http://AstroWiki.jmhastronomy.com/index.php?title=Barrel_Size&feed=atom&action=historyBarrel Size - Revision history2024-03-28T16:42:56ZRevision history for this page on the wikiMediaWiki 1.31.0http://AstroWiki.jmhastronomy.com/index.php?title=Barrel_Size&diff=63&oldid=prevPhpdevster: Created page with "==Overview== Most eyepieces conform to one of three standard barrel sizes: * 0.965 inches (obsolete, no longer made except for very cheap telescopes) * 1.25 inches * 2 inch..."2019-05-20T04:21:32Z<p>Created page with "==Overview== Most eyepieces conform to one of three standard barrel sizes: * 0.965 inches (obsolete, no longer made except for very cheap telescopes) * 1.25 inches * 2 inch..."</p>
<p><b>New page</b></p><div>==Overview==<br />
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Most eyepieces conform to one of three standard barrel sizes: <br />
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* 0.965 inches (obsolete, no longer made except for very cheap telescopes)<br />
* 1.25 inches<br />
* 2 inches<br />
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The modern standard sizes are 1.25" and 2".<br />
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It should be noted that 2" barrels are not inherently an upgrade to 1.25" barrels the way say, a faster CPU or a camera with more megapixels might be. Different sized barrels are more akin to different sized paint brushes. Sometimes you need a larger paint brush, sometimes you don't. <br />
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==2" vs 1.25" Barrels==<br />
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The reason for the existence of 2" barrels is typically to provide a wider [[field stop]], which allows for a wider [[field of view]] at lower magnification. For example, a 40mm eyepiece in a 1.25" barrel can only have an apparent field of view of about 42 degrees, but in a 2" barrel it can have an apparent field of view of about 70 degrees. <br />
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But since not all eyepieces have such long focal lengths, it is not always necessary to house their optics in a larger 2" barrel. For example, the 13mm Tele Vue Ethos has an extremely wide 100 degree apparent field of view, but only requires a 1.25" barrel to achieve it. Conversely, the 17mm Ethos requires a 2" barrel in order to provide that apparent field of view at that focal length.<br />
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If you have a 2" focuser, don't select eyepieces based on their barrel size..The barrel size is merely a consequence of their optical design and characteristics. Instead, choose eyepieces for their focal length and apparent field of view, and don't worry about what the barrel size is. <br />
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==Common Maximum Apparent Fields of View By Barrel Size and Focal Length==<br />
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===1.25" Barrels===<br />
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* 40mm ~42 degrees<br />
* 32mm ~52 degrees<br />
* 24mm ~68 degrees<br />
* 16mm ~82 degrees<br />
* 13mm ~100 degrees<br />
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===2" Barrels===<br />
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* 40mm ~70 degrees<br />
* 32mm ~82 degrees<br />
* 25mm ~100 degrees<br />
* 9mm ~120 degrees (technically could be a 1.25" barrel, but the size and weight requires a 2" barrel to hold securely)<br />
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===3" Barrels===<br />
* 30mm ~100 degrees<br />
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==Misc==<br />
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===Dual Barrel Eyepieces===<br />
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It's common for some 1.25" eyepieces to also have an outer 2" barrel. The reason for this is so that they can be used in 2" [[focuser]]s without the need for an adapter. It has no bearing on the eyepiece's optical performance, it simply is a matter of convenience. <br />
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It should be noted however, that depending on the design of the barrel, using these dual barrel eyepieces in 2" mode might result in damage to a prism or mirror diagonal if it's too shallow to accept the eyepiece. <br />
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In some cases, the focuser might not have enough outward travel for the eyepiece to come to focus in 2" mode, since 2" mode puts the eyepiece's [[focal plane]] much deeper in the focuser than the 1.25" mode does.<br />
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===Other Sizes===<br />
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The only mass produced 3" eyepiece on the market right now is the Explore Scientific 30mm 100 degree eyepiece. Generally speaking, 3" eyepieces are not common as there is not a strong need for them. The only reason to have a 3" barrel is to achieve 100+ degree apparent fields of view in focal lengths longer than 25mm. However, the sheer size and weight of such eyepieces means they are only suitable for large [[Dobsonian]]s or strongly mounted refractors that have large focusers. However, large Dobsonians tend to have short [[focal ratio]]s, and 30mm eyepieces may result in [[exit pupil]]s that are too large to be usable.</div>Phpdevster