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Electronographic Photometry of Star Clusters in the Magellanic Clouds
Published online by Cambridge University Press: 14 August 2015
Extract
The high quantum efficiency, linear response, large dynamic range and large number of picture elements make the electronographic camera the most suitable of the presently available detector systems for the study of the colors and magnitudes of stars in the clusters associated with the Magellanic Clouds. Owing to the remoteness of these objects and the consequent small angular separation of the stars within them, the best method of determining electronographic magnitudes of the cluster stars is still that of measuring the star image profiles on one-dimensional scans made using a Joyce Loebl microdensitometer. While this technique does not utilize all of the information contained in the star image, it provides better correction for overlapping star images and variable background than is achieved with two dimensional methods that simulate observation through a focal-plane diaphragm. By this method, it has been possible to measure a star of B =23.8 on a 180 min exposure in 2.0 arcsec seeing taken with a Spectracon image converter on the 1.5 m Tololo reflector. The probable error of this measurement is ±0.12 mag, while the error due to photon statistics is ±0.06 mag. Thus, using electronographic cameras on larger telescopes and in good seeing, it should be possible to study the stellar content of the Magellanic Cloud clusters down to about the luminosity of the Sun. Further, current studies indicate that if a sufficient number of electronographic magnitudes are determined in a cluster, these can then be used to set up calibration curves as a function of radial distance from the cluster center, permitting photographic photometry of a large number of stars to be carried out, free of the usual limitations imposed by the effects of the unresolved background light of the cluster on the photographic measures. Using this technique, cluster observations can be carried out effectively using a small-field electronographic camera such as the Spectracon, which may be much smaller and less expensive than the large-field electronographic cameras, and thus more practical to use at distant observing sites.
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- August 29 Open And Globular Clusters
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- Copyright © Reidel 1980