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SV: ATM Barlow Lens

Tom,

There is one problem with shorty barlows, that I have tried to illustrate
in ASCII graphics. Incoming light (converging towards a point off center of
the focal plane, near the edge of what should be the true field of view)
will be refracted as to come from a point near the (first) focal point of
the Barlow negative lens, and emerge diverging. Light may diverge so
strongly as to pass inside the eyepiece field stop and still miss the
eyepiece field lens and be lost, this will appear as vignetting. I know
others have had problems with shorty Barlows using long-focus eyepieces,
but sensitivity to the problem must depend on field stop and field lens
sizes and may vary with EP design (and of course focal length of Barlow).

<html><pre>
            Neg.Barlow lens   EP field stop 
                    \    /       |       ___---lost light
Incoming light      |    |      _|_---~~~   /\
 >--------__________ \ _/_---~~~           |  |
             _ _- - ~|~ |                  |  |
---------~ ~---------|--|------------------|--|----optical axis-----
        ^Focal pt of neg lens          EP field lens  

</pre></html>
(thanks Bratislav, hope that makes the HTML work for the archive this time)


Nils Olof

20 degrees north of you