A Dual-Channel Pulse-Counting Astronomical Spectrophotometer

A spectrophotometer constructed and used by the author simultaneously compares 15 Å centered at Hα with 100 Å of flanking continuum, also 15 Å centered at Ca K with 100 Å of flanking continuum; the readout employs pulse-counting electronics and an automatic digital printer. Wavelengths other than Hα and CaK may he easily selected. Design criteria and relevant details of construction are discussed. A special cooling pump used to refrigerate the photomultiplier tubes is described, together with a system for preventing condensation of moisture on the optics and photomultiplier. The performance of the apparatus is discussed—in particular its approach to 1% accuracy in individual Hα line-intensity readings, and its ability to work through several magnitudes of varying clouds and extinction with essentially unimpaired accuracy.