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What
is the difference between an Optical Corrector (OC) and a standard
Barlow?
In
short. If you buy a true optical corrector or OCA the maker will tell
you how to use it and what magnifications you will get in a binoviewer..
Now in long.
A barlow is just that.
Usually a telenegative type lens that is used with a single eyepiece to
achieve a present amount of magnification. You never use an
optical corrector with a single eyepiece. The term "optical
corrector" or the acronym "OC" usually followed by an
additional acronym for the different specific companies' almost
exclusively refer to binoviewer use. This term became popular when
binoviewers started to be supported by specific optical devices rather
than borrowing a telenegative lens from a barlow. Hence the term
"Optical Corrector" as it relates to binoviewing was born.
So in short an optical corrector is a lens system specifically designed
for a binoviewer regardless of the manufacturer. The magnification
amount is not specifically important, although most "OC's"
start at lower magnifications then is possible when a simple barlow is
used. OC's can be a simple telenegative doublet if that doublet is
designed and specifically made to offset the optical path of a
binoviewer with a preset magnification factor.
Here I will try to
help you understand better why simple barlow elements respond the way
they do when introduced to a binoviewer application. Most standard
x2 barlows of average length use an element
"telenegative" of roughly -100mm's focal length. That
negative value represents the overall strength of the element or
the effect it will have on the optical train. It's placement in
the telescope optical path and distance in relation to the eyepiece will
determine the magnification factor that you finally get. There are
two ways to change the magnification factor. Increase the distance
between the eyepiece and barlow element. When focus is then
achieved the magnification will be higher. If you keep the
distance relatively the same extra magnification can also be achieved by
increasing the strength of the negative element. A typical shorty
barlow is trying to achieve a greater magnification factor in a shorter
distance. These elements are typically -50mm to -75mm fl lens.
Because of the extra power or greater divergence of the element less of
a distance is required to achieve x2.
In a 1.6x barlow of
normal length the focal length is likely in the -140mm fl. This
explains the less magnification of that particular barlow. Now
make a 1.6x in a shorty barlow and likely you have started out with a
normal -100mm fl in a typical barlow and simply shortened the optical
path between the eyepiece and element. Now 1.6x is achieved
because of not having a full optical path as would be the case in a x2
barlow. If this element is applied to a binoviewer it's
effect will be likely the same as a standard length x2 barlow.
Not to get overly
windy, but more sophisticated optical correctors will use additional
elements normally a positive and a negative set to first increase the
focal length entering the corrector and then reducing the focal length
exiting the corrector. This makes the intervening space take the
place of the binoviewer's optical path. This makes it possible to
offset greater distances such as in the case of the optical path of the
binoviewer, but at a lesser magnification factor. The formulas for
these type of correctors can be somewhat complicated which makes it
necessary for special optical elements to be used.
So there you have it.
A mostly complete explanation of what an optical corrector is.
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