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    Default Qs about EPs & Barlows.



    Take for example a 21mm Ethos, and a 2X barlow, will I still have a 100 degree field of view with it barlowed? in other words, will it be the same as a 10.5mm Ethos? (I know a 10.5mm Ethos doesn't exist, but suppose it did)

    Also, is 1.3 degree field of view enough to see the biggest nebula?

    Oh and what's the difference between a Barlow and a powermate of the same power and barrel size? is a powermate better? (it is more expensive)

    Thanks in advance!
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  2. #2
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    Default

    Hi Leo, wanna see a visual illustration of the view before and after a barlow is used? Look here (top post):

    Hello, from a Hyperion zoom lover!

    The 1st image is a shot of the view at 8mm or 50x without a barlow. The 2nd image is barlowed 2x or 100x (using a TV 2x barlow). The apparent field of view (AFOV) remains the same, but the actual area seen at 2x (which is the true field or real field) is reduced as compared to the true field without the barlow.

    One of the biggest nebula, the North American Nebula (NGC 7000) spans around 3 degrees so the 1.3 is not sufficient to cover the entire expanse.

    A Powermate is a Televue product, (I have coming a TV 2.5x Powermate) that can be thought of as an enhanced barlow in that it has two extra lenses or 4 in all (the regular barlow has only 2 lenses which is a negative doublet) that corrects diverged rays and aberrations from a barlow by using a positive doublet (sort of a counter-active lens) and restores the light rays to a telecentric operation with the field rays restored back to their original direction, thus preventing vignetting, edge aberrations and altered eye relief/pupil movement.

    Will one see the difference between a regular barlow and a Powermate/telecentric barlow?

    Tele Vue makes a series of Barlow-like devices called Power-mates. In
    effect, a Powermate is a standard 2-element Barlow with a second doublet lens added to minimize vignetting (darkening the edge of the image)
    and excessive extension of eye relief when used with long focal length
    eyepieces. Power-mates are available in 1.25" 2.5X and 5X models
    ($190) and 2" 2X and 4X models ($295). We don't doubt that Powermates are excellent products, but we've never been able to tell any difference in image quality between a Powermate and a high-quality standard Barlow."
    The above quote comes from the book "Astronomy Hacks" (by Robert & Barbara Thompson) p. 261. Will have to see for myself if this holds true once I receive the TV 2.5x Powermate and compare it with the TV 2x barlow; will report findings.

    Best,

    Hernando

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  4. #3
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    Default

    Thanks a lot Hernando, that's all I needed to know!
    Looking forward to your report.
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    Default

    The important point about the PowerMate isn't about being a "better" Barlow (the Thompsons' quote in "Astronomy Hacks" is a really odd thing for them to say) - the PowerMate is about solving certain specific problems with using a Barlow:

    • 32mm Plossls (for example) normally do not Barlow well, because of excessive eye-relief. They should Barlow fine with a PowerMate
    • Barlows don't work well with binoviewers - and provide far more magnification than normal. Most PowerMates (although, curiously not the 5x one) will still provide the same magnification when used with a binoviewer.
    • PowerMates provide consistent magnification with different eyepieces. Conventional Barlows' magnification will vary depending on the eyepiece design.

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    Default

    I have the Hyperion zoom,..and bought the 2.5X powermate to go with it because I wanted to know I was getting the best view possible through the zoom.,.not only is it great with that E.P..,it is great with the rest of my E.Ps (nothing fancy) It cost $70.00 more than the T.V. 2X barlow.,.but I consider it money well spent,.,O+O

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