Next Contents Previous

II. OBSERVATIONS

The equipment and techniques used for these observations are described fully in Paper I. Briefly, all measurements were made with an InSb detector cooled to 55° K. To measure the strength of the H2O absorption, two filters cooled to either 77° K or 55° K were used. They have effective wavelengths and full widths at half maximum of 2.00(0.08 µm) and 2.20(0.11 µm); the latter filter is the "continuum" filter used in measuring the CO index (Paper I). Thirty-seven galaxies selected from Paper I were observed in the spring of 1976 with the 60-inch (1.5 m) and 100-inch (2.5 m) telescopes on Mount Wilson, and the 200-inch (5 m) Hale reflector. Most of these observations were made at the same time as the CO and JHK observations of Paper I. The stellar calibration of the H2O index was determined with these telescopes and with the 60-inch (1.5 m) Tillinghast reflector of Mount Hopkins, Arizona. Several globular clusters were observed on the 1.3 m telescope of Kitt Peak National Observatory and the 1.5 m telescope at Cerro Tololo Interamerican Observatory. As in Paper I, all measurements were made with the focal plane apertures centered on the optical nuclei and the visual centering was confirmed by maximizing the infrared signal. Sky chopping was always in the north-south direction with the "reference" beam typically 2 to 3 aperture diameters away from the "signal" beam. Most of the measurements were made on only one night. Repeatability of the standards, however, shows that the combined error from uncertainties in the calibration, statistical measurement error, and systematic errors arising from beam profile effects are on the order of 0.02 mag. A full discussion of the sources of error in the measurements of the CO index was given in Paper I, and applies equally well here.

Next Contents Previous