3.6.3. Solid Angle of the Program Observations
The solid angle is a function of both the angular pixel scale (spatial direction) and the angular slit-width (dispersion direction). The pixel scale was measured empirically by taking spectra of two stars with known angular separation while they were simultaneously aligned in the slit. The measured separation in pixels was then compared to the known angular separation of the stars. Four pairs of stars were observed in this way with angular separations in the range 44-82arcsec. Each pair of stars was observed with the slit at 3 positions differing by less than 1 degree, in an attempt to obtain truly parallel slit alignment. The stars used for this purpose are in the field of M67, for which the relative astrometry of members is known to better than 0.3 mas (Girard et al. 1989). The pixel scale was found to be 0.5843 ± 0.0035 arcsec pixel-1 (1 error).
The slit-width of the spectrograph is adjusted by a manual micrometer while the instrument is on the telescope. The calibration and repeatability of the micrometer was verified using a microscope. The repeatability of the width setting was tested by opening the jaws to their maximum extent between each of several sets of measurements. At the micrometer setting used for our program observations, the slit width was measured to be 1.536 ± 0.002 arcsec (1 error) at the center of the jaws. The total solid angle of each pixel is therefore 0.8975 ± 0.0054 arcsec2 (1 error).