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Thread: Blurry Barlow

  1. #1
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    Default Blurry Barlow



    I have a 2X meade barlow witch looks fine from the outside, but whenever I use it under an eyepiece whether its a 26mm or a 9mm the image gets real blurry and you have to line the object directly center to be able to make anything out. I'm wondering if the lens is just dirty or perhaps scratched? I have a Meade DS-2114S telescope, that is a D=114, F=1000, f/8.8
    Last edited by Savage750; 03-23-2009 at 05:33 PM.

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    Default

    Hey sav . Is it the barlow that came with the scope ? If it is , its just an inexpensive one and will just give degraded views .The one that came with my 114eq is junk and has long since been discarded . I think my kids burn ants with the thing .
    16in Night sky
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    Default

    thats good to know, do you recommend any specific one and do you know how well the adjustable zoom eyepieces do that celetron offer?

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    Default

    I have heard to stay away from zoom eyepieces . As far as barlows go check these guys for something inexpensive . You want at least a fully multicoated one and possibly ED glass ..It is good quality glass and the coatings help with letting light pass through ... Each time you add a glass element in to the viewing path it degrades the view . Meade makes some good glass but you could go with a GSO or ANTARES .both are good quality as is the meade #128 . I would not go with anything bigger than 2x ... Anyway check here . www.agenaastro.com They are the cheapest place i have found for accessories although i havent bought from them before .
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    Default

    Re Zoom EPs

    Yeah the cheaper end are really not so great, as they tend to only perform in a part of the available range.

    The more expensive brands are OK like Vixen and Baader. Also Antares "Speers Waler", and Televue, but note that the Speers Waler and Televue only zoom over a narrow range.

    The cheaper zooms like Celestron/GSO etc are OK I guess for a spotting scope, but for astro not so hot, better to stick with standard fixed focal length EPs.

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    Default

    so for looking at saturn and different nebulae would I be better off using my 25mm with a 2X barlow? And does using a barlow rather than a high power EP leave the picture brighter and more defined?
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    Default

    Hi again savage . If your looking at saturn or planets for that matter , you can use a barlow and amp up the power with a higher power eyepiece like your 9mm and barlow , But when looking at nebulas and other deep sky objects dont use a barlow , Low power is where its at for those . Nebula are gas clouds and they tend to be big and a barlow wont really let you see the whole thing .
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    Default

    I think what's happening with your barlow Savage, is field curvature.
    You do get that with cheaper barlows.
    Any lens or focussed mirror creates a sharp, in focus, image at a certain distance behind itself - you focus your eyepiece onto that image to get your view.
    Inherent in all optics, is a bit of curvature to that sharp image - in other words... If a lens of, say, 900mm focal length produces an image 900mm behind itself, that figure is for the centre of the image - due to field curvature, the edges may come to focus at around 899.5mm - and the further you get from the centre, the lower that figure, due to the curve of the 'field'.

    Some scopes produce a more curved field than others - for instance, fast scopes generally give more of a curve than slow scopes - and certain designs produce a more pronounced curve than others.
    Some advanced designs such as the 'Ritchie-Cretien', or 'Dall-Kirkham', are corrected to produce a very flat field (Hubble is a RC) - and unfortunately, barlows, especially cheaper ones - have the effect of curving the field.
    Another thing with barlows, is that they also generally create a very narrow 'depth' to the field too - this means that for instance, if you could get acceptable focus, within say, 0.5mm in front, or behind the focus point (tolerance) - add a barlow, and that comes down to, say 0.1mm 'tolerance'.

    Difficult to put into words - so here's a diagram to show what I mean


    Barlows (especially cheap barlows) tend to curve the field even more, and to narrow the depth of field.
    So if you imagine a similar thing coming the other way from your eyepiece - you get sharp focus where the opposing fields overlap.
    And if the scope/barlow combo is producing a highly curved, and narrow, field - it's easy to see why the edges can be fuzzy - the field is falling short of meeting the in-focus field of the eyepiece.

    This same principle also partly explains why widefield eyepieces don't generally work too well in fast scopes.
    (Fast scope = more curved field..combined with wider view of the EP)

    It also explains why compound newtonians (with a cheap barlow built into the focuser) struggle to give sharp images at high magnifications, and are notoriously difficult to find the very narrow focus-point.
    Last edited by Carlos_dfc; 03-25-2009 at 05:12 AM.
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