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Orientation of spectra

The detectors are frequently aligned on the Spectograph such that the central order falls (mainly) parallel to the detector rows or columns. The orders themselves however, are often not parallel to each other. The algorithms used for tracing orders assume the orders run approximately in the horizontal direction. It is therefore necessary to rotate any column-parallel frames before reduction (through 90 degrees).

The package is very flexible as to the exact order orientation and highly sloped and severly distorted orders can usually be extracted without difficulty provided that a tracing frame of sufficient quality is provided. This will generally in the form of a bright star exposure, or a template trace frame such as an exposure of a continuum lamp (with a narrow dekker preferably).

If the target object under study is sufficiently bright, then the object frame itself can of course be used as the trace template frame. The package is capable of tracing very poor object frames, but with the result that the modelled order positions will be less well determined, impacting on all subsequent phases of a reduction. The user is therefore strongly encouraged to provide the best order tracing template possible.

Mon Mar 14 16:50:31 GMT 1994