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  1. #1
    kevint's Avatar
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    Default Another Barlow question



    I am using a 102mm f/7 refactor and a set of TV Plossls - 32, 20, 15, 11 and 8 mm. I want to increase the magnifications for planet and moon viewing. I looked at the TV 3x barlow which would put the 8m at about 270x. But this barlow is 5" long and I wonder if I will run out of backfocus when I put it in a diagonal. Plus the stability of having this thing sticking out of a diagonal.

    If I put the barlow before the diagonal does this increase the magnification factor? If so by how much? If it is enough I may be able to get by with a 2x barlow in this position instead. Is this a reasonable thing to do? Are there drawbacks to doing it?

    One more question... Is there an advantage to using a 2" diagonal in a 2" focuser with 1.25 eyepieces instead of a 1.25" diagonal?

    Thanks, Kevin

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    Default

    Not to sure about the back focus. I'd call televue and ask them. If I remeber, the barlow before the diagonal will and another 50% mag. So the 5x would be 7.5X. Nothing will gained with a 2" barlow in your situation. The advantage of the 2 incher would be if you bought some 2" eps in the future you could barlow them as well. Cheers.
    name: Derek

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    and such.

  3. #3
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    Default

    Hi Kevin, for the back focus situation, I'd say (albeit not 100% sure) but although it's quite long this should still be okay, I've not heard of running out of back focus with teles before, sorry I guess this isn't a straight yes or no!
    Nexstar 6SE, Celestron Ultima LX eyepieces, Canon 5D MKII
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    Default

    Your 8mm with a 3X barlow is going to give you a very small exit pupil (8/(3*7) = 0.38). You might see a lot of floaters in your eyes at that exit pupil, especially when looking at bright things like the moon. Just something to consider.
    Sky-Watcher 8 Inch Collapsible Dob with Astrozap Shroud

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  5. #5
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    Default

    My question is exactly what telescope do you own...

    I own both a 4 inch f/6 Refractor and a 5 inch f/9 Refractor (Orion and Meade both Achros)

    NEITHER can be pushed up much past 150x even on pretty good nights...I just have my doubts if your scope could handle that much magnification...

    Bob G.
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  6. #6
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    Default

    I have used a 2X Barlow before the diagonal to increase the magnification up to maybe 270X om my Skywatcher 90 refractor when watching Saturn. Used it with a 10 mm eye piece. It works but I had to move the focusing tube a long way out. According to the specs the maximum usable magnification of this scope is 180X so this was also a way of avoiding to buy an extra, maybe too extreme, eye piece.

  7. #7
    kevint's Avatar
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    Default

    Quote Originally Posted by Bob327 View Post
    My question is exactly what telescope do you own...

    I own both a 4 inch f/6 Refractor and a 5 inch f/9 Refractor (Orion and Meade both Achros)

    NEITHER can be pushed up much past 150x even on pretty good nights...I just have my doubts if your scope could handle that much magnification...

    Bob G.
    My scope is an Explore Scientific 102mm APO f/7 (FL=714mm) refractor. Using the 50x per inch rule of thumb for max magnification puts this size scope at 200x with good seeing.

    I decided against the 3x barlow and went with a 2x. Without the barlow an 8mm eyepiece will give a magnification of about 89x. The 2x barlow will increase this to about 178x. The 11mm eyepiece will give a barlowed magnification of about 130x. I have the 2x barlow now, but haven't had any clear skies to try it out.

    Thanks,
    Kevin

  8. #8
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    Default

    Good idea to go with the x2 barlow. Very nice range of magnification you will have.
    Josh L.

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