When échelle observations are completed, the data are sent by FTP to a Sun workstation. There, the reduction of the data is done in a semi-automated manner by means of an IRAF1 script. Processing consists of bias subtraction, flat fielding, aperture extraction (in that order), and wavelength calibration by means of a Th-Ar lamp. The data product is formatted in FITS. Header keywords for heliocentric correction are supplied, but the correction is not done.
The reduction procedure is described in detail in a 1996 M. S. thesis by Christopher L. Mulliss.The Roper Scientific camera's images are saved in TIFF format and converted to FITS by means of the Mac OS X version of ImageMagick. Then, the data are processed with IRAF1. An average dark frame with the same integration time as the stellar spectrum, or interpolated to the same exposure time, is subtracted for first-order removal of stray light. Then, standard background subtraction is done. The stellar spectrum is divided by the background-subtracted flat lamp, which is set equal to 1 below a fairly high threshold, and then extracted with variance weighting. Because of strong vignetting and optical distortion at the ends of the spectrum, the signal-to-noise ratio is inferior there. Wavelength calibration utilizes a Ne lamp (in the red) or an Fe-Ar lamp (in the yellow) and is carried out by standard methods. The reduced data are saved in FITS format, and header keywords giving heliocentric velocity correction, heliocentric julian date, and airmass are provided.
Correction for telluric lines is the responsibility of the user of the data, but a library of artificial telluric spectra will be provided in the on-line data archive for both spectrographs. The templates are created from spectra of rapidly rotating A- and B-type stars taken at high airmass and/or high humidity, frequently enough so that a the library will include a standard star that is roughly contemporaneous with any given observation.
All the telluric lines in the spectrum of the standard star are fitted with gaussian or voigt functions with the IRAF task splot. Then, the parameters of the fitted functions are read into the IRAF task mk1dspec and superimposed on a flat, unit continuum. Since the templates are noise-free, noise is not introduced when the stellar spectrum is divided by them. Telluric line removal can be done with the IRAF task telluric or by any other suitable method.
The telluric standard spectra themselves will also be in the archive when it is complete, so that users can make their own templates if desired.