Lee A. Breakiron

A preliminary look at astrometric accuracy as a function of photon counts

The importance of photon counts as a factor in the accuracy of astrometric positions is illustrated by a preliminary look at data collected with the Allegheny Observatorys photon-counting detector, the Multichannel Astrometric Photometer. Twenty estimates are plotted of the standard errors, per hour, of astrometric positions obtained with the photometer and the blue-light lens of the observatorys 30-inch refractor. The agreement between the estimated standard errors in the X and Y coordinates supports the conclusion that, as predicted by Drake, the accuracy is dependent upon the count rate.

A New Astrometric System

The 30-inch Thaw Refractor of the University of Pittsburgh’s Allegheny Observatory has been completely rebuilt. Changes include a new objective lens, a new detector system, and computer control and data acquisition. With an observational accuracy exceeding one-thousandth of an arc second, the new system will greatly expand the domain of astrometric research.

The Astrometric Search for Neighboring Planetary Systems

Extensive testing suggests that astrometric techniques can be used to detect and study virtually any planetary system that may exist within 40 light years (12.5 parsec) of the Sun. Three years ago the astrometric group at the Allegheny Observatory began an intensive survey of 20 nearby stars to detect the nonlinear variations in their motion that planetary systems would induce. Several tests conducted to further our understanding of the limitations of this survey indicated that the photographic detector itself is responsible for most of the random error. A new photoelectric detector has been designed and a simplified prototype of it successfully tested. The new detector is expected to be able to utilize virtually all of the astrometric information transmitted through Earth’s atmosphere. This is sufficient to determine relative positions to within an accuracy of 2 mas/hr. Such precisions exceed the design capabilities of the best existing astrometric telescopes; thus a feasibility study has been conducted for the design of an improved instrument. The study concludes that a new ground-based telescope and a new detector combined should be able to study stars as faint as 17th magnitude with an annual accuracy of a few tenths of a milliarcsecond. However, to obtain the ultimate accuracy possible from current technology, we must place an astrometric system above Earth’s atmosphere. A spaceborne instrument utilizing the new detector would in theory have sufficient accuracy to detect any Earth-like planet orbiting any of the several hundred stars nearest the Sun.