Eyepieces/Exit pupil

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2844az

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Thank you for answering my other questions on eyepieces. Here is another one that puzzles me. I realize that the theoretical maximum power of a telescope is not practical. Telescope companies often give you a 4 mm EP, that is useless. I think one good rule that I have read is: that you multiply the scopes aperture in mm times 1-1.5 and that will give you the practical power of the Telescope.<br /> I have been debating over the Exit pupil’s (ExPu) practical limits. Here is one equation just to remind some of you how to obtain the Exit Pupil’s size.<br />1. ExPu can be determined by taking the EP focal length and dividing it by the scope’s f/ratio. Example . EP 25mm/f/5=5mm ExPu<br /><br /> You need at least 0.5-1.0 ExPu to see. Question: Who can see with a ExPu of 0.5-1.0? Is that practical or should we look for eyepiece that will yield at least a 3 mm ExPu? And remember if you use a 2 x Barlow lens your f/ratio doubles and that decreases your ExPu in half. Thanks, Gary<br />
 
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2844az

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Sorry about the double post. The screen froze up and I wasn't sure the post went through. Gary
 
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