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.



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