CONTINUUM NORMALIZATION OF THE SPECTRA

The combination of narrow slit, field rotation, observations taken in non-photometric conditions, and limitations in setting the slit to the parallactic angle makes flux calibration of our data problematic. Therefore, to smoothly join the different grating settings taken over many years and to provide a continuum shape suitable for automatic classifications algorithms, we fit each observation to a spectral energy distribution standard with a close match in spectral type. This requires a library of flux-calibrated spectra that covers both a large range in spectral type and the 3400 - 9500 Angstroms wavelength range of our spectra.

Unfortunately, there is no available library of spectrophotometry that covers our extended wavelength region as well as providing full coverage of the stellar atmospheric parameter space. In fact, the existing spectrophotometric libraries primarily cover MK spectral types and luminosity classes, thus do not incorporate metal-poor stars. We have chosen the Pickles (1998) compilation as our spectrophotometric database. Note that this library is normalized to unity at 5550 Angstroms and, hence, our library is also normalized to this point.

In order to handle the 14 cases for which there is no library spectral energy di stribution (SED), such as for the C and S spectral types, or no known spectral type, such as some of the fainter metal-poor stars, we added a dummy SED with unit continuum everywhere. This SED is in the same format as rest of the library so that the continuum matching is performed in the same way for all stars.

The continuum matching is done by integrating the SED standard and the observation into adjacent 10 Angstroms bins. Bins that cover strong spectral features and/or telluric features are excluded. (The complete set of bins used may be found here. A smooth transformation function, specifically, a 10th order Chebyshev (with iterative rejection to remove deviant points), is fit to the ratio, expressed in magnitudes, of the observed and standard continuum values. The function is then applied to each pixel value in the spectrum to produce the continuum calibrated versions of the data.

The Pickles continuum standard applied is identified in the header information for the spectrum, as well as in the accompanying data table. For stars with spectral type earlier than F0, our strategy is to find the closest Pickles star in spectral class, since the Balmer discontinuity and spectral shape is more temperature sensitive than gravity sensitive at the higher temperatures. For stars of type F0 and later we place first emphasis on attaining the correct luminosity class. As noted above, there are 14 stars with a flat continuum applied. Users of our library also need to be aware that the assignment of a continuum standard for metal-poor stars is of limited validity.