Sky & Telescope - July 2024 - 75

fails to show additional detail beyond
what less magnification can. So, if you
have a 3-inch (76-mm) scope, it'll top
out at around 150×. Indeed, you'll often
use half that amount even when looking
at the Moon or planets. On most
nights, all you'll accomplish by upping
the magnification is to turn a small,
sharp image into a big, fuzzy one. And
since the size of the field of view and
magnification are related, quite often
you'll opt for low power just so you can
fit the entire Moon or a splashy cluster
into your eyepiece.
A second but much more important
magnification, the finer the detail you'll
be able to perceive. Whether we're trying
to make out tiny lunar craters, surface
markings on Mars, splitting close
double stars, or observing individual
stars in a tightly-packed globular star
cluster - a telescope's resolving power is
key. But because (as stated earlier) any
telescope can be made to magnify any
amount, aperture is effectively the only
variable we need to consider.
As a crude measure of resolving
specification is a telescope's lightgathering
power. Here again basic math
tells us everything we need to know
to compare one scope with another.
Light-gathering is almost entirely a
function of the instrument's aperture,
be it the objective lens (in a refractor)
or primary mirror (in a reflector).
Bigger is better - it's as simple as that.
But there's a subtlety to the calculation.
You might expect a 6-inch telescope to
collect twice as much light as a 3-inch,
but it actually collects four times as
much. That's because it's the total surface
area of the objective that matters,
not the diameter. Doubling the aperture
quadruples the surface area.
Light-gathering power is important
because the more photons your telescope
delivers to your eye, the fainter
the celestial objects you'll be able to see,
and the more obvious a given object will
appear. So, while a 3-inch scope can
show you stars as faint as magnitude
12.6 under a dark sky, a 6-inch lets you
reach down to around magnitude 13.7.
That might not sound like much of an
improvement, but it means being able to
pull in three times as many stars! And
the more light your telescope gathers,
the better it can handle higher magnification.
In other words, when you have
a brighter image to begin with, you can
READ THE LABEL A telescope's focal
length is usually printed somewhere on the
instrument itself. In the case of this vintage,
3-inch reflector (right), the label tells us the
instrument's focal length is 700 mm. The retaining
ring of this refractor's 66-mm objective lens
(far right) tells us its focal length is 388 mm.
sk yand tele scope .o r g * JULY 2024 75
LITTLE DETAILS The Moon is a wonderland
of fascinating features. The quality of a telescope's
optics, the size of its aperture, and the
magnification used all play a role in determining
how much detail you'll be able to see.
magnify it more before it starts to break
down and look dim and fuzzy.
You'll note that earlier I said that
light-gathering power is almost entirely
down to the size of the aperture. That
" almost " qualifier is simply to acknowledge
the fact that no telescope - no
matter how well made - delivers 100%
of the light it gathers to the eyepiece.
Some percentage is lost to dust on the
optics and/or inefficiencies in optical
coatings. But generally, we're talking
about an amount that would be difficult
for even an experienced observer to
perceive. For all practical purposes, it's
a telescope's aperture alone that governs
its light-gathering potential.
Our final telescope power is resolution
- an instrument's ability to reveal
fine detail. Resolving power is determined
by the scope's aperture and magnification.
In a nutshell, the bigger the
telescope's objective and the higher the
power, we can employ something called
the Dawes limit. A simplified version of
the formula is R = 4.56/D, where D is
the telescope's aperture (in inches) and
the result (R) is expressed in arcseconds
- the smaller the number, the better.
So, our 3-inch scope can resolve a pair
of stars separated by 1.5 arcseconds, but
our 6-inch can get down to 0.8 arcseconds.
Once again aperture wins the day,
but the true resolution of your scope
depends on a number of variables, most
crucially the quality of its optics. Better
lenses and mirrors produce sharper
images, which allow us to see greater
detail. The specific optical design of the
telescope also plays a role, though not
as great as aperture and quality.
But the most significant limitation
in your ability to see fine planetary
features or resolve close double stars is
usually the steadiness of our planet's
churning atmosphere - a property
astronomers refer to as seeing. The air
above us can smear details and make
a good telescope seem bad. But when
seeing conditions are steady, your
scope will present its very best views of
whatever celestial sight has caught your
fancy. And that truly has the power to
take your breath away. ¢
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Sky & Telescope - July 2024

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