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difference between barlow and telenegative?

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#1 earthbot1

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Posted 06 September 2010 - 09:00 AM

I purchased a 2x telenegative and am wondering if it has any different limits/applications than a barlow? :question:

#2 Greg Keys

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Posted 06 September 2010 - 11:27 AM

A true tele extender is a device for astro-photography. Some are empty tubes to allow attaching a camera in conjunction with an eyepiece, some have lenses inside to further play with the magnification factor. See this writeup by our host: Tele Extenders
The real question here is what exactly did you purchase?

#3 earthbot1

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Posted 06 September 2010 - 01:30 PM

Thanks. I got a Meade 4000 2x Telenegative....I posted the wrong name first time.
One on left.
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#4 j schmo

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Posted 06 September 2010 - 02:05 PM

It's Meade's version of a "short" barlow. If I recall correctly there were two models released of differing quality. Do you know which model you have?

#5 Greg Keys

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Posted 06 September 2010 - 02:12 PM

That, my friend, is a barlow :)
Tele - distance (or length)
Negative - to ruduce - minus
A barlow plays with the focal length of an eyepiece by reducing it (negative distance)
A 25mm EP becomes a 12.5 with a 2X barlow
Magnifiction of a scope is the scope FL divided by the EP FL
So, a 2X barlow reduces an eyepieces FL by half, a 3X reduces the EP FL to 1 third.

#6 earthbot1

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Posted 06 September 2010 - 03:17 PM

Ok...thanks for the info!

#7 sixela

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Posted 07 September 2010 - 06:57 AM

I purchased a 2x telenegative and am wondering if it has any different limits/applications than a barlow?


Well, technically a barlow is one form of telenegative. In other words, not all telenegatives are barlows, but a barlow is one kind of telenegative (with just one cemented doublet).

Of course, it doesn't help that some "barlows" are really telextenders/telenegatives but not really barlows.

My Zeiss barlow is a barlow, but my Baader VIP barlow isn't, since it has an air-spaced doublet.

My former TMB barlow was a barlow, but my former Orion Shorty plus "barlow" (three lenses) and former Powermate┬╣ (two doublets) weren't.

In practice, I used all four without worrying too much about the number of lenses and lens groups ;).

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┬╣(TeleVue is a bit more precise in its use of words and won't refer to a PowerMate as a barlow)


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