Hi Joe, Based on the way I understand it, you are correct. The Earth's atmosphere and the curvature of same, is the main reason the focus changes over long periods of viewing. The focus of an object changes as it climbs higher into the night sky where the atmospere is preceived to be thinner. I know when I was at Bryce this past fall, I noticed I didn't have to play around with the focus near as much as I do here in the valley. Temperature plays a part in focus as well. As the OTA heats and cools, it, like everything else, will shift creating focus problems. I'm not sure about the Hubble, but I believe that once outside of the influence of Earths atmospere, where the temperatures are stable, focus problems are less of an issue. But I may be wrong... Guy Quoting Joe Bauman <bau@desnews.com>:
Thanks, Jim, that's one reason! But while you have the view focused without wearing your glasses, why isn't that setting good enough for everything in the heavens above? Why do you have to refocus when you look at something else, or when you take a look another day? The only reason I can come up with, and it doesn't seem like a complete answer, is that the atmosphere might act like a lens and distort the view depending on the angle of the telescope to the curvature of the atmosphere. Is that the reason? Does Hubble need to focus when it points to a different target? -- Joe
This question I can at least partially answer. I wear some pretty strong glasses (4.5 diopters). Sometimes I view through the telescope with glasses removed. The focus adjustment allows me to make this work (well, the astigmatism is still frustrating...).
Jim
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