This license is commonly used for video games and it allows users to download and play the game for free. There are many different open source licenses but they all must comply with the Open Source Definition - in brief: the software can be freely used, modified and shared. Programs released under this license can be used at no cost for both personal and commercial purposes. Open Source software is software with source code that anyone can inspect, modify or enhance. Freeware products can be used free of charge for both personal and professional (commercial use). As recently as the 1980s, even professional astronomers could only dream of such instantaneous communication and proc- sing ability.Freeware programs can be downloaded used free of charge and without any time limitations. Developments can be e-mailed to other interested amateurs in real time, during an observing session, so that when a cataclysmic event takes place amateurs worldwide know about it. In the modern era digital images can be obtained in minutes and analyzed ‘on the fly’ while more images are being downloaded. However, even those telescopes took hours to reach such limits, and then the photographic plates had to be developed, fixed, and examined by eye. Such limits would have been within the realm of the 100- and 200-inch reflectors on Mount Wilson and Mount Palomar in the 1950s, but no other observatories. An amateur equipped with a 30-cm telescope and a CCD camera can easily image objects below magnitude 20 and, from very dark sites, 22 or 23. Today, at the start of the twenty-first century, amateurs are better equipped than any professionals of the mid-twentieth century, let alone the nineteenth. The brighter variable stars were monitored, but photography was in its infancy and digital imaging lay a century in the future. The Moon and planets, as well as bright comets, were the key objects of interest. In the Victorian era – or for non-British readers, the mid-to-late nineteenth century – amateur astronomy tended to center on Solar System objects.
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