Hi there, welcome to the first in 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 the various types of telescopes and what you need to know
about them to either make the best choice for purchasing one, or how to better use the
telescope you may have right now.
So let's get started!
First, we will look at the three major types of amateur telescopes.
Refractors, reflectors, and compound – or catadioptric – telescopes.
When most people think “Telescope” they probably imagine a
refractor, like this one.
They have a large glass lens at one end, through which light passes and refracts – or bends
– the light to a focal point.
As the light passes the focal point, here, we
place an eyepiece, which then focuses the light into a magnified image we can see.
Though usually a “star diagonal” is placed here to bounce the light 90
degrees for more comfortable viewing.
So far so good, right?
A reflector, on the other hand, is often called a Newtonian telescope.
These can be mounted on equatorial or Dobsonian mounts.
And yes, this type of telescope was named after Isaac Newton, who first came up with
this telescope concept.
As the name implies, light is reflected off of a mirror in this kind of telescope, usually
placed at the bottom of a tube.
As it is reflected, it is towards a focal point
as well, but note where that happens: Near the top of the tube!
That makes it hard to place an eyepiece there, as both the focuser and your head would
block incoming light.
So a reflector uses a “Secondary mirror” - a small, diagonally-shaped flat mirror to
bounce the light out the side of the tube, like this, where it enters the focuser and
reaches the eyepiece.
The third type of telescope is a compound telescope, or catadioptric.
“Cat-a-what?”
I know, that's a big word: It means, “an optical system that involves both the reflecting
and refracting of light, in order to reduce aberration.”
So this Schmidt-Cassegrain telescope, for example, utilizes a “Corrector lens” on
the front of the telescope, a strongly curved primary mirror at the back,
and then another outwardly curved mirror in the middle of the corrector lens before light
finally goes through the center of the back of the telescope to the eyepiece.
It basically can take a very long focal-length light path, and “fold”
it into a shorter tube.
There are other versions of catadioptric scopes, Schmidt-Cassegrains and Maksutov-Cassegrains
being the most prominent amateur types available.
Let's take a quick overview of the strengths and weaknesses
of each:
For refractors, here are some advantages: Unobstructed design and ease of manufacture
allows for theoretically better optical quality Eyepiece location is convenient in smaller
models Tend to be portable in apertures of 100 mm
or less and are easy to aim
Some disadvantages include Chromatic abberation in achromats, where not
all wavelenghts of light focus in the same place
Apochromatic designs, though overcoming chromatic aberration, are usually 3 to 5 times as expensive
In longer focal lengths, the long tubes can sway in wind or on inexpensive mounts
Larger models place eyepiece uncomfortably low
Reflectors have their own set of advantages: No chromatic aberration
Easier and less expensive to make in larger apertures
Faster focal ratio systems provide wide fields of view
Light grasp in larger models is excellent Cost per inch of aperture is best
But they also have their drawbacks: The optical aberration “coma” occurs in
faster models, unless specialized eyepieces or correctors are used
Light loss due to multiple mirrors is greater than refractors
Central obstruction due to secondary mirror can cause diffraction and contrast loss
Larger models can be bulky and heavy Eyepiece can be in awkward positions
Catadioptrics, likewise, have pluses and minuses.
On the plus side, they
Have reduce optical aberrations to minimal levels
Provide good light gathering power Eyepiece generally in very convenient location
Offer excellent portability
On the downsides, cats are The most expensive per inch of aperture
Have the greatest light loss due to multiple lenses and mirrors
Often have the largest central obsctruction Tube currents due to sealed systems can cause
poor images before fully cooled to outdoor temperatures
So that is a quick look at the three major types of telescopes, and some of their better
features and downsides.
Now let's look at some other aspects of telescopes; in part two of this series,
I'll talk about magnification and focal ratio.
Or if you're ready to see what's up in the night sky, see 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 the
types of telescopes used for amateur astronomy.
Keep your eyes on the sky and your outdoor lights aimed down, so we can all see, what's
up.