If you’re thinking of buying a telescope, or just want to know how they work, then this thread is for you! Just pardon my poor image creation skills.
All telescopes work in one of two systems. They are the horizon system and the equatorial system. The horizon system works in altitude and in azimuth,
which constantly changes. The equatorial system works in declination and right ascension, which never changes.
Altitude: The position up/down with respect to the horizon.
Horizon = 0 degrees
Zenith (directly overhead) = 90 degrees
Azimuth: The position along the horizon from North
N = 0 deg. E = 90 deg. S = 180 deg. W = 270 deg.
When using the horizon system a good approximation of an object’s position in the sky is by using the width of your fist. The width should be
approximately 10 degrees. Since the sky is on constant motion, all objects positions in altitude/azimuth are also constantly changing.
Declination: North/South position from the Celestial Equator (line in the sky which corresponds with the Earth’s equator.) Rotates around the North
and South Celestial Poles (directions in space of the Earth’s axis of rotation.)
CE = 0 deg.
NCP = +90 deg.
SCP = -90 deg.
Right Ascension: East/West time
position of an object.
0 or 24 hours RA = Vernal equinox (March 21)
6 hours RA = Summer solstice (June 21)
12 hours RA = Autumnal equinox (September 21)
18 hours RA = Winter solstice (December 21)
There are 24 hours of RA, with 1 hour equaling 15 deg. of the sky.
Telescope Math and Types
First off, the main task of a telescope is to gather light, despite the popular idea that it is to magnify an image. In this section I’ll cover how
telescopes work, using simple mathematical formulas (read: “don’t get scared off because of math!”), the three types of mounts, and most importantly
the two types of telescopes and their differences.
Light Gathering Power (LGP)
The LGP of a telescope is very simple to determine using this formula:
(Diameter of telescope)²
(Human eye aperture)²
The human eye aperture is pretty much constant at 4 inches. So, using that formula and the constant of the human eye, you can figure out how much more
light any given telescope gathers than the human eye.
A telescope with an 8 inch diameter.
= 4x more LGP
A telescope with a 20 inch diameter.
= 25x more LGP
Magnifying Power (MP)
The MP of a telescope is also very easy to determine. It can be done by using this formula:
Focal length of telescope
Focal length of eyepiece
The shorter the focal length of the eyepiece the higher the magnification. For more clarity in the image, a limit of 50x MP per inch of diameter
should be used. For example, a telescope with a 4 inch diameter should not have an MP greater than 200x. Also, as MP goes up, the field of view of the
telescope goes down.
A telescope with a focal length of 200 mm, using an eyepiece with a focal length of 2 mm.
= 100x MP
A telescope with a focal length of 328 mm, using an eyepiece with a focal length of 8 mm.
= 41x MP
There are two popular telescope types. They are the refractor, and the reflector. Each uses unique methods of gathering light, and each has their own
distinct advantages and disadvantages.
A refractor telescope is a basic design for a telescope, much like a spyglass you see in movies. There are two or three lenses at one end and an
eyepiece at the other end. A refractor with two lenses is called achromatic, while a refractor with three lenses is called apochromatic. The
difference between the two is the clarity of the image, with apochromatic being the clearest.
A diagram of a basic refractor telescope:
(1) Light enters telescope, (2) passes through set of lenses, (3) is focused down the tube, (4) focused light enters eyepiece for viewing.
The strengths of refractor telescopes are that they give highly detailed images, making them ideal to use for viewing planets and binary star systems.
They also have a very long focal length, giving a higher possible magnification. On the downside they are fairly expensive and the large glass lenses
are only supported on their edges. Also their large size keeps them immobile.
Reflector telescopes use a system of mirrors and corrector plates to focus the image and gather light. There are three main types of reflectors:
Newtonian, Schmidt-Cassesgrain, and Schmidt-Maksutov.
A diagram of a Newtonian reflector:
(1) Light enters the telescope and travels to the (2) primary parabolic mirror, the light is then (3) focused and reflected towards the (4) secondary
mirror, (5) the light then reflects out the side of the telescope to the (6) eyepiece.
A diagram of a Schmidt-Cassegrain reflector:
(1) Light enters the telescope, (2) passes through a thin color corrector plate and continues to the (3) primary parabolic mirror where the light is
focused and (4) reflected forwards to the (5) secondary mirror which, in turn, (6) reflects the light towards the back and center of the telescope and
(7) out the eyepiece.
A diagram of a Schmidt-Maksutov reflector:
(1) Light enters the telescope, (2) passes through a thick, parabolic color corrector plate and continues to the (3) primary parabolic mirror where
the light is focused and (4) reflected forwards to the (5) secondary mirror which, in turn, (6) reflects the light towards the back and center of the
telescope and (7) out the eyepiece.
The strengths of the reflector telescope are that it is compact (fitting more focal length in less space) and that their smaller size allows them to
be transported from location to location with little effort. The main weakness is that the images are not as detailed as in a refractor.
So what do these telescopes sit on? Well more than likely it’s one of three different types of mounts. These are the Dobsonian, German Equatorial, and
the Yoke (or fork).
This mount is fairly simple, only works with the horizon system, and is used mainly for Newtonian reflectors. The (1) telescope has a (2) joint that
it is connected to. This joint allows the telescope to move in altitude. That whole assembly then sits on the (3) box-like mount. This mount swivels
on its (4) base, allowing the telescope to move in azimuth.
This mount is fairly simple to use and understand, and can be used on any kind of telescope. It is most commonly used for refractor telescopes, which
keeps these types of mounts, for the most part, immobile. (1) The telescope sits on one end of a (2) long rod. On the other end of the rod is (3) a
counterweight, used to balance the telescope. This entire assembly sits on top of (4) a tripod (for mobile use) or central pier (immobile.) Also, if
you hadn’t guessed by the name, this type of mount utilizes the equatorial system.
This mount can be used in both equatorial and horizon systems. (1) The telescope, like the Dobsonian, has (2) a connected joint. This joint allows the
telescope to move in declination and in altitude. From this joint the telescope sits on (3) a pier, which connects to (4) the wedge. The wedge is the
most important part of the telescope when using the equatorial system. This is because it is what you use to align the telescope. The wedges should be
set to what degree of latitude you live at. For example, I live near the 42nd N, so the wedge of my telescope would be set at 42 degrees. This whole
assembly then also sits upon (5) a tripod.
So Now What?
Well now that you know just about everything you would need to know in order to operate a telescope, you probably want to get out there and use it!
Now once you have it set up you should start off by looking at the brighter objects in the sky. Why’s this? Because they’re the easiest to find! A
good investment would be to purchase some simple star charts, a red-lensed flashlight (red lights don’t ruin night vision as badly as white lights),
or even some software for a computer such as Starry Night.
Also, don’t expect to see images like those taken by
giant telescopes or the Hubble.
Observing the Moon
Despite popular belief, the best time to view the full moon is not while it is full. On the contrary the best time is actually while the moon is in a
crescent stage. This is because the other stages are too bright to view finer details on the lunar surface. While looking at the moon it’s best to
observe craters and canyons. Also, one of the neater things about viewing a crescent moon is that the crater walls and cliffs cast intricate shadows
onto the moon’s surface.
Observe Saturn and Jupiter
When looking at Jupiter through even a small telescope the largest moons can be seen. They appear as bright stars to either side of the planet. Also,
some bands of gas can be seen. Using a higher magnification more bands can be seen.
When looking at Saturn through a small telescope the rings can be seen. On lower magnifications the rings look like something sticking off of the
planet itself. On higher magnifications the rings will separate from the planet, and on even higher magnifications divides in the rings can be
Observe the Messier Objects
The Messier Objects are a collection of 110 star clusters, nebulas, and galaxies. A great list of them, including images and information can be found
So now that you know what to do, go enjoy your night sky!
Using a Telescope
The Munich Archive Astromaps
[edit on 8/19/2004 by cmdrkeenkid]