The Application of Digital Techniques to the Optimization- of Plate Exposure Time in Stellar.spectroscopy

The photoelectric exposure meter described in this thesis was designed to monitor a small, fraction of the light passing through an astronomical spectrograph, and to provide a continuous indication of the integral value of the light flux being monitored as measured from some reference time determined by the operator. Since the sample com­ prises a fixed, percentage of the total flux passing through the instru­ ment, the displayed information is directly related to the total flux incident on the photographic plate used for recording the data. Thus i the operator may optimize the exposure in spite of time variations in the light flux value due to external conditions. After evaluation of the system constraints, a digital approach was selectedo The system was designed to have a relatively wide linear dynamic range, and particular attention was given to the optical design considerations from the viewpoint of obtaining good imaging, and also source-detector spectral matching. The various parameters and proper­ ties of photomultiplier tubes were investigated. Evaluation tests of the photoelectric exposure meter described have been carried out on the 36" Cassegrain spectrograph and on the 84" Coude spectrograph at the Kitt Peak National Observatory. The results indicate that the system requirements have been met.