Sky & Telescope - March 2025 - 72

ASTRONOMER'S WORKBENCH by Jerry Oltion
The Itty Bitty
Radio Telescope
Have great fun with a surprisingly small instrument.
RADIO TELESCOPES ARE huge things,
right? We've seen dishes that dwarf the
buildings around them, even fill entire
valleys. We've seen arrays of dishes that
spread for miles, or in the case of the
Event Horizon Telescope that images the
accretion disks around black holes, span
the width of the entire Earth. Given the
enormous size and investment of these
behemoths, what chance does an amateur
astronomer have of getting into
radio astronomy?
A very good chance, it turns out. Several
years ago, Society of Amateur Radio
Astronomers (SARA) members Chuck
Forster and Kerry Smith came up with a
design for a compact, even portable, and
inexpensive radio telescope that you can
The IBT consists of four basic parts: The
dish antenna, the signal meter, the power
source, and the mount.
Detecting device
(feedhorn)
Shadow
at focus
point
build yourself. It's called the Itty Bitty
Telescope (IBT), or sometimes the Little
Bitty Telescope.
Oregon ATM Michael Caba recently
built one, and he has had great success
with it. Not in detecting black holes,
but in getting people excited about radio
astronomy at daytime outreach events.
How does the IBT work? The concept
is pretty simple. It consists of a satellite
TV dish, a signal meter, a power source,
and a mount. When the dish is aimed at
a radio source, the signal meter swings
up the scale and (if the meter has an
audio feature) a tone sounds.
What can you detect with it? The Sun,
primarily. The Sun is by far the " loudest "
radio source in the sky, and it has
the added advantage of
emitting visible light
as well, so you can aim
the IBT at the Sun simply
by positioning the
shadow of the feedhorn
(the radio detector at
Signal
meter
the dish's focal point)
at the geometric center
of the dish. For a typical
satellite TV dish,
the geometric center
is actually at its edge,
which leads to some
interesting discussions
about geometry as well
as radio astronomy.
Power
source
When the shadow falls
on the sweet spot, the
signal meter registers
the influx of radio
waves, and you know
you've detected the Sun
in the radio band.
72 MARCH 2025 * SKY & TELESCOPE
pThe Itty Bitty Telescope is a popular item at
daytime astronomy outreach events.
Which radio band, you might ask?
The IBT is sensitive to signals between
12.2 and 12.7 gigahertz, which is the
2.5-centimeter band. Astute readers will
recognize that this is in the commonly
called " microwave " range of the spectrum,
which is still considered radio.
You might think that aiming a
dish at the Sun and watching a needle
move on a gauge (and hearing a tone
sound) would be pretty bland fare at
an outreach event, but Mike reports
that the IBT is often more popular
than H-alpha or white-light views of
the Sun. Because it's so simple to use,
people love aiming it themselves and
they're delighted to " find " the Sun with
a scientific gadget.
Is your solar viewing event clouded
out? No problem. The IBT can detect the
Sun right through the clouds. Finding it
is a bit trickier without the feedhorn's
shadow, but the reward is that much
more exciting.
It gets even better. Any warm object
emits energy in the radio spectrum.
That means the IBT will detect a warm
tree or a building . . . or you! If you
ALL IMAGES COURTESY OF MIKE CABA
Sky & Telescope - March 2025

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