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  1. #1
    AndrewButler56's Avatar
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    Default New to telescopes - barlow.



    Really sorry - I bet this has been asked before. My wife bought me a skywathcer EQ1. I put in the wide angled lens and got a great view of the moon. I then tried the barlow and got a fuzzy mess!
    The next day in daylight I tried again, I focussed in on a distant building - the barlow gave a good view. Back to the moon that evening and a fuzzy mess again. I am sure there is an easy answer - what is it please????
    Andy
    Northampton - UK.

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    A few things come to mind when I read your post. Number one......what is the power of the EP your using and what is the power rating of your Barlow? < Well that is my few things that come to mind.....


    You may be trying to use too much power and you would get just that, fuzziness because you are beyond the scopes limit of HUM (Highest Useful Magnification).
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    I looked up the scope you stated and if I did find the right one the HUM is rated at 165x at a 700mm FL (Focal Length).

    If it came with the 25mm & 12 mm plossls and a 2x Barlow then you get with the EP's alone a 28x for the 25mm and a 58x for the 12mm. These should work in the scope. When you Barlow them then you get a 56x for the 25mm and a 116x for the 12mm. Both should have worked.

    But what was your seeing conditions as far as atmosphere and artifical variables like light pollution?

    Also keep in mind that a scope manufacturer will rate scopes on the high end in theory for HUM to have as a selling point. So if your scope says 165x HUM then I would myself drop that to about 130x on the safe side. That would be a 20-23% decrease in what they rate it at. I always go for a 25% decrease to be well on the safe side myself. Call me cautious but in my line of work I like being cautious.
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  5. #4
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    Default Thanks!

    Wow - what alot of information!

    The seeing conditions were excellent - full moon, by the sea, no artificial light. I still cannot understand why the set up worked in daylight but not at night time?
    Andy

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    Yeah - I also think that atmospheric steadiness (or unsteadiness) is probably the problem.

    If you look at a distant building, you are probably looking through maybe a mile, or even less, of air.
    Looking up into the sky, you are looking through 60 miles of air, or maybe much more.
    If the Moon was low above the horizon, you could be looking through 5 or 6 times that much air.

    Imagine looking at a penny on the bottom of a swimming pool - it's the same effect looking up throuhgh many miles of moving air, at high magnifications.

    It's often quoted not to go beyond 2x of magnification per mm of objective lens (or mirror)
    But that is under ideal conditions - If an object is low in the sky, that figure can come down quite a lot.
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  8. #6
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    Thanks Carlos!
    So does that mean my barlow is not really suitable for my telescope and I should use the wide angled lens supplied?
    Andy

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    It depends on the telescope Andrew.

    It would be useful to know the diameter of your telescope's objective, and what eyepieces you have.

    The atmospherics change from one night to the next - A lot of the time in UK, you can't get much above 150x, even when looking straight up, without atmospheric 'wobble' - But on some nights it's steady enough to push beyond 300x
    Astronomers call this the quality of 'seeing'
    If the 'seeing' is bad, then it doesn't matter what equipment you own, you won't get a good high-power image - but it's worth having some high magnifications available for those rare nights when the seeing is very steady
    Also, the height in the sky can make a big difference to the 'seeing' - If you look low above the horizon, you're looking out of the atmosphere at a shallow angle, and therefore looking though 5 or 6 times as much air as if you look straight up.

    As well as for getting high magnifiactions - Barlows can also be used to effectively give you a bigger range of eyepieces.
    You've probably noticed that the shorter eyepieces (High powers) tend to have very small eye-lenses that you need to squint into...
    Well if you use a 2x Barlow with - say - a 10mm eyepiece, you'll get the same magnification as using a 5mm eyepiece....
    However, the 10mm eyepiece wil have a bigger eye-lens, and will be more comfortable to look into.
    So the 'barlowed' 10mm, will be nicer to use than a non-barlowed 5mm - but still give the same magnification.
    And by barlowing your wide angle, lowest power eyepiece, you'll get a useful in-between magnification, with a nice big eye-lens to look into.
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