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Thread: Barlow Lens Vs Telecentric Lens

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    Default Barlow Lens Vs Telecentric Lens



    I have a Sky-Watcher 10" Dobsonian. It has a 1200mm focal length, and a focal ratio of f/4.7. I have found that the 2.5x barlow I purchased with the scope is too powerful for a 10mm EP to focus, and am going to return it and get a 2x.

    I am considering getting an Explore Scientific 2x Focal Extender (2") which is a Telecentric lens (as opposed to a Barlow), and I'm wondering if this type of lens has the same effect of reducing the focal length as a Barlow does. My concern is that if it does not reduce the focal length, then I will still not be able to use it with a 10mm eyepiece.

    Many thanks.
    Roger
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    Default Re: Barlow Lens Vs Telecentric Lens

    It should work but will be close to as far as you can push it with any regularity. It works with my AD12 at f/4.7 using my 11mm ES 82.
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    Default Re: Barlow Lens Vs Telecentric Lens

    Thank you Ozman13. It's reassuring to hear from someone who it using the gear.
    Roger
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    Default Re: Barlow Lens Vs Telecentric Lens

    Quote Originally Posted by rogercorbett View Post
    I have a Sky-Watcher 10" Dobsonian. It has a 1200mm focal length, and a focal ratio of f/4.7. I have found that the 2.5x barlow I purchased with the scope is too powerful for a 10mm EP to focus, and am going to return it and get a 2x.

    I am considering getting an Explore Scientific 2x Focal Extender (2") which is a Telecentric lens (as opposed to a Barlow), and I'm wondering if this type of lens has the same effect of reducing the focal length as a Barlow does. My concern is that if it does not reduce the focal length, then I will still not be able to use it with a 10mm eyepiece.

    Many thanks.
    Celestron Luminos is a great EP line, I have 3 EPs and love them on DSOs. However they have a soft focus and will not produce sharp images above ~150x. So using your Luminos with 2x barlow for 240x will not be productive.

    You might be better off getting EP in 5-7mm focal range instead. Depending on you budget I would suggest Explorer Scientific 82 deg 6.7mm or Celestron X-Cel LX 7mm or 5mm. These EPs produce sharp images even at high magnification. I pushed them upto 400x on Jupiter in my 8" SCT.

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    Default Re: Barlow Lens Vs Telecentric Lens

    Interesting, I got the impression from different sources that 10mm is the maximum magnification that would work with my scope: 10" (254mm) mirror, 1200mm focal length, f/4.7 focal ratio. Is there any basis for this? Would a 5mm eyepiece function with my scope?
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    Default Re: Barlow Lens Vs Telecentric Lens

    Quote Originally Posted by rogercorbett View Post
    Interesting, I got the impression from different sources that 10mm is the maximum magnification that would work with my scope: 10" (254mm) mirror, 1200mm focal length, f/4.7 focal ratio. Is there any basis for this? Would a 5mm eyepiece function with my scope?
    For telescopes large than 8", maximum magnification is limited by viewing conditions rather than optical limits. Most nights it will be ~150x, but there will be a few nights with excellent seeing when you can push it to 200-400x. 5mm will give 240x in your scope, so it is definitely doable. Also on the Moon you can do 240x pretty much any night (or day ).
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    Default Re: Barlow Lens Vs Telecentric Lens

    The general rule of thumb is that on typical nights, your eyepiece focal length shouldn't be any shorter than approximately your scope's focal ratio. In your case your dob is f/4.7, so according to that general guideline then an eyepiece of 4.7mm would be the shortest you would likely be able to use on average nights. Of course as BigZ said, your seeing conditions is what truly imposes the limits. All general rules aside, there will be times when you can go higher in magnification than what a 4.7mm will yield in your scope, and there will be nights when you will not even be able to use a 4.7mm eyepiece. It is up to you to test the seeing waters so to speak to find your limits at a given time. Seeing varies night to night, hour to hour and often minute to minute. As you step your way through increasing magnification and depending upon the nature of the object you are viewing, you will reach a point where the view goes soft or blurry no matter how much you fiddle with the focus. Then you know you've gone too far, and you drop back to the point where you get the best view. Its always an experimental thing as conditions are always changing, even if minutely.


    Also, one of the differences between a regular barlow and a true telecentric device is that the barlow can shift the focus point outward (or extend the eye relief). With shorter focal length eyepieces that may not be a bad thing as some can have shorter eye relief than is desired for some folks. But with some longer focal length eyepieces, it may push the focus point out more than desired making eye positioning more critical. A true telecentric device retains the native eye relief of the eyepiece.
    Last edited by KT4HX; 05-18-2016 at 03:58 AM.
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    Default Re: Barlow Lens Vs Telecentric Lens

    A 5mm will be as high as you can go with good seeing. There will be those exceptional nights where you can do better, but most of the time, no. How much $$ do you want to spend to be prepared for those short times of exceptional seeing? It's a personal preference and decision. If budget is a concern, I'd stop at 5mm.
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    Default Re: Barlow Lens Vs Telecentric Lens

    Thanks everyone for your input. I have plenty to think about before deciding to get any more glass.
    Roger
    Sky-Watcher 10" SynScan Dobsonian. Explore Scientific 82° 6.7mm, 11mm, 18mm, 30mm.

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    Default Re: Barlow Lens Vs Telecentric Lens

    Quote Originally Posted by KT4HX View Post
    Seeing varies night to night, hour to hour and often minute to minute. As you step your way through increasing magnification and depending upon the nature of the object you are viewing, you will reach a point where the view goes soft or blurry no matter how much you fiddle with the focus. Then you know you've gone too far, and you drop back to the point where you get the best view.
    I very much agree with Alan's quote here. It probably sums up the reasons why I'm very much a fan of zoom lenses. My Baader Hyperion Mk111 8mm-24mm zoom straddles very nicely the F/10 ratio of my C6 SCT. I zoom in and get to the soft or blurry point that Alan speaks of and then back off as far as is necessary to get the sharper view. I find it far more convenient than constantly changing eyepieces. One caveat on zooms though is its only the better ones that remain parfocal (or nearly so) while you zoom in or out.

    Alec.
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