), and 0.6% at 2800 Angstroms (E230H). Errors in emission line spectra are typically 1% of the peak line flux and can affect adjacent orders.

We have implemented the Bowers and Lindler algorithm as an option within the "x1d" task, though this enhancement has not yet propagated to the standard "Calstis" distribution or to the OTFC pipeline used to process archive requests. For now, the only way to run the two-dimensional scattered light algorithm is to download the software and new reference files from the ftp site linked below, install the files on your computer, and then reprocess the data yourself.

Before downloading the new software and reference files, please read the release notes to make sure the software will run on your machine. Then ftp the gzipped Unix tape archive (tar) file (66Mb). If you have questions about the two-dimensional scattered light correction algorithm, please contact help@stsci.edu