Australian Sky & Telescope - June 2018

72 AUSTRALIAN SKY & TELESCOPE July 2018

ASTRONOMER’S WORKBENCH by Jerry Oltion

G-Tracker is a work of Art

Simple. Elegant. Good Enough.

WHEN I WAS INTO astrophotography,

I used to spend half an hour or longer

polar-aligning my telescope. Nowadays

all I do is visual observing with a

trackball scope, and polar alignment

is a matter of figuring out more or less

where the pole is and plopping the

mount down sort of aligned with it.

It would be nice if there were a

simple, elegant way to make alt-azimuth

mounts track with equal facility. Not

necessarily perfect, but good enough.

Art Gamble has figured out just such a

device, and it’s as simple as you can get.

It looks a little like a barn-door

tracker: two slabs of wood, a hinge,

and a screw that slowly tilts one of the

wooden slabs upward. In order to put

the hand-knob close at hand while he’s

observing, Art added a flexible shaft

made from two grease gun hoses. Since

Art is a retired machinist, he also made

a tracker out of metal. That one comes

in two pieces and works more or less the

same, using the back feet of the moving

part as the hinge. A third design uses a

strap hinge and might be the simplest

of the lot.

To use the G-Tracker, as Art has

dubbed it, you simply put it under the

east leg of your tripod or under the

east foot of your Dobsonian, and turn

the knob attached to the screw. That

side of your tripod or ground board

rises upward, and your scope moves

westward. If you’re aimed anywhere

near the meridian (the north-south

line that runs directly overhead), your

tracking is pretty much spot-on. The

farther you stray to the east or west, the

more your target takes a diagonal path

through the field of view.

When that happens, place your

object a little off-centre and let it drift

through centre toward the other side

of the field while you’re tracking. Even

near the horizon, that motion is much

slower than having no tracking at all.

No, it’s not perfect, but it greatly

increases the amount of time your

target stays in the field of view. As Art

says, “This device is meant to be an aid

to give you more time at the eyepiece;

astrophotographers need not apply”.

There are ways to improve its

accuracy, though. If you’re going to

spend much time in a particular part

of the sky, you can always adjust the

mount so the east leg becomes the

northeast leg, which corrects its aim for

objects in the northeast or southwest;

or make it a southeast leg, which helps

in the northwest or southeast.

At some point you have to stop and

reset the screw or you risk tipping over

your telescope. You can double your

tracking time by raising the west side

of your mount by that same height,

starting out low on the east side and

tracking through level to high.

If you start 2.5 cm low and go until

you’re 2.5 cm high, that’s ten minutes

— plenty of time for most observations.

It’s a great improvement over no

tracking at all. And that’s the name

of the game when it comes to ATM

projects: improvement.

■ Contributing Editor JERRY OLTION

aims for ‘good enough’ when tracking.

ART GAMBLE (3)

S The G-Tracker design lends itself to many

different forms; three variations are shown

here.

S The plywood version uses a piano hinge on

one end and a metal plate on the other for the

adjustment screw to rest on.