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  1. #1
    wgwrigs's Avatar
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    Default A bit confused, please help!



    Im trying to educate myself a bit and this all seems so complicated sometimes. Im excited about my new scope Astromaster 130mm eq. Im getting it for x mas and figured it was great starter scope!

    my confusion lies in lenses, would i be able to see jupiter better in a 10mm lens with a 2x barlow OR a 32mm plossl by itself for a wider field of view? or with a 2x barlow? or just buy a 5mm with no barlow...


    i read so many different opinons on this scope, any info would be appreciated!

  2. #2
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    Default

    a 32mm will,as you noted give you a wider view,
    and less magnification,so jupiter will appear
    quite tiny.barlow it,you get 16mm,better.
    if you barlow a 10mm,you get 5mm,the only benefit
    over the 5mm ep alone,is you retain eye relief.
    be aware though,higher mag can make the view blurry.
    what you gain in magnification,you will lose in contrast.
    i would not use the 5mm with your scope,it would be too
    much power on all but the best viewing nights imho.
    your 10mm would be as high as i would go,unless
    you get a 16-17mm,then you could barlow that.
    clear skies,

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    andy

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    revelation 20X80 bino's,camlink tp-2500 camera/bino tripod.
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  4. #3
    Pingu's Avatar
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    Default

    Andy brings up a very good point. I agree the 5mm would be to much power. I don't like barlows so I won't go there. But if you dig'em, maybe a 16mm with a 2x barlow would best suit you. Remember, you still have to contend with the atmospheric conditions in addition to your telescope's capabilities. The best views I get with Jupiter is with a 8mm on my 127mm and 90mm telescopes. After that, it gets blurry and details are lost.
    Cal
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  6. #4
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    Default

    I have to disagree with the above posts. That is an f/5 scope, so it should be able to handle a 5mm eyepiece under most conditions. That will be its practical limit, though. In truly excellent conditions, you might be able to barlow the 5mm.

    Generally, for planetary viewing, you want the most magnification that your scope can deliver in that night's conditions, because planets are pretty tiny. Sometimes, especially if conditions aren't great, a lower magnification eyepiece will give a clearer view.

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  8. #5
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    Default

    Awesome! thanks for the help! WOuld that same rule apply for saturn? Since its further then jupiter, would a stronger lens help or would the same rule apply for my scope.. maybe a 16mm with a 2x barlow still be best?

    should have asked that in my first post!

    thanks!

  9. #6
    Pingu's Avatar
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    Default

    Keith. Perhaps but I wouldn't chance it. I would stick to an 8 and maybe and barlow it or work my way up to a 5 if I felt comfortable increasing power. Otherwise, you gonna have an EP chiilin out doing absolutely nothing for you. Except for those once in a blue moon episodes.
    Cal
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  10. #7
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    Default

    I'm THINK... I'm with Keith on this one...in Theory anyway....

    In Real life however it may be a different story.

    The scope is an f/5 650 mm with 130 mm of aperture... using a 5mm eyepiece it will yield 130x....that is all of 26x per inch of aperture.. so it should be usable on any night that is not raining ...

    HOWEVER ... getting all the way up to 130x in my 100 mm f/6 600 mm refractor (almost the same spects as the Astromaster 130) is difficult on all but the best of nights ..The Astromaster does have an extra inch of aperture so he may be able to push it up to 130x.. but only on above average nights..

    That scope (like my 100mm refractor is not designed to be a Planetary telescope.. its a great wide field low powered telescope that would be used for viewing DSO's and would be a fantastic scope to use when viewing Open clusters..(what I mainly use my 100 mm refractor for)

    Also the Eyerelief on a standard 5 mm Plossel however may be "slightly" restrictive and a royal pain to use if one wore glasses...

    Bob G.
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  11. #8
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    Default

    Quote Originally Posted by wgwrigs View Post
    would i be able to see jupiter better in a 10mm lens with a 2x barlow OR a 32mm plossl by itself for a wider field of view? or with a 2x barlow? or just buy a 5mm with no barlow...
    Jupiter will be smaller, brighter, floating on a brighter sky background in the 32mm, and less sensitive to atmospheric disturbances.

    Jupiter will be larger, dimmer, floating on a darker sky background in the 5mm (or Barlowed 10mm), and significantly more sensitive to atmpspheric disturbances.

    There will be essentially no differences between a 2X Barlowed 10mm and a 5mm eyepiece, excepting for eye relief.

    As a general consequence of atmospheric disturbances, the EP selection at the higher power end of the FL spectrum will want more focal lengths from which to choose than in the wide field end of the EP collection. Thus, while a 3/2 (or 2/3) ratio exists between EPs at the wide low power end, as you get donw towards 8mm you start wanting 1mm steps to tune the magnification to what the sky is permitting tonight.

  12. #9
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    Default

    Quote Originally Posted by KeithBC View Post
    I have to disagree with the above posts. That is an f/5 scope, so it should be able to handle a 5mm eyepiece under most conditions. That will be its practical limit, though. In truly excellent conditions, you might be able to barlow the 5mm.

    Generally, for planetary viewing, you want the most magnification that your scope can deliver in that night's conditions, because planets are pretty tiny. Sometimes, especially if conditions aren't great, a lower magnification eyepiece will give a clearer view.
    Keith are you saying in truly excellent conditions a f/5 scope can handle a 5mm lens barlowed to 2x, whcih would equal an 2.5mm EP? -Greg
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  13. #10
    KathyNS's Avatar
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    Default

    Quote Originally Posted by GWING View Post
    Keith are you saying in truly excellent conditions a f/5 scope can handle a 5mm lens barlowed to 2x, whcih would equal an 2.5mm EP? -Greg
    Yes.

    The general rule is that you will hit the diffraction limit of the scope with an eyepiece that has a focal length in mm equal to the focal ratio of the scope. 5 mm for an f/5 scope. (Most people perform the calculation by converting aperture to magnification and back to eyepiece focal length, but that is way too much math, and gives the same result anyway.)

    In excellent conditions, you can double the magnification of the diffraction limit (2.5 mm for an f/5 scope), because although you can't get any more resolution, greater magnification in steady atmospheric conditions will allow you to see the existing detail more clearly.

    Obviously there are other variables besides the atmosphere. The quality of the optics, for one thing. However, that is a general rule of thumb.

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