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that are being shown on the screen. But if galaxies (for example) are currently turned off, you're obviously unable to click on one in order to turn galaxies on. In such cases, you need to use the Data Shown dialog. It's also convenient if you want to change the display of many types of object at once. In the Data Shown dialog, you are given a list of fifteen classes of objects controlled by the Data Shown menu/dialog: Messier NGC + IC Galaxies Nebulae Planets Variables NSV Open clusters Asteroids Gal Clusters Comets Globulars Planetaries Dark Nebulae Satellites Each object class is followed by buttons to turn that class "on", "off", "auto", or "fixed". When a particular class of objects is turned On, all objects of that class will be shown, regardless of magnitude. When Off, none will be shown. When set to Auto or Fixed, all objects brighter than the magnitude limit for that class will be shown. The difference between 'fixed' and 'auto' is a simple one. In 'auto' mode, Guide will automatically adjust the limiting magnitude as one zooms in or out, in an attempt to maintain a constant density of objects. (This is always done for stars, for example.) But in 'fixed' mode, you are essentially telling Guide that you want a class of object to be shown to a particular limiting magnitude, and that the zoom level should be ignored. If an object class is turned On or Off, the magnitude limit is irrelevant, and is therefore grayed out. Next to these four radio buttons, there is (usually) a labelling button and a button to select the color for that class of object. For "Gal Clusters", the panel shows an extra three-way switch between Zwicky On, Abell On, and Abell + Zwicky On. The Abell catalog of clusters of galaxies covers the entire sky, and contains over 5,000 objects. The Zwicky catalog only covers the northern half of the sky, but it has over 9,000 objects. Some objects are found in both catalogs. Depending on where you are in the sky, you may wish to see the Abell
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