Sky & Telescope - June 2020 - 72

ASTRONOMER'S WORKBENCH by Jerry Oltion

Finder
Finders
Here's a wide-ﬁeld helper that
puts you in the ballpark.
THOSE OF US WHO USE optical ﬁnders
on our telescopes know the frustration
of trying to ﬁgure out exactly what
part of the sky the ﬁnder is pointed at.
Optical ﬁnders 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 ﬁeld
around it - but they're surprisingly
difﬁcult to aim anywhere near your
objective to begin with. That's because
optical ﬁ nders don't have anything
convenient to sight along. Like trusstube Dobsonians or ball scopes, there's
no structural component to use as a
general aiming device.

72

JUNE 2 02 0 * SK Y & TELESCOPE

Enter the "ﬁnder
ﬁnder" - a simple
sighting device that lets
you aim the telescope
in the right general
direction before you
look through the
optical ﬁnder for your
target.
One obvious solution would be to
mount a red-dot ﬁnder
alongside the optical
ﬁnder, but that requires
a second mounting
bracket. And if you
use multiple scopes,
it requires a second
bracket on each scope.
Worse, because mounting brackets never seem
to point exactly in the
same direction, that means re-aligning
two ﬁnders instead of just one when
you change scopes.
I prefer mounting my ﬁnder ﬁnder
right on the main ﬁnder'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 ﬁnder has six adjusting 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 difﬁcult.
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 more

angular precision, and if you align the
components carefully when you glue
them on the ﬁnder body, when you sight
from one to the other you'll be looking
down the ﬁnder's optical axis. Paint
them white or use glow-in-the dark
paint or tape to make them stand out.
A peep sight reﬁnes the concept a
step further. A ¼-inch hole in back
through which you must look forces
your eye into the proper position, guaranteeing that whatever the front sight is
aimed at is also what the main ﬁnder 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 ﬁnder's optical axis as the
peep hole you're looking through.
An even fancier reﬁnement is a splitpupil ﬁnder. Described in our June 2013
issue (p. 66), and in more detail on my
website at https://is.gd/splitpupil, a
split-pupil ﬁnder 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 simultaneously 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 height
q A split-pupil finder finder takes a little getting
used to, but once you learn how it works it's a
joy to use.

A LL IM AG ES BY THE AU THOR

t A peep-sight finder is surprisingly accurate. Note the
bright target ring and bright
perimeter around the peep
hole, which help you find
the finder finder at night.
https://www.is.gd/splitpupil
Sky & Telescope - June 2020

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