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The accuracy of parallaxes obtained with ground-based CCD cameras now often reaches or exceeds 0.5 milliarcseconds. This improved accuracy permits us to measure significant parallaxes and determine distances for the nearest planetary nebulae. At present, the central stars of 11 planetary nebulae have been observed as part of the USNO parallax program. We now have determined distances with accuracies better than 20 percent for four central stars and better than 50 percent for five more. This paper gives the present status of the program, a brief interpretation of the results, and future prospects.
We present new results on RR Lyrae stars and anomalous Cepheids in the Draco dwarf spheroidal galaxy. We have increased the number of double-mode RR Lyrae stars and found three new anomalous Cepheids. With period-magnitude and period-amplitude diagrams, we discuss the Oosterhoff classification of Draco. Contradictory results were found in that Draco appears to contain both Oosterhoff I and II type RR Lyrae populations.
The Wide-Field and Planetary Camera (WF/PC) is a CCD imaging instrument which is part of the Hubble Space Telescope. Ground-based observations have been made with a CCD system similar to those of the WF/PC in order to establish the standard star sequence to be used for in-flight photometric calibration. Because the WF/PC passbands differ from those in previous photometric use, the filters and CCDs will define a new photometric system. We outline here the procedures used to establish the calibration fields to be used in flight (see Harris et al. 1988 for additional details).
The following comments have been received on Mr. E. S. Calvert's paper, published in the last number of the Journal. A further selection of comment, including a note by Captain F. J. Wylie, and possibly a reply by Mr. Calvert, will be published in the next number.
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