|Improving Performance With A Barlow Lens
I purchased the Televue 2x Barlow lens thinking that I
could just place it into my 1.25" eyepiece adapter
right out of the box. Unfortunately, when used in the
Swarovski AT80HD spotting scope, the Barlow lens,
as it comes out of the box, reduces infinity focus to
about 20 feet.
This led me to do some research about Barlow lenses.
For one thing, the 2x is not an absolute magnification
factor. The glass portion of the Barlow lens has it's
own focal length, and magnification power depends
on 1) the placement of the Barlow lens in relation to
the telescope objective lens and 2) the placement of
the eyepiece in relation to the Barlow lens.
This led me to remove the glass portion of the Barlow
lens by simply unscrewing it from the extension tube
that came with it. The top end that attaches to the
extension tube is a thread size unique to Televue, as
far as I know, but the bottom of the glass portion of
the Barlow lens is threaded for standard 1.25"
astronomy filters. Also available are 1 inch eyepiece
extension tubes that use the same threads.
To configure the Barlow lens to work with my
spotting scope I took the 1 inch extension tube
and reduced it's length to 3/8 of an inch by
sanding. I then screwed this into the bottom of
the Barlow lens and turned it upside down so the
top was now the bottom. This is possible with this
Barlow lens as a Barlow lens usually consists of
two equal negative lenses.
The next thing I did was take a prime focus /
eyepiece projection adapter for a Single Lens
Reflex camera and removed the entire top thin
section by sanding it down to the lower thick
section. I then removed the very bottom 1.25"
barrel by unscrewing it. The remaining part of the
adapter then slipped very snuggly over 3/8 inch
extension tube on top of the Barlow lens, and the
40mm Plossl fits into the top of the modifed
adapter and locks in with a set screw.
To keep the Barlow lens glass element from
falling out, once the left hand assembly in the
picture to the right is made, a narrow piece of
good quality tape around the top circumference
of the extension tube will keep it from falling out
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Copyright Stephen Lindquist