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  1. #1
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    Default Newbie having trouble with a barlow lens



    Hello everyone, I just bought my first Dobsonian telescope (8 inch aperture 1200mm focal length). I've been able to use it twice now and it has delivered some pretty spectacular views. I cannot understand why it is that when I use my barlow lens with the lowest powered eyepiece (30mm) I get a defocused image. No matter how much I try to focus the image it remains out of focus. Can anyone help me understand how to use my barlow lens effectively?

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    That used to happen to me but it turned out to be dew on the eyepiece. I would make sure you have the eyepiece all the way in the barlow and the barlow all the way in the scope. I get wacky views when I use mine sometimes, it has a bit of a learning curve.
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  3. #3
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    Hey Eden, Welcome to the forum. Not all barlows have the same focus distance. Try turning your focus tube all the way in, then put your barlowed EP in and, without tightening the set screws, pull it out while looking through it to see if you can achieve focus. Try this with the focus tube set half-way if you can't. Some combinations of eyepieces and barlows, and indeed even some eyepieces themselves, cannot achieve focus when they are fully inserted, and must be left out a bit. Sometimes you have to change to a shorter or longer barlow.

    In the photo below, you can see that in order to achieve focus with this particular combination of eyepiece, and, in this case, not a barlow but an erector prism for terrestrial viewing, (but the principal is the same), I had to pull the assembly out of the focus tube a bit.

    You can buy rings with a set screw for setting this distance.
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  5. #4
    Eden's Avatar
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    Thanks WWPierre. I will try this as soon as conditions allow. We've got a storm moving into Southern California and so I will not be able to see anything for the next 5 days or so.

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    Let us know how it goes. Try it in daylight.
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  7. #6
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    I had a problem with focusing on my Dob. One thing I did to fix the problem is the position of the primary mirror. You have approx .5 to .75 inch of adjustment while columinating your primary mirror. If my calculations are right this equates directly in focual point of your eyepieces. My problem was a burr on one of the screws limiting adjustment, which I repaired. Now all my EP's focus while being fully seated.

 

 

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