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What's The Difference Between 26mm 1.21" And 29mm 1.21"??

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Posted by Martin on June 9, 2001 06:02:41 UTC


I'm building my first telescope and I'm in the business of ordering eyepieces. I have a 9.8" focal length 1225mm (around f/5) telescope but I don't know what kind of eyepieces I should choose? I'm buying the Harry Siebert eyepieces which I believe is a great bargain. I've ordered the 10mm, 21mm, 35mm and the variable barlow. Now after having used Newt for a while I realized that the exit pupil on the 35mm would be 7mm! This is probably much bigger than my eye will ever get?! Now exactly what does this mean? Will it be difficult to use, and perhaps difficult to keep the eye in the right place?
So what I was planning to do was to get either the 29mm or the 26mm instead. Now this would give me an exit pupil of 5.8mm & 5.2mm. Now here comes the tricky part. The 29mm gives an actual field of view of 1.211" and the 26mm 1.253"??? This does not compute in my little head :? There just has to be a difference between the two right? Otherwise everyone would get the 26mm. And how can the same actual field of view be shown at 5.2mm and 5.8mm? Wouldn't that make the picture of the 26mm smaller while it actually shows more and at higher magnification?!

Please I'm going crazy so someone explain this to me before I blow up!


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