72 AUSTRALIAN SKY & TELESCOPE May | June 2018
ASTRONOMER’S WORKBENCH by Jerry OltionHunter’s zoom finder
This innovative design solves several problems at once.
HUNTER DAVIDSON LIVES NEAR
a large, light-polluted city. He also
enjoys manually aiming his telescope,
which offers little success when the
magnitude-4 sky reveals maybe a dozen
stars. He has tried different types of
finders, but found few that worked well.
Hunter needed an optical finder that
would offer some gain. The trouble is,
this would lead to a narrower field of
view, which would reduce the number
of useful field stars. He began asking
himself, “So what provides both wide-
angle at about naked-eye field of view
as well as some magnification when
needed?” The answer — a zoom finder.
With the words ‘zoom finder’ in his
head, he went into his workshop and
mated a Sony 20- to 80-mm f/2.5 TV
zoom lens to an inexpensive Amici roofHUNTER DAVIDSON (2); EXPLODED DIAGRAM: GREGG DINDERMAN /S&Tprism and the lenses from a 20-mm
Plössl eyepiece. He couldn’t use the
entire eyepiece because the focal point of
the zoom lens lies inside the prism, so he
had to mount the eyepiece lenses so their
focal point reached the same plane.
The result is a finder that zooms
outward to give him a 50° field of view,
which allows easy orientation even with
only a few reference stars. Zooming to
the narrowest field of view, 12.5°, adds
about 3 magnitudes of gain, bringing
out more stars as the field narrows.
Designed for TV cameras, the lens has
a smooth-acting lever for zooming, so
Hunter can shift back and forth easily
without jiggling the scope.
At its widest, the finder’s field of
view includes the front of the telescope,
making it difficult to judge where the
centre is. Even at full zoom, its 12.5° field
is still a lot wider than the 1° field of a
typical low-power eyepiece, and Hunter
soon realised the biggest hiccup in his
design: With the focal plane inside the
Amici prism, he couldn’t add cross-hairs
to indicate the centre of his finder’s field.
So he augmented his zoom finder with
a second, conventional 5° optical finder.
Together they let him zero in on just
about any target visible in his sky.
Thenonenighthehadanepiphany:The zoom could function like cross-
hairs. An object that’s not centred will
drift to the side when zoomed, but an
object in the centre of the field will stay
put. Hunter reports, “The long hand
lever of the lens makes adjustment
very easy, so I just zoom in and out,
tweaking the scope position until the
target star doesn’t move. This is quicker
than one might imagine and became
second nature.”
Now he says the most serious
downside is that “this optics
combination provides, to be kind, not
the sharpest star images. But I find the
images good enough, and ‘good enough’
seems okay for this application.”
These zoom lenses are available on
eBay fairly frequently. Others might
work as well, but make sure they come to
focus far enough from the rear element
to allow room for a prism and eyepiece.
For more information, you can
contact Hunter at hunterdavidsonjr@
gmail.com.■ JERRY OLTION uses all kinds of
inders. A zoom may soon join them.20 mm Plössl LensesLens cellBracket
clamps to
lens barrelRing set screws engage lens
dovetail allowing diagonal rotation.Eyecup and
focus adapter
Modiied diagonal
eyepiece holderHunter Davidson uses his
zoom inder to locate guide
stars in his light-polluted sky.The zoom lever
is easy to reach
and operate in
the dark.The zoom inder
is relatively easy
to assemble, and
simple to use.