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  1. #1
    Barry S. Brown's Avatar
    Barry S. Brown Guest

    Default decreasing length of Televue 3x barlow- what will happen?



    In a cost effective effort to come up with a 2.5x barlow, I wonder what
    would happen if I fashioned a barlow tube about 1/2 the length of the
    current one on the Televue 3x? This barlow is in three, unscrewable
    sections. The black top portion has a 3" length plus another chrome 1.5"
    that the lens screws into (4.5" total length from the tip of the black tube
    to the face of the barlow lens). If I half this distance to 2.25" total
    length, what will happen to the barlow's magnification? Can the barlow be
    used in this modified form without adversely affecting images?

    Thanks,
    Barry



  2. #2
    Carsten A. Arnholm's Avatar
    Carsten A. Arnholm Guest

    Default decreasing length of Televue 3x barlow- what will happen?

    Barry S. Brown wrote:

    Barry,

    I have the Televue 3x as well. By reducing the distance to the image plane,
    you reduce the magnification factor. If you half the distance I cannot tell
    exactly what the resulting factor will be, but Peter Katreniak has some
    formulae at http://www.pk3.org/Astro/scopes_barl...al_reducer.htm

    Unless I read his formulae incorrectly, perhaps the effect in your case
    would be a resulting factor of 2x. It seems to me you need to reduce the
    distance by a factor ~0.77x (i.e. to 3.5") to achieve the 2.5x factor. But I
    am certainly no expert and can be very wrong on things like this.....

    I agree a 2.5x barlow would be desireable, 3x is a bit much on many cases.
    Maybe some sort of mock-up is the easiest way to resolve it. You also need
    to verify that you can reach focus in the new configuration.

    Clear skies
    Carsten A. Arnholm
    http://arnholm.org/
    N59.776 E10.457



  3. #3
    Vladimir Sacek's Avatar
    Vladimir Sacek Guest

    Default decreasing length of Televue 3x barlow- what will happen?

    "Barry S. Brown" <barrysbrown775y78r@yahoo.com> wrote in message news:<SJwIc.3497$sV2.252@newsread2.news.atl.earthl ink.net>...

    For Barlows with a single lens or lens group there is a simple
    relation
    between magnification (M) and the separation (S) between the lens and
    the top of Barlow (where supposedly rests eyepeiece field stop). It is

    M = 1 + S/f

    with "f" being the f.l. of Barlow lens (it is taken as positive number
    for the sake of simplicity). If your current magnification is 3 with
    S~4.2" (mesured from the middle of the lens group), the lens f.l. is
    2.1" (from f=S/(M-1)).
    Needed separation S for M=2.5 is given by S=(M-1)f, which comes to
    3.15".
    With S reduced to ~2", the lens would give magnification of ~2x.

    Vlad

  4. #4
    Vladimir Sacek's Avatar
    Vladimir Sacek Guest

    Default decreasing length of Televue 3x barlow- what will happen?

    "Barry S. Brown" <barrysbrown775y78r@yahoo.com> wrote in message news:<SJwIc.3497$sV2.252@newsread2.news.atl.earthl ink.net>...

    For Barlows with a single lens or lens group there is a simple
    relation
    between magnification (M) and the separation (S) between the lens and
    the top of Barlow (where supposedly rests eyepeiece field stop). It is

    M = 1 + S/f

    with "f" being the f.l. of Barlow lens (it is taken as positive number
    for the sake of simplicity). If your current magnification is 3 with
    S~4.2" (mesured from the middle of the lens group), the lens f.l. is
    2.1" (from f=S/(M-1)).
    Needed separation S for M=2.5 is given by S=(M-1)f, which comes to
    3.15".
    With S reduced to ~2", the lens would give magnification of ~2x.

    Vlad

  5. #5
    Lawrence Sayre's Avatar
    Lawrence Sayre Guest

    Default decreasing length of Televue 3x barlow- what will happen?

    Barry S. Brown wrote:
    Might I suggest simply selling your Televue 3X and purchasing an Antares
    3X barlow. The one I had was excellent, and the consensus on the
    'Excelsis' astro equipment rating site is in full agreemement with me.
    See it at:

    http://www.excelsis.com/1.0/section.php?sectionid=27

    Where 38 people have (on average) rated it a 9.5 out of a possible 10
    points max. You can read their comments (mine among them somewhere, I
    gave it a 10) on the same site.

    This is a shorty 1.25" 3X barlow with a wide 27mm clear aperture, and it
    is the only (non Powermate) barlow I ever owned which did not move the
    focus points of my various eyepieces inward much, if at all. The one
    you want is the model UB3S. I rated mine to be superior to my 2X
    Celestron Ultima barlow. $65 new as I recall. I only sold mine because
    my 2.5X Powermate is a wee bit better overall, and (just as importantly
    to me) the 2.5X factor suites my combination of eyepieces and my scope
    (as a combination) better overall than 3X does.

    Lawrence Sayre

    --
    My philosophy, in essence, is the concept of man as
    a moral being, with his own happiness as the moral
    purpose of his life, with productive achievement as
    his noblest activity, and reason as his only absolute.

    Ayn Rand (in the appendix to 'Atlas Shrugged')


 

 

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