Optical
Paths of Various Types of Telescopes
Here
is one of the most interesting parts of astronomy and star
gazing: the equipment. There are four different types of
basic telescope designs that are in use today. One of these,
perhaps the most widely known, is the Refractor. The refracting
telescope was first utilized for astronomy by Galileo Galilei
in 1609, but it was invented by the German Dutch lens maker
named Hans Lippershey in 1608 (Zoom Inventors and Inventions).
There are also three other types of common telescopes. These
include the Shmidt Cassegrain, Maksutov Cassegrain, and
the Newtonian Reflector. In terms of design, they all have
different advantages and disadvantages (including cost considerations)
that have to be taken into account if one were to decide
on a particular design to purchase. Ultimately, the physics
behind the the different designs will have an effect in
influencing a purchasing decision. But before the four different
designs are discussed with greater detail, a few technical
terms must be introduced and defined.
Central
Obstruction
- Refers
to the secondary mirror present in certain telescope optical
systems (Shmidt Cassegrain, Maksutov Cassegrain, Newtonian
Reflector) that can add "noise" to the observed
image.
Focal
Length - The distance that it takes for the light passing
through a refractor objective or over a parabolic telescope
mirror to reach the focal plane (or its focal point).
F-Ratio
- This is the ratio of the focal length to the diameter
of the primary objective. For example, a 10'' telescope
with a 100'' focal length is said to be an "F/10"
instrument, which can be calculated as (focal length)/(Mirror
Diameter) or 100''/10''. This tells you wether or not a
telescope optical system is "fast" (low focal
ratios like F/5) or "slow" (long focal ratios
like F/10). The higher the focal ratio, the more magnification
you acheive with a given eyepiece and the more restricted
your field of view becomes.
Aperature
- The diameter of the primary mirror or primary objective.
This is crucial because it determines how much light enters
the telescope's optical system. The more light, the more
detail you will see and the more resolution you will
have. This is contradictory to many manufacturer's advertising
that says their telescope can acheive a magnification of
600x, but may only have an aperature of 4''. This is a very
unlikely scenario for such a small aperature with poor optical
components and a short focal length.
Collimation
- The alignment of the telescope's optical components. This
is discussed further in the performance
section.
The
Refractor
The
basic design of a refracting telescope lays its foundation
upon the following schematic:
Image
courtesy of Telescope
Basics
In
this schematic, it can be seen that there is an objective
lens that bends the light (see light
refraction) into the shape of a cone. This cone of light
is projected into the center of an eyepiece holder, where
an eyepiece intercepts the "focal point" of the
light. The focuser allows the eyepiece to be moved closer
to and farther away from the main objective, allowing the
light to be intercepted at varying points along the cone.
A
refracting telescope has various advantages and disadvantages
in terms of its underlying physics. The first thing to point
out is that the objective lense is solid glass. This leaves
no central obstructions to impede the incoming light as
it travels down its optical path (see "newtonian telescope"
below) which yields a sharper image. However, these types
of telescopes can become extremely expensive. The more inexpensive
kinds ($500 range, called "Achromatic Telescopes")
will often yield a purple halo around astronomical bodies
(such as jupiter, saturn, and bright stars) due to the fact
that red light bends at a different angle than does blue
light. This phenomena is normally minimized through the
use of various coatings (Telescope Basics). In order to
eliminate this purple fringing (or chromatic aberrasion,
see performance) the Apochromatic
Telescope Objective (APO) was created. In an APO telescope
objective, this chromatic distortion is completely eliminated
by using higher quality glass, such as Florite, or it bends
the light through multiple lenses so that the light path
is corrected more than once (How Stuff Works). But, in order
to obtain such an objective, a 4'' diameter objective could
cost as much or even more than $2500, where as an identically
sized Achromatic refractor would cost around $300. So ultimately,
the physics of how the light bends through this type of
objective determines how much you will need to spend in
order to obtain "the best image".
The Newtonian Reflector
The
basic design of a Newtonian Reflecting telescope can be
seen in the following schematic:
Image
courtesy of Telescope
Basics
In
this schematic, it can be seen that the optical system is
vastly different from the refractor telescope listed above.
The primary difference lies in the fact that the light is
reflected rather than refracted through the use of a parabolic
primary mirror (see mirror design).
Due to this fact, the eyepiece can not be positioned to
intercept the focal plane directly without the users head
getting in the way of the light path when they go to look
into the eyepiece. This yields the reason for the flat secondary
mirror which intercepts the cone of light at a 45 degree
angle, reflecting it up to the side of the tube. This central
obstruction consists of a secondary mirror that is held
in place by what is called a "spider vein". These spider
veins are what cause the distortion typically seen in a
Newtonian optical system. This distortion appears, on bright
objects, as narrow bands of light emanating from the object
that is being viewed. An image of this can be seen below.
Image courtesy of Telescope
Basics
So,
this is one major disadvantage for a Newtonian telescope,
however, it has one major advantage. This advantage consists
of the fact that the primary mirror in these kinds of optical
systems is relatively inexpensive and simple to manufacture,
and only requires a thin coating of aluminum in order to
make the surface reflective. Hence, the cost per inch of
aperture is substantially lower than that of a refractor.
For instance, an telescope with an 8'' diameter parabolic
primary mirror costs around $500, and a 10'' can be found
for $650, as opposed to a 4'' Apochromatic which may run
you $2500.
The
Schmidt Cassegrain and Maksutov Cassegrain Reflector
The
schematic for the Schmidt Cassegrain and Maksutov Cassegrain
can be seen below:
Image
courtesy of Telescope
Basics
Image
courtesy of Telescope
Basics
As
can be seen, these designs are very similar to the Newtonian
Reflector seen above. Both designs have a primary mirror,
but in this case the secondary mirror is convex instead
of flat. The convex nature of this mirror allows the focal
length to be extended since the light is traveling a longer
distance (by a factor of 3 times) (Telescope Basics). Essentially,
the light is being forced to overlap itself multiple times.
This allows these telescopes to achieve a longer focal length
in a shorter tube. For instance, a 6'' diameter telescope
mirror with a 72'' focal length (which would make approximately
a 72'' long telescope) can be compressed into an instrument
with the same focal length, but with a telescope assembly
that is only 20-24'' long. Also found in this design is
the presence of the corrector lens. This lens refracts the
light slightly so that the light will hit the primary mirror
evenly. This is necessary because the primary mirror, ultimately,
has a very short focal length. Short focal length mirrors
are harder to produce so that they form a perfect parabolic
shape since so much glass must be ground out of the middle
region of the mirror, hence, the corrector lens is needed
to compensate for it.
Due
to the extra glass in the design and the precision that
the lens elements must be ground to, these telescopes run
more costly than do Newtonians, but are still cheaper per
inch of aperture than Apochromatic refractors. These telescopes
can have a tad bit of chromatic aberration, but it is barely
noticeable and well worth the savings over an Apochromatic
refractor. However, these designs still have a central obstruction
and it can be noticed, but it will not produce the drastic
emanating veins of light that the Newtonian Reflector creates.
Furthermore, these types of telescopes focus in a different
manor than do the Newtonian or Refracting telescopes. Instead
of moving the eyepiece in and out of the focal plane, the
primary mirror is moved forwards and backwards within the
tube. This shifts the cone of light by moving it closer
to and further away from the eyepiece, which remains stationary
at the back end of the tube.
One
more major disadvantage to these telescopes is the fact
that the cooldown time is considerably increased due to
the presense of the miniscus. See the preformance
section under "Air Turbulance" for why this is
an important consideration.
