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Focal extender/Tele extender/barlows! What’s the difference?

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i am a novice at this game and this is my first post (apart from saying hello) and it will no doubt be the first of a number of questions I have:


Whilst looking at Barlow lenses I have seen adverts for Barlow Lenses, Focal Extenders and Tele Extenders! They all seem to be going the same thing (increasing magnification), but I am a little confused as to weather there is actually a difference between them.

The question therefore is weather there is a difference, what the difference is and weather one is therefore better than the other?

My interest is purely visual observing and not AP (in case that is relevant).


Thanks to anyone who can shed light on this matter.



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Barlow lenses were invented by Peter Barlow and are a form of diverging (negative) doublet lens that effectively increases the focal length of the telescope it is used in. Some modern Barlows have three or more elements.




AFAIK tele extender lenses use at least four elements and work on a slightly different principle, even though the end result is basically the same.




As I see it the primary advantage of the extender/amplifier over the traditional Barlow is that as the light rays are more parallel there is no change to the eye relief of the original eyepiece. A normal Barlow can increase eye relief, which may not be preferred. 

Edited by Nightspore
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