Microphotometry-Computer Classification of Objective-Prism Spectra

Abstract

Three-dimensional classification of late-type stars is obtained through computer-processed digitized microphotometer data. Plate transmissions are converted to relative intensity through spot-sensitometer calibration data. A pseudo-continuum consisting of several straight-line segments is formed by connecting high points in the spectrum. Absorption-line strengths and line ratios are then measured. The temperature-and-luminosity-sensitive ratios are relatively insensitive to seeing effects. For plates with a dispersion of 108 Å mm^-1, preliminary results indicate an accuracy in derived absolute magnitude comparable to, and possibly better than, that of MK classification.

The CN anomaly serves as the abundance index. The CN index was defined in a manner to give the largest figure of merit (the ratio of total range of the index to the mean error). The adopted method is the same as that of Yoss and Lutz (1971) and has a figure of merit over twice that of the index similar to that of Griffin and Redman (1960).

For field stars, plate X and Y positions are converted to equatorial coordinates and printed out in order of increasing right ascension, making quick and easy identification of the Henry Draper numbers.