Showpieces Missed by Herschel
As diligent and thorough an observer as he was, William Herschel passed over a number of clusters, nebulae, and galaxies that are routinely viewed by deep-sky observers today using typical backyard telescopes. In some cases this is surely understandable – as for those objects apparently too small or too large in angular size for the telescopes he was using. In other cases, it is a real mystery how he ever missed them. Perhaps changing atmospheric conditions, the cooling of the metal mirrors and their varying reflectivity as they tarnished, and fatigue of the observer are among the explanations for these. There is also the problem of sweeping near the zenith using altazimuth-mounted telescopes as were Herschel’s, which create “dead spots” in the sky. But surely he would have allowed for this. (And speaking about observing near the zenith, we can only imagine what it was like attempting to look into the mouth of the vertically pointing tube when used as “front-view” as Herschel did. In the case of his mammoth 40-foot reflector, this placed him more than 40 feet above the ground, the supporting structure itself towering some 50 feet high!)
KeywordsOpen Cluster Angular Size Apparent Size Planetary Nebula Double Star
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