Improvements to the PDS microdensitometer of the Dominion Astrophysical Observatory.

Modifications to improve the photometric response, positional accuracy, resolution, and reliability of a Photometrics Data Systems (PDS) model 1010 microdensitometer are described. The modifications resulted in a greatly improved utility of the machine in the areas of stellar photometry, astrometry, and measurement of stellar spectra. The original photomultiplier and logarithmic amplifier were replaced with components of higher performance and stability, the drive motors and screws on both axes were replaced, and the optical encoders on both axes were replaced with a laser interferometer system. The original control computer and electronics were totally replaced, along with the control and scanning software, increasing the reliability and versatility of the instrument. Encoders were fitted to all front-panel controls, allowing all system parameters to be recorded during measurements. Other improvements include the addition of new plateholders and the installation of improved sub-stage lighting.