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  1. #1
    Chris McC's Avatar
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    Default Eyepiece Selection



    Hi,
    I've had my Celestron Astromaster EQ130 since Christmas and I am already getting the urge to buy more stuff for it. I Had a great view of Saturn recently but, with only the 20mm and 10mm eyepieces that came with the telescope, i only get 66X magnification. I would like to buy a 2x Barlow. I know it won't benefit the 20mm EP as i have a 10mm but i think it would be a benefit if i select any new lenses carefully.
    So my questions:
    What would be the best high power lens for me? (assuming i have the 2x Barlow)
    The spec sheet for the telescope states the 'Highest Useful Magnification as 306X. Can i expect to be able to use this level of magnification or should i buy a lens that doesn't get to the limit?

    Thanks Chris

  2. #2
    Stormrider51's Avatar
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    Default

    Chris,
    I believe the focal length of your telescope is 650mm. Divide that by the focal length of your eyepiece to find magnification. If we assume the 10mm EP then you are effectively getting 65X mag. Add in a 2X Barlow and you are at 130X. Not anywhere near the theoretical limit.

    Now before you go running off to buy a much smaller EP let me give you some things to think about. Telescopes aren't really about magnification. They are about light-gathering. That "highest useful magnification" is an optimistic number. All scope makers want to max out that number to attract buyers. It assumes, among other things, nearly ideal viewing conditions and the scope being perfectly collimated. It's safe to bet that a really small EP will spend most of its' time sitting in the case instead of being used because you will discover that while the object in it is bigger it isn't clearer. I don't know about you but a tiny disk is just as interesting as a big blurry one. Can't tell much about either.

    I'd suggest you getting a good quality 2X Barlow and see how it works with the 10mm EP you have. Double check the collimation of your scope and wait for a really good night.

    John

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  4. #3
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    Default

    I second the barlow option, and suggest Seibert Optics for some good info on barlows. I got mine there after much deliberation. My scope is 750mm and and these baelows seemed to fit my scope specifications. They are hand crafted so delivery is a wait. The curse of clouds hasn't lifted since my barlow arrived so it hasn't seen first light yet!
    Eric
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  6. #4
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    Default

    John:

    To be brutally honest I own 4,5,8 and 11 inch Telescopes and no matter what scope I am using on any given night it is rare for me to go over 250x on anything except the moon...

    It is not my telescopes that limit my magnification its the darn sky...

    I suggest that you buy an eyepiece that when barlowed gave you about 225-250x as your initial purchase... Also I think most beginners immediately (I know I did) want to buy high powered eyepieces and do...but soon learn they use lower powered eyepieces 2-3 times as much... so I'd buy accordingly and utilize the barlow for high powered observing...

    Bob G
    CPC1100 housed in a slotted domed observatory (Exploradome) 4 and 5 inch refractors for use from the lawn, a 8" Sct (NS 8i) for star parties...
    I Hate the winter so I use heated Motorcycle clothing to stay warm while observing in winter
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  8. #5
    Chris McC's Avatar
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    Default

    Thanks for the good advise - i think i'll be going for a 6.5mm and a 2x Barlow. That will give me mag of 33, 66, 100, 132, and 200.
    I think i will get 32mm also for a wide field of view

  9. #6
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    Default

    Hi chris .I aggree with bob327 on this one ..I have a 2000mm 8in scope and they say you can get 600 power out of it , Yea right ..Not on a good night or ever ..The most i got out of it was 450 or so but the seeing has to be great... One thing you need to consider is exit glass ..some high power eyepieces have real small exit glass .... Another thing to look for is fully multi coatings and blackend edges ...And field of views ....
    Last edited by roverich; 03-16-2010 at 12:45 AM.
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