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  1. #1
    Adoniram's Avatar
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    Post Probably ridiculously simple questions, but...



    So I'm trying to develop a strategy for what I'm going to buy, and when, but I need a little guidance.

    I've settled on either the C6-NGT or C8-NGT, only leaning toward the C8 because of its diameter and 2" eyepiece compatibility. However, I am having trouble finding 2" eyepieces as they seem much more uncommon. So... my questions...

    1. I understand that a smaller focal length for an eyepiece means greater magnification. Is the magnification the same for 2" and 1.25" eyepieces of equal focal length? In other words, would a 9mm 2" eyepiece have the same magnification in the same telescope as a 9mm 1.25" eyepiece?

    2. Is there a mathematical formula I can use to determine magnifying power for each eyepiece focal length, given the stats of a particular telescope? Well, actually, I'm sure there is, but I'd like to know if anyone here knows it

    3. Is there any reason to get a 3x barlow, or is a 2x more useful? It seems much more common, and makes me wonder why more people don't use a 3x.

    4. Is "eye relief" the diameter of the viewing side of the eyepiece? (i.e. how big the "hole" is that you look through)

    Thank you very much for your help!

  2. #2
    Phil Leigh's Avatar
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    eye relief is how far away from the surface of the eyepiece element your eyeball needs to be/can be. You don't want to have your eyeball pressed against the lens (0mm eye relief!). Very important if you wear glasses.

    Choice of Barlow depends on what eyepieces you have... Not much point in getting a 2x Barlow if you have 20, 10 and 5 mm eyepieces!

    Magnification = focal length of scope / focal length of eyepiece.

    2 inch eyepieces can actually be fitted to either scope you mention if you attach a suitable visual back+diagonal...although there is little point since the internal baffle tube is less than 2 inches!
    You do not need 2 inch eyepieces for those scopes.
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  3. #3
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    There is no advantage to 2" eyepieces over 1.25" ones, except that it makes wide angle eyepieces possible. If it isn't a wide angle eyepiece, it won't be offered in a 2" format. You just use an adapter to insert the 1.25" eyepiece.

    Magnification is calculated as the focal length of the scope divided by the focal length of the eyepiece. So, a 27mm EP in an 800mm scope gives a magnification of 800/27, or 30x. The diameter of the barrel has no impact on magnification.

    Which barlow to get depends on which eyepieces you have and what scope they are going into. You want to avoid over-powering the scope. The limiting magnification of a scope is equal to the aperture in millimetres. In exceptionally good conditions, you might be able to double that.

    Eye relief is the distance between the lens and your eye at which you can see the entire field of view.
    Last edited by KathyNS; 05-02-2012 at 06:32 PM.

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  4. #4
    magic612's Avatar
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    Hi there Adoniram,

    Those are really good questions. First, I'd suggest watching these videos (though you probably won't need to watch video 1):

    Informational astronomy videos

    The one thing I don't cover in those is a good question you asked:

    "Is the magnification the same for 2" and 1.25" eyepieces of equal focal length?"

    The answer is "Yes... BUT" because though the magnification is the same, the field of view will likely be larger in the 2" eyepiece. See the videos, and hopefully that will help you get a better understanding. Then ask any follow up questions you may need to ask.
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    , but I am getting help. Every time I look at the heavens, it helps.
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  6. #5
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    ..2 inch eyepieces at longer focal lengths, like 40mm, can give a larger FOV... a 1.25" 40mm largest field of view is around 43 degrees.. a 2 inch 40mm can have 62 degrees or more FOV..so i think for 40mmto 30mm range the 2 inch may be better....plus there may be a comfort factor with the 2 inch also..
    Last edited by thoany; 05-02-2012 at 09:14 PM.
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  7. #6
    Adoniram's Avatar
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    Thank you all for the helpful replies! That really helps me understand, and those videos are outstanding! Are they made by someone who has an account on this forum?

  8. #7
    magic612's Avatar
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    Quote Originally Posted by Adoniram View Post
    Thank you all for the helpful replies! That really helps me understand, and those videos are outstanding! Are they made by someone who has an account on this forum?
    Yes, I made them. I need to finish up Part 6, but... it was clear tonight, despite what the forecast said. So... I was outside looking at the sky.
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    , but I am getting help. Every time I look at the heavens, it helps.
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  9. #8
    Adoniram's Avatar
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    Quote Originally Posted by magic612 View Post
    Yes, I made them. I need to finish up Part 6, but... it was clear tonight, despite what the forecast said. So... I was outside looking at the sky.
    Well, glad to meet you then

    Thanks again for taking the time to make such easy to understand videos that also happen to be so comprehensive!

  10. #9
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    Quote Originally Posted by Adoniram View Post
    Well, glad to meet you then

    Thanks again for taking the time to make such easy to understand videos that also happen to be so comprehensive!
    Nice to meet you too.

    I wouldn't classify the videos as being comprehensive necessarily (though I appreciate the compliment!). What I really tried to do was give an overview of each concept/item with enough information so that those who are new to telescopes can have a good understanding and therefore ask better questions on forums like these, or make better decisions about purchases.

    Hopefully I succeeded in doing that.
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    , but I am getting help. Every time I look at the heavens, it helps.
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