A double beam astronomical photometer.

A double beam photoelectric filter photometer has been designed and constructed to make simultaneous measurements of two astronomical objects. Both objects are imaged in the same focal plane so that the same aperture-filter-detector system is used for both beams. The device greatly reduces errors due to variable atmospheric extinction. Photometric measurements can be made under conditions normally suitable only for spectroscopic work. Measurement precision is also increased, and accuracies of a few tenths of one percent are routine under thin cloud conditions. A description of the instrument and its operation is given in this paper.