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Thread: Just curious...

  1. #1
    ssgt1time's Avatar
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    Default Just curious...



    This might not make sense (or just a dumb question all together) but I am going to ask anyways....

    Hypothetically... Let's say I am using 2 of the same telescopes, same conditions, and in the exact same spot (by means of an alternate space/time, of course){stargate ftw} Also, assuming accessories are same brand,same series, etc.

    Telescope 1: 1000mm focal length / 4mm eyepiece giving me 250X Mag

    Telescope 2: 1000mm focal length / 8mm eyepiece w/ 2X Barlow Lens also giving me 250X Mag (If I am understanding the barlow lens correctly)

    Would these views be perfectly identical?

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    Default

    Hello,

    I am still trying to wrap my head around optics but IMHO, I would say yes....(in theory). However, I was led to believe that anytime you add optics (additional glass / lenses)....that may change the appearance. However, I would think that the change would be so small, it would not be evident in visual observing.

    Whether or not the addition of the barlow would change the orientation...I wouldn't think so. Then again, I never considered the question before now.

    Good question.

    Tom

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    Default

    One thing a barlow can do is allow you to use lower power eyepieces with more eye relief and viewing comfort. Say your using a 20mm plossl and 10mm plossl. The diameter of the viewing lens in the 20mm is much bigger than the 10mm. Using a barlow lets you obtain the larger mag without having to look through a pin head. I can't tell much of a difference between a barlowed ep vs. the higher mag, but there is a little bit. You do add more glass in the light train so you do loose a little light transmission.
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    Default

    Quote Originally Posted by ssgt1time View Post
    This might not make sense (or just a dumb question all together) but I am going to ask anyways....

    Hypothetically... Let's say I am using 2 of the same telescopes, same conditions, and in the exact same spot (by means of an alternate space/time, of course){stargate ftw} Also, assuming accessories are same brand,same series, etc.

    Telescope 1: 1000mm focal length / 4mm eyepiece giving me 250X Mag

    Telescope 2: 1000mm focal length / 8mm eyepiece w/ 2X Barlow Lens also giving me 250X Mag (If I am understanding the barlow lens correctly)

    Would these views be perfectly identical?


    Depends on the eyepieces also.
    One thing you have to know is a barlow while it magnifys it also converges the lightcone which means more eyerelief but a hard to find sweet spot for the eye that gets smaller causing blackouts & kidneybean effect etc...
    Is true you retain the larger eyepiece glass of the 8mm but this cone effect must be considered as a counter effect. The edge of field may become more apt to coma & distortions also.
    Only real way to correctly magnify is with a TV powermate or similar device since its built with 4 internal lenses so that the light exiting to the eyepiece stays correct as it should and parallel light exits,.. not a cone so no having to keep the eye precisly centered over the eyepiece. Just pure magnification & you retain the original eye relief & AFOV, they are not cheap tho as a barlow is so you get what you pay for. A powermate in 1.25" size runs $180 and a regular barlow can be had on the used market for peanuts now & then.
    Don't get me wrong barlows are great for what they do but their use is limited somewhat compared to a powermate. For the highest mag & highest contrast the barlow looses so its ultimatly the end users decision if its worth the extra conveniance of eyepiece function maintained while being magnified.
    I used to own a powermate & plan to buy another soon so im speaking from exp, not from just TV print. A powermate is one of the few pcs. of equipment that once procured you keep it usually for good unless moving up to all 2" eyepieces etc. then a larger powermate is necessary.
    This is the only way the images will stay the same to answer your question in reguard to the view at the eye without the problems I mentioned.
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    Default

    Yes the magnification per inch of aperture would be equal. Would be difficult for the 'view' to be identical - if as you say same series, your eye placement would be changed by the Barlowing - but in theory, yeah, sort of same. As mentioned above if the eye relief was long to begin with - it may be problematic for eye placement. Might depend more on if you are on a tracking mount and looking more at the central 50% of the ep or if on undriven and watching object transit or nudging. Or using a G&G alt/az mount with slo-mo... Unless I'm testing eyepieces I am usually looking at something in the central region. There's a bit more IMO to the view through a scope than just the optics being used. I've had some unexpected results in the performance of ep's when testing with barlows - most notable was the 2x barlowing of an 8mm Brandon and comparing to my ZAOII 4mm, the Brandon through the Barlow performed so near the ZAOII that I was a bit astonished. At it's native 8mm there was more scatter and BLC (Bright Low Contrast) detail a bit less. Both ep's have pretty tight eye placement to begin with.

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    Default

    If the barlow is of exceptional optical quality then yes. If it's a cheap one and not the best optics then NO.

    Televue would be a good brand with good glass.
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    I'd 2nd getting a GOOD barlow. I like the eye relief from using them oppose to the needle hole of a 4mm EP.
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    I have the televue barlow and notice the slightest change in light transmission. Not much, but noticable.
    Josh L.

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    Default

    I personally would prefer to use a barlow to keep the "usual" extra eye reielf a lower powered eyepiece gives me...especially since I wear glasses....

    The Comments on Barlows (Good vs ok vs not so darn oood" however is not an easy one to understand... I've been in this hobby for over 50 years now and to be honest have owned dozens of barlows over the years and the difference between a good one (Televue) and a cheap but NOT PLASTIC barlow is NOT all that different...

    You will loose some brightness with a Barlow...BUT I bet your own eyes hardly notice the loss... it is very minor from my experience...

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