Sky and Telescope - January 2017 - 72

ASTRONOMER'S WORKBENCH by Jerry Oltion

Build Your Own On-Axis Guider
Eliminate a host of guiding problems with one surprisingly easy project.

game) in favor of auto-guiding with a
guide camera replacing the human eye
and thumbs. As in the old days, that's
most often done with a separate guide
scope, but that still leaves a serious
problem: due to differential flexure and
mirror shift, the guide scope and main
scope don't necessarily point in exactly
the same direction in all orientations.
The solution is to aim the imaging
camera and the guide camera through
the same telescope. That usually means
off-axis guiding, in which you guide
with a star that's just out of the camera's field of view, but such stars tend
to be scarce and aberrated. On-axis
guiding, in which you select a guide
star from within the same field you're
photographing, is a more elegant solu-

p Don Rea with his fully assembled telescope and guide setup. The imaging camera is on the bottom, with the guide camera to the right.

72

J A N U A R Y 2 0 1 7 * SK Y & TELESCOPE

º Do you have a telescope or
ATM observing accessory that S&T
readers would enjoy knowing about?
E-mail your projects to Jerry Oltion
at j.oltion@sff.net.

tion, but how do you get light to both
cameras at once?
The answer is to optically separate
the light from the telescope into two
spectral bands, using one to guide
on and the other to create the photographic image you're interested in.
Don's research led him to a near perfect
solution: a cold mirror.
A cold mirror is not what it sounds
like. Rather than being cooled like cameras often are, the mirror merely reflects
"cold" wavelengths. A cold mirror
reflects visible (cold) light while letting
near-infrared (warm) light pass straight
through. You guide off the infrared
image while your imaging camera collects photons in the visible range.
The Edmund Optics #64-451 3.3mm cold mirror proved ideal for this
application. It reflects better than 95%
of incoming light from ultraviolet to
700 nanometers (beyond deep red), and
it passes about 95% of anything longer.
To hold the mirror and provide
camera attachments, Don found that
the housing of Orion's 5523 flip mirror
worked perfectly. He pulled the guts out
of it and mounted his cold mirror inside
on brackets angled at precisely 45°. A
50-mm square mirror just fit. Behind
that he mounted a smaller circular plain
glass plate of the same substrate to serve
as a corrector plate to remove the astigmatism in the guider image introduced
by its light passing through the 45° mirror. (Without the corrector plate, the
stars in the guider image would be badly
distorted and guider accuracy would be

DON RE A

WHEN I THINK OF do-it-yourself
projects, I seldom think of astrophotography. Astrophotography is so exacting
that it's hard to imagine anyone making
photo gear in their home shop. So I was
a bit flummoxed when California ATM
Don Rea showed me his homemade onaxis guider. Could it really be possible?
It could. And it is. Inspired by the
early work of ATM Kaz Tanaka and the
commercial on-axis guider offered by
Innovations Foresight, Don built a precision gadget that completely eliminates
the most vexing problems with guided
photography. It turns out to be surprisingly simple and reasonably low-cost.
Most astrophotographers have
abandoned manual guiding through an
eyepiece (the world's most boring video



Sky and Telescope - January 2017

Table of Contents for the Digital Edition of Sky and Telescope - January 2017

Contents
Sky and Telescope - January 2017 - Cover1
Sky and Telescope - January 2017 - Cover2
Sky and Telescope - January 2017 - 1
Sky and Telescope - January 2017 - Contents
Sky and Telescope - January 2017 - 3
Sky and Telescope - January 2017 - 4
Sky and Telescope - January 2017 - 5
Sky and Telescope - January 2017 - 6
Sky and Telescope - January 2017 - 7
Sky and Telescope - January 2017 - 8
Sky and Telescope - January 2017 - 9
Sky and Telescope - January 2017 - 10
Sky and Telescope - January 2017 - 11
Sky and Telescope - January 2017 - 12
Sky and Telescope - January 2017 - 13
Sky and Telescope - January 2017 - 14
Sky and Telescope - January 2017 - 15
Sky and Telescope - January 2017 - 16
Sky and Telescope - January 2017 - 17
Sky and Telescope - January 2017 - 18
Sky and Telescope - January 2017 - 19
Sky and Telescope - January 2017 - 20
Sky and Telescope - January 2017 - 21
Sky and Telescope - January 2017 - 22
Sky and Telescope - January 2017 - 23
Sky and Telescope - January 2017 - 24
Sky and Telescope - January 2017 - 25
Sky and Telescope - January 2017 - 26
Sky and Telescope - January 2017 - 27
Sky and Telescope - January 2017 - 28
Sky and Telescope - January 2017 - 29
Sky and Telescope - January 2017 - 30
Sky and Telescope - January 2017 - 31
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Sky and Telescope - January 2017 - 33
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Sky and Telescope - January 2017 - 37
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Sky and Telescope - January 2017 - 41
Sky and Telescope - January 2017 - 42
Sky and Telescope - January 2017 - 43
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Sky and Telescope - January 2017 - 45
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Sky and Telescope - January 2017 - 50
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Sky and Telescope - January 2017 - 56
Sky and Telescope - January 2017 - 57
Sky and Telescope - January 2017 - 58
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Sky and Telescope - January 2017 - 60
Sky and Telescope - January 2017 - 61
Sky and Telescope - January 2017 - 62
Sky and Telescope - January 2017 - 63
Sky and Telescope - January 2017 - 64
Sky and Telescope - January 2017 - 65
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Sky and Telescope - January 2017 - 81
Sky and Telescope - January 2017 - 82
Sky and Telescope - January 2017 - 83
Sky and Telescope - January 2017 - 84
Sky and Telescope - January 2017 - Cover3
Sky and Telescope - January 2017 - Cover4
Sky and Telescope - January 2017 - SGA17RAL
Sky and Telescope - January 2017 - 90
Sky and Telescope - January 2017 - SGA17R
Sky and Telescope - January 2017 - SGA17RA
Sky and Telescope - January 2017 - SGA17EAL
Sky and Telescope - January 2017 - 94
Sky and Telescope - January 2017 - SGA17E
Sky and Telescope - January 2017 - SGA17EA
Sky and Telescope - January 2017 - SGA17SAL
Sky and Telescope - January 2017 - 98
Sky and Telescope - January 2017 - SGA17S
Sky and Telescope - January 2017 - SGA17SA
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