Hi there, welcome to Part two of the video series, “Telescope Basics .” My name is
David Fuller, host of the “Eyes on the Sky” weekly stargazing series that can be
found at eyes on the sky dot com. In this video, I will explain how magnification and
focal ratio work in amateur telescopes.
{ Take a look at these three amateur telescopes;
it may not appear so at first, but all three are the same size. ["Billionaire" Character]
“I want this host fired!” No, really – let's take a
closer look. These two telescopes have similar sized lenses; this one is 76 millimeters,
and this one is 70 millimeters. This short reflector has a mirror of 76
millimeters, so they are all within 6 millimeters – or ¼ of an inch – in diameter of each
other. Why does that matter?
Well when we talk about how “powerful” a telescope is, or can be, what we really
mean is how much light it gathers. There is a reason why the Hubble Space Telescope
has a large mirror, and other ground-based telescopes are measured in meters. Larger
aperture gathers more light, and that allows us to see dimmer objects and increases
the resolution of the optical instrument.. But let's not get too far into the weeds:
We'll look at these three similarly sized telescopes to understand magnification first.
To determine magnification of a telescope, we perform a simple calculation. Take the
focal length of the telescope in millimeters, and divide that number by the
focal length of the eyepiece you are using. So for example, let's take our three telescopes
and look at each, assuming a 20 millimeter eyepiece.
This 76 millimeter refractor has a 1200 millimeters focal length. So if we divide that by a 20mm
eyepiece, the magnifcation is 60x. For our shorter refractor, it's
focal length is about 600 millimeters, so dividing that by 20mm, we get 30x. And for
a short, stubby little reflector, it has a focal length of just 300 millimeters,
so dividing it by 20mm we get just 15x! And think about this for a moment: If we change
to a 10 millimeter eyepiece, we see the change in magnification for each, but
the RATIO of magnification doesn't change among the telescopes. We'll talk those ratios
about that in a moment.
But first, how does each telescope's length alter what we can SEE in each telescope different?
Well, it actually changes it rather substantially. Look at this: In the
long refractor, we would see only this much of the well-known Pleiades star cluster. That's
not much; the field of view is only three-quarters of a degree, and the Pleiades
is much larger than that. Moving to our mid-sized refractor, we can now see quite a bit more,
just barely fitting the entire cluster in the field of view. But the reflector
is where we can see much more; look at how this view frames the cluster nicely. So the
lower magnification and shorter focal length with it's wider field of view can actually
help us see more.
How does that work? It works by understanding a bit about focal ratios. Remember, all of
those telescopes were nearly the same aperture, but different lengths.
This time, if we divide the focal length by the aperture, we arrive at some different
numbers, but these are fixed numbers for every telescope, and do not change.
The long refractor is nearly 16 to 1, as we divide 1200 mm into the aperture of 76mm and
arrive at 15.8 for an f-ratio, or about f/16 - quite a long focal ratio telescope.
The mid-length refractor is a shorter f-ratio: 600 millimeters divided by 70mm is f/8.5.
And the short little refractor has the shortest focal length of all: 300 divided
by 76 is 3.9 or close to f/4 – f/3 to f/5's are definitely short f-ratio scopes.
So the longer f-ratio telescopes provide more magnification with the same eyepiece, but
less field of view; this makes it easier to get high magnification for planets,
but harder to see wide field views. Middle ones can still provide fairly high magnification,
decent fields of view as well; no wonder many f/7 to f/10 instruments are popular.
And short focal length telescopes provide the widest field of view, but the least magnification
using the same eyepiece, though in larger apertures, the focal ratio tends to get
faster so that tube lengths do not become unwieldy and difficult to use or mount..
Of course, we can get more magnification with shorter eyepiece focal lengths or using barlow
lenses, and wider fields of view using other types of eyepieces; but see Videos
3 and 5 for more information on those topics.
So that is a quick look at magnification and focal ratios. Now let's look at some other
aspects of telescopes; in part three of this series, I'll talk about various types of eyepieces.
To see what's up in the night sky, check out the weekly night sky videos at eyes on the
sky dot com. Thanks for watching; I hope that has provided you with a good overview of magnification
and focal ratio. Keep your eyes on the sky and your outdoor lights aimed down, so we
can all see, what's up.