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FOS reduction is essentially a two stage process at the moment. The initial stage, as a quick look at the data, is undertaken on the Perkin Elmer instrument computer, and handles object extraction and wavelength calibration. The full FOS reduction package has been rewritten and runs on the main Vax data reduction computers. Details may be found in the WHT FOS userguide. Alternatively it is possible to reduce FOS data using standard packages such as FIGARO, which for many users is the preferred option.
The aims of the FOS on-line reduction package are to provide
1) fully reduced and calibrated spectra so you can see what your data represent.
2) facilities for interactive classification and redshift determination.
The steps taken to fulfill 1) are easily completed in a few hundred seconds, enabling the observer to do so while the next exposure is in progress. The following points should be noted :
1) Modes A and B (beamswitching ) do not have their own dedicated software, but can be reduced using some of the standard CCD arithmetic routines in conjunction with the FOS software.
2) Default wavelength and flux calibration is provided. The former should not worry the observer who can easily take comparison lamp exposures to check the calibration. The latter however, is not able to take account of variations in extinction, and so it is recommended that standard stars are observed for this purpose.
Following the upgrade of FOS in May 1988, considerable changes were made to the software, most of which will be invisible to the observer (except that now the routines work properly). The most obvious external change is that the FOS chip is displayed left to right across the Lexidata rather than up and down This means the full 590 rows are now visible, rather than only 512. The display is such that the longest wavelengths are to the right. The following routines have been modified, as it was found they did not always handle the arithmetic correctly:
The following routines have been removed:
The following routines are original:
For the sake of completeness, several stages of on-line data reduction are described which are rarely ever used, as all that is necessary usually is to extract the spectrum and wavelength calibrate it. The stages which can easily be ignored are Defect removal, Cosmic ray removal, and Flatfielding. So unless you really want to use these facilities, skip on to Spectrum extraction and Sky subtraction.