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  1. #1
    Tyler Welch's Avatar
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    Default Cannot focus with barlow



    Hello,
    I need some help here figuring out this problem.
    I recently purchased a Televue BigBarlow lens. Problem is, when I use it with my eyepieces (Orion Stratus 5mm 1.25", Explore scientific 100 degree 9mm 2"), I am unable to bring my focuser IN far enough to make an image.
    My telescope is a 10" celestron dobsonian with a closed tube.
    Would I be okay if I purchased a low-profile focuser?
    Thanks in advance for any input. Cheers!

  2. #2
    chainek's Avatar
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    Default

    Ugh. That sounds awful. I'm surprised nobody has come in with advice. I have no idea.

    Good luck.
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  3. #3
    deaman49's Avatar
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    Default

    You may have to simply move the Barlow or the eyepiece up or down to focus. Don't get frustrated and overcomplicate an otherwise simple thing. You've probably shove the Barlow all the way in, and the eyepiece too. Relax, nothing is broken.
    Lee

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  5. #4
    Tyler Welch's Avatar
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    Default

    Right now the barlow, they eyepiece, and the focuser are brought in as far as possible, and I am still not able to make an image. I'm not sure how much further in I need to bring the eyepiece, but my focuser won't get me there. Anybody have this problem before?

  6. #5
    jenniferchristine's Avatar
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    Default

    I don't think you have much chance using a 2.5mm with a 1250 focal length 10 dob.

    It exceeds the useful magnification - the focus point would be so hard to find, you've probably shot past it without realising
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  8. #6
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    Default

    the "sweet spot" for focusing at that magnification
    will be very small,so as jen has said,you may be racking
    the focuser too quickly and actually missing it all together.
    i wouldn't bother trying the 5mm and barlow,for the reasons
    stated by jen.
    the 9mm,i would only barlow on a REALLY good night,which will
    be few and far between.
    try lifting the ep and barlow up in the focuser tube,there are people
    who have this problem,and this tends to cure it.a little bit of trial
    and error will help find the "ideal" position.
    also,high mag is not neccessarily the be all and end all.
    so many have the misconception that the highest mag will offer
    the best views.the opposite is true,as the higher the mag,the more contrast
    you lose.
    the secret is to train your eyes to see more detail at lower mag.
    in all honesty,it would be very rare that i would go any higher than your
    9mm ep on it's own.
    hope this helps.
    clear skies,

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  10. #7
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    Default

    Hi, 500x with your telescope is to much power, you will have a hard time coming to focus.

    Tom

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  12. #8
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    Default

    Excellent advice here.
    After a year and a half observing I just came to the realization that big magnification is a tricky thing at best.
    Forgive the pun, but my eyes were opened when I went the other way and started appreciating the views that the added light of low power afford.

    Good luck.
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  13. #9
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    Default

    Two things will affect your attempt to focus. First, the seeing conditions are seldom good enough to permit 282x (1270/4.5 ). Second, the diffraction caused by your aperture kicks in at 254x. Both of these will prevent the image from being very sharp. Under excellant seeing conditions you could probably go to about 350x to 380x before diffraction would limit your focus.

    If you can focus with longer focal length eps, you don't have a focus problem. It's the weather.

  14. #10
    Tyler Welch's Avatar
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    Default

    thanks for the advice from everyone, it's nice to get feedback. I've only recently been checking out star clusters and really appreciating them, and the barlow isn't going to help with that. I just wanted to get as much magnification as possible for lunar and planetary viewing. I think I will still purchase a better focuser with a 10:1 micro focuser knob in order to get that perfect sharpness.

 

 

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