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Me again... Powermate vs. Barlow

Started by astroboy314, 12/09/2002 11:50PM
Posted 12/09/2002 11:50PM Opening Post
Can someone explain to me what the difference between a Barlow and a Powermate is? I appreciate your patience and hope no one minds me using this forum as an alternative to searching for stuff on on the internet.
Posted 12/10/2002 07:56AM #1
A Barlow has a negative power achromat at the
bottom of a long tube. It increases the effective
focal length, but also causes the light rays to
diverge. This sometimes has bad effects
on eyepiece performance (usually vignetting
for low-power eyepieces, and some increase in
eyerelief at low powers).

A PowerMate has a negative power achromat at the
bottom of a long tube, and a positive achromat
in the middle of the tube. The first lens
grouping increases the focal length and
diverges the light, just like a Barlow.
But the second grouping converges the light
and brings the rays back into near-parallel
paths. So while it has the efect of a Barlow,
the behavior of the light rays is nearly the
same as if there were no added lens. Eyepieces
behave closer to optimal performance.