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  1. #1
    Younge's Avatar
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    Default Unable to get perfect focus on Jupiter



    Hey Folks,

    I have only had my Celestron 127 SLT for a couple of weeks now, and I am loving it (when the nights are clear enough to use it!!!) but I have a query that I have just noticed.

    Last week I bought a TeleVue 2x Barlow, and, using it with the two EPs that come with the scope (9mm and 25mm), all seems to work well - except with the 9mm EP.

    I have spent most of my time looking at Jupiter, and I have noticed that I cannot get a perfect focus with the 9mm EP and the 2x Barlow together - it is always just a little bit blurry. When I use the Barlow with the 25mm EP I can get a very good, clear focus on the planet.

    Is there something I am doing wrong? Is there something I am not doing that I should be doing?
    Celestron NexStar 127 SLT
    That's all for now!!!

  2. #2
    ferkijel's Avatar
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    I don't have experience with this particular scope, but with its 1500mm focal length you are getting x333 magnification through your 9mm x2 Barlowed. It's a bit high for your 5 inch scope, so I would assume it's quite normal for it to lose crispness.
    Zhumell Z12 Deluxe 12" Dobsonian
    Celestron Astromaster 130 EQ
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  4. #3
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    Default

    hello young,
    as ferkijel said the magnification is too high for the 5" telescope, in general the maximum reasonable magnification for your telescope = 127mm * 2 = 254 but thats also depends on many other factors such as the weather, light pollution in your area,...
    i have an 11" telescope and the maximum magnification i use is x280 when conditions are perfect, try to use lower magnifications to get a steady sharp view
    goodluck
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  5. #4
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    Quote Originally Posted by khaledb View Post
    hello young,
    as ferkijel said the magnification is too high for the 5" telescope, in general the maximum reasonable magnification for your telescope = 127mm * 2 = 254 but thats also depends on many other factors such as the weather, light pollution in your area,...
    i have an 11" telescope and the maximum magnification i use is x280 when conditions are perfect, try to use lower magnifications to get a steady sharp view
    goodluck
    Would you please tell me what the optimal magnification for the Celestron AstroMaster 130EQ? Here are the stats:

    Celestron AstroMaster 130EQ Telescope Features:
    Telescope Type: Reflector.
    Magnification: 33x (20mm EP) , (65x 10mm EP).
    Focal length: 650mm.
    Focal ratio: 5.
    Aperture: 130mm.
    Eye piece: 20 mm.
    Optical coating: Aluminium.
    Mount: CG-3 Equatorial.
    Weight: 12.7 kg.

    When I upgrade my eyepieces, I would like to get the most bang for my buck. I can currently view Jupiter with minimal clarity. I would like to see the GRS and the different colors of the stars.

  6. #5
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    Default

    maximum reasonable magnification = Aperture in mm * 2
    = 130 * 2 = x260
    but still depends on other factors
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  7. #6
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    Default

    I would stick to x25/inch under PERFECT conditions (some would even say x20). In Riverside, which is a red area (or a 7/8 in the Bortle scale), I would suggest max x15 to x18. That means around x100. For the Astromaster that equals a 6.5mm EP. If you go to a 5mm EP, you will have quite significant sharpness and brightness loss. You can always go higher, but whatever "size" you gain will be a quality trade-off.

    Now...let me guess what you're thinking: "this scope is a piece of junk...I can only get so little magnification !". If that is the case, calm down a bit...you know what is my favourite eyepiece for my 12" ? A 30mm, which gives me a magnification of "only" x50. The FoV you can see through it is fantastic. And the brightness of the objects is excellent.

    Magnification doesn't necessarily mean better. Unless you are looking at M57 through a 45mm EP in a 12"...you don't really need a lot of power. You can see quite a bit of banding in Jupiter even at the apparent small size you can get out of the Astromaster 130.
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  8. #7
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    Quote Originally Posted by khaledb View Post
    maximum reasonable magnification = Aperture in mm * 2
    = 130 * 2 = x260
    but still depends on other factors
    Khaled, I think that is too much. The Astromaster cannot possibly give you x260. The absolute max you can push it is x30/inch, a max of x160. Above that it TRULY is useless, unless you're looking at the moon or your next door neighbour. Not that I do that !
    Zhumell Z12 Deluxe 12" Dobsonian
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  9. #8
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    Default

    hello ferkijel,
    i meant under perfect conditions, at least 50km away from any light pollution sources, steady winds,low humidity ... please read my first reply my friend
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  10. #9
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    You're MAXIMUM would be 5.12*60 = 307.2X (60 times the aperture in inches)...rounded up that would be a 3mm EP. Don't run out and buy one: you will get to use that eyepiece twice a year in "Darkside, Montana." I would shoot for something in the 140X-150X magnification; so you're looking at around a 5mm EP.

    Technically I could use a 4mm EP in my 1200mm focal length scope (300X), but the most optimal views I've had with a high power piece have been in the 7mm-8mm range. Any higher is very dim, ridiculously hard to focus, and hard to keep objects in the field of view.
    Allen
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  11. #10
    WWPierre's Avatar
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    Another factor is that at high magnifications the focus "sweet spot" gets smaller and smaller. So small that it will get lost in the slop of a cheap focuser. Most astronomers observe at magnifications below 150x most of the time, unless they have perfect conditions. The sweet spot also is smaller the lower the focal ratio of the scope.
    Last edited by WWPierre; 12-05-2010 at 09:08 AM.
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