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R.L. Marcialis (LPL/U. Arizona), W. J. Merline (SWRI)
In 1948--1950, G.P. Kuiper (1950.\ {\sl PASP\/} {\bf 62}, 133--137) used a disk meter to ``measure'' the diameter of Pluto. This instrument (Camichel, 1944.\ {\sl Comptes Rendus\/} {\bf 219}, 21--22) is designed to produce an artificial luminous disk of controllable brightness, color, and diameter, allowing the user to estimate angular diameters. Kuiper's result, 0\farcs 23 (0.46~d\oplus or 5\thinspace 870~km), stood as the definitive diameter determination for over a quarter century.
We now know that at the time of the 1950 observation, Pluto's disk actually was 0.\!\!\prime\prime0445, with 0.\!\!\prime\prime0248 Charon offset by (\rho,\,\theta) = (0.\!\!\prime\prime6767,\, 46.\!\!\circ6). Most people have discounted the observation as faulty, in spite of the fact that Kuiper and Humason repeatedly measured 0.\!\!\prime\prime23 for Pluto, and 0.\!\!\prime\prime11 for a nearby mag 11 star. Can the result be reconciled with what they actually observed? Surprisingly, it can!
We have run numerical simulations of the experiment. The ``realistic'' CCD model of Merline and Howell (1995.\ {\sl Exp.\ Astron.} {\bf 6}, 163--210) was modified to simulate the human eye. Synthetic images were generated for a wide variety of conditions (seeing, sky brightness, eye response, Pluto/Charon brightness ratio, {\sl etc.}). A double-blind experiment was then performed. Under the assumption of a radially-symmetric point spread function ({\sl i.e.}, a single source), widths (FWHM) of these simulated images were extracted for both the Pluto--Charon blend and the nearby comparison star. We conclude that, although Charon was not resolved, it did indeed have an unanticipated, but understandable effect on Kuiper's measurement. The result of our modelling confirms that Kuiper should have seen approximately what was measured. It is not necessary to invoke changes in seeing between Pluto and star measurements.
Kuiper's result is further consistent with a Pluto/Charon light ratio of \lower.5ex\sim\raise.5ex\llap{>} \!8.2:1. This compares favorably to the ratio required for no stellar occultation in 1965 (Halliday {\sl et al.} 1966.\ {\sl PASP\/} {\ bf 78}, 113--124; Marcialis 1996.\ {\sl BAAS\/} {\bf 28}, 1080), but is much gre ater than the current value of ~4.8:1 at comparable rotational phase.
The author(s) of this abstract have provided an email address for comments about the abstract: umpire@lpl.arizona.edu