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Does TFOV change when Barlow added?

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#1 lagagnon



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Posted 30 June 2013 - 01:25 PM

I have calculated the True Field of View of my 4 eyepieces using the stellar drift method. Now I am wondering if I add my 2X Barlow to any of these will the TFOV change?

I am assuming it will as the diameter of the Barlow lens is probably different than that within the eyepieces??


#2 FirstSight


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Posted 30 June 2013 - 01:36 PM

YES. You don't get to effectively halve the focal-length of an eyepiece (and double the magnification) without the inescapable corollary of also cutting in half the diameter of the TFOV of the native eyepiece. A 2x barlow, in principle, effectively creates a virtual eyepiece of half the focal-length of the native eyepiece, with all the accompanying parameters. In practice, this is not quite true, since the eye relief may change with a barlow, which is one advantage of using a modified (but usually more expensive) design, which (in the Televue brand version thereof) is called a "Powermate".

BTW: don't confuse a 2x barlow with a 2x focal reducer, which changes the effective focal-length (and focal-ratio) of the telescope itself, rather than the eyepiece it is used with. Shortening the focal-ratio of a telescope does have the effect of correspondingly increasing the true field of view yielded by any given focal-length of eyepiece used with the scope, but again there is no free lunch; any given focal-length eyepiece used in a telescope with e.g. a 2x focal reducer will also correspondingly yield only half the magnification it did at the telescope's native focal-length & focal-ratio. And so, if an eyepiece of given focal-length is used in a telescope equipped with a 2x focal-reducer, you would need a 4x barlow to reach the same magnification you could without the 2x focal reducer, but the same eyepiece/same telescope/2x barlow.


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