ASTRONOMER’S WORKBENCH by Jerry Oltion
Finder
Finders
Here’s a wide-fi eld helper that
puts you in the ballpark.THOSE OF US WHO USE optical fi nders
on our telescopes know the frustration
of trying to fi gure out exactly what
part of the sky the fi nder is pointed at.
Optical fi nders are great for zeroing in
on an object once you’re in the right
general area — you can often nail your
quarry to within a few arcminutes if
you know your target or the star fi eld
around it — but they’re surprisingly
diffi cult to aim anywhere near your
objective to begin with. That’s because
optical fi nders don’t have anything
convenient to sight along. Like truss-
tube Dobsonians or ball scopes, there’s
no structural component to use as a
general aiming device.qA split-pupil fi nder fi nder takes a little getting
used to, but once you learn how it works it’s a
joy to use.tA peep-sight fi nder is sur-
prisingly accurate. Note the
bright target ring and bright
perimeter around the peep
hole, which help you fi nd
the fi nder fi nder at night.ALL IMAGES BY THE AUTHOREnter the “fi nder
fi nder” — a simple
sighting device that lets
you aim the telescope
in the right general
direction before you
look through the
optical fi nder for your
target.
One obvious solu-
tion would be to
mount a red-dot fi nder
alongside the optical
fi nder, but that requires
a second mounting
bracket. And if you
use multiple scopes,
it requires a second
bracket on each scope.
Worse, because mount-
ing brackets never seem
to point exactly in the
same direction, that means re-aligning
two fi nders instead of just one when
you change scopes.
I prefer mounting my fi nder fi nder
right on the main fi nder’s body. That
way the two are always aligned. And I
like making mine without electronics,
so there are no batteries to die when I
inevitably leave the switch on.
The simplest design is akin to a
gunsight. If your fi nder has six adjust-
ing screws, you have probably already
discovered that you can get rough
alignment by sighting across two screw
heads. The screw heads are usually
dark, however, and blunt, which makes
aiming diffi cult.
You could paint the screws white,
and that would help a little. You could
glue small triangular caps to the screws
and paint those white, which would
help a little more. But the baseline, the
distance between the screws, is really
too short for much accuracy. You’re
better off putting a post all the way up
front and either a post or a wedge in
back. The longer baseline affords moreangular precision, and if you align the
components carefully when you glue
them on the fi nder body, when you sight
from one to the other you’ll be looking
down the fi nder’s optical axis. Paint
them white or use glow-in-the dark
paint or tape to make them stand out.
A peep sight refi nes the concept a
step further. A ¼-inch hole in back
through which you must look forces
your eye into the proper position, guar-
anteeing that whatever the front sight is
aimed at is also what the main fi nder is
aimed at. The front sight can either be
another larger hole or a simple post. The
important thing is to make the part you
sight on (the center of the hole or the
end of the post) the same distance from
the main fi nder’s optical axis as the
peep hole you’re looking through.
An even fancier refi nement is a split-
pupil fi nder. Described in our June 2013
issue (p. 66), and in more detail on my
website at https://is.gd/splitpupil, a
split-pupil fi nder involves a lens at the
rear and a glowing arrow at the front.
With your dilated eye, you look through
the lens at the arrow while simulta-
neously looking over the lens at your
target in the sky. The arrow must be
placed at the lens’s focal point, and the
lens’s center must be the same height72 JUNE 2020 • SKY & TELESCOPE