Sky & Telescope - April 2022 - 72

ASTRONOMER'S WORKBENCH by Jerry Oltion
A Plate-Solving Finder
Get spot-on pointing with equipment you may already own.
SEVERAL OF MY observing buddies use
Go To telescopes. One of their key measures
of a good night is whether or not
the computer puts their target objects in
the field of view. They're always fiddling
with the alignment: sometimes starting
over from scratch, often retraining the
scope after they center an object, or even
nudging a tripod leg to make the computer's
internal star map match reality.
If targets are completely out of the field
of the eyepiece, they're " lost in space. "
What if you could get your Go To
scope to put an object dead center in the
eyepiece every time? Sure, recent-generation
scopes with plate-solving technology
will photograph the sky, figure
out where they're pointed, and center
the scope on the target, but what about
those millions of previous-generation
scopes still in use? Are they left out of
the plate-solving revolution?
Thankfully, no. Virginia amateur
Robert Capon, last seen in these pages
with his incredibly compact travel scope
(S&T: Jan. 2021, p. 74), has figured out a
way for people with previous-generation
Go To scopes to join the party. And if
you're an astrophotographer, you might
very well be able to do it with equipment
you already have on hand.
Rob says, " Virtually every astrophotography
program today can plate-solve
and recenter after a slew. I'm using a
ZWO ASiair Plus for astrophotography,
but popular programs like The SkyX,
N.I.N.A., and SharpCap also work. "
The trick is to mate the software
and camera with a guidescope or
finderscope that provides an optimalsized
field of view. Rob's first attempt,
using a Svbony SV165 Mini Guider
Scope 30mm/120mm F4 he bought for
about $50 and his wide-format ZWO
ASI294MM camera, provided way too
wide a field of view. It put thousands of
stars in the field, combined with distortion
in the corners, so the software was
pThe ASiair Plus computer easily fits on the side
of the telescope and communicates wirelessly
with a smartphone or tablet for telescope control.
unable to recognize the images. Switching
to an inexpensive ZWO ASI120MMS
guide camera with a much smaller
chip solved the problem. That scope
and camera combination has a suitable
2.3°-by-1.7° field of view, which
results in reliable plate-solving. To keep
the finder from dewing up, Rob added a
homemade dew shield.
To operate the computerized autocentering
finderscope, Rob mounted
both the finder and ASiair Plus minicomputer
on small finderscope dovetail
brackets, connected the computer to
the mount's external control port, and
issued his Go To commands from his
tablet, with the ASiair Plus set to platesolve
and recenter after each slew.
It's reasonably fast. It takes a couple
seconds to shoot the image, another couple
to download the image to the comLeft:
Rob Capon's plate-solving finder is a simple, easy solution for exact Go To pointing. Right: The blue rectangles in this SkySafari screen shot
show the field of views of the two cameras Rob tried. The wider field contained too many stars for the software to plate-solve, but the narrower field
was just right.
72 APRIL 2022 * SKY & TELESCOPE
ALL IMAGES COURTESY OF ROBERT CAPON

Sky & Telescope - April 2022

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