|Simplifying the Use of a Barlow Lens
I decided to modify my initial Barlow configuration in the interest of hardware simplicity, and in the interest of
attempting to gain some control over the magnification factor of eyepieces for the purpose of digiscoping, while
retaining the additional eye relief gained from using a Barlow lens at a distance less than it's focal length.
The first thing that I did was order a negative achromat from Edmunds Industrial Optics. They sell them in several
different focal lengths, magnesium flouride coated, BK7 glass. I ordered one with a focal length of -150mm, the
longest focal length, in an attempt to be able to limit my Televue 40mm Plossl, Coolpix 5400, Swarovski AT80HD
digiscoping combination to a 35mm equivelant maximum focal length of about 1550mm or slightly less.
Experimenting with two different Barlows has lead me to conclude that there is a very limited acceptable placement
of a Barlow lens in the optical path of my spotting scope. There is the physical limit of the sealed piece of optical
glass that prevents placing the Barlow any closer to the objective, which in turn limits the magnification factor that can
be obtained with the two Barlows that I have used. There must be a Barlow that has an optimum focal length to be
used with this scope, as the 20x-60x zoom lens manufactured for it appears basically to be a fixed focal length
eyepiece that zooms by raising or lowering a 3x Barlow as the zoom mechanisim is turned. However, I value the
Barlow lens for digiscoping for it's ability to increase eye relief over it's magnification ability.
To make my Barlow I used a 1 inch eyepiece extension tube that I ordered from Scopetronix, 1.25 inch diameter,
and placed the -150mm negative achromat at the very bottom. To make it fit snug in the tube I used black tape
wrapped around the outside diameter of the negative achromat. To insure that it doesn't move in the extension tube, I
went to the hardware store and bought some black rubber O-rings, outside diameter 1 1/8 inch, inside diameter
15/16 of an inch. The O-rings were then pressed into the extension tube, each one snug against it's neighbor, until
there was enough room at the bottom of the extension tube for the thickness of the negative achromat plus 3/32
inch. The lens is then pressed in to the extension tube, and one more O-ring is pressed in at the very bottom to help
hold the lens in the tube. I didn't apply glue at this point as I might wish to take the lens out for periodic dust removal.
To help insure that the lens, which is held in place by friction, doesn't fall out, I keep the bottom of the Barlow
capped, and the cap is secured with a Velcro strap when not in use in the scope.
The one inch extension tube turned Barlow is then screwed into the bottom of the 40mm Televue Plossl, which
appears to be the correct distance for both the eyepiece and the Barlow when inserted as far as it can be inserted, in
the 1.25" eyepiece adapter for my scope. Configured this way, the Barlow yields a magnification factor of about
1.12x. If I wanted greater magnification from the Barlow, I would select a negative achromat lens with a shorter focal
©2005 Stephen Lindquist