|
Optica Design
Activities such as astronomy, nature studies and viewing sports must often be done from a distance. For various reasons we cannot get close enough to the subjects to view them in the detail that is needed. Our eyes are general purpose tools and their resolution is limited, their magnifying properties are minimal and they are limited in how much light that they can gather. We must use optical devices such as telescopes and binoculars to increase our visual range.
A telescope is an optical device which makes distant objects appear closer. It samples a small area of view, a field, and then magnifies it so that distant objects appear larger. Parallel light rays entering the telescope are focussed to a single point, called the focus or focal point. These focussed rays are then magnified with a very powerful lens, or more commonly a set of lenses, called an eyepiece, to give enlarged views of distant objects. The eyepiece acts in the reverse direction to the telescope lens, taking the focussed rays and sending them to the eye as parallel rays. |
| |
|
REFLECTOR
The second method of focussing light is to reflect the rays off of the surface of a curved mirror, producing a type of telescope called a reflector. The most common reflectors in use today are called Newtonians because this design was pioneered by Isaac Newton.
A mirror is made by coating the front surface of a concave piece of glass with a reflecting material. Light rays entering the telescope reflect off of the mirror and since they never pass through the glass no false colour is produced. |
The surface of the mirror of a high focal ratio reflector can be shaped or figured to that of the surface of a sphere. This works for small reflectors and those with focal ratios of f9 or higher. However, with large reflectors and those with focal ratios of f8 or lower, these spherical mirrors do not bring all of the light rays to the same focal point. The rays from the mirror’s perimeter are focussed at a different point from it’s centre, resulting in an image which lacks contrast due to spherical aberration. To overcome this defect, mirror surfaces are shaped during polishing to a paraboloidal shape which focusses all of the light rays to the same point.
Since the light rays are reflected back up the optical tube by the primary mirror, they must be redirected in order to be viewed. A secondary mirror, which has a flat surface is mounted at a 45 degree angle in the centre of the tube to reflect the rays to the focal point. The secondary is usually oval in shape because this presents a circular shape when viewed from a 45 degree angle. Obstructions, such as secondary mirrors, have a limited visual effect when placed in the path of the light entering the telescope. They modify the diffraction patterns, which can cause very minute loss of contrast, and they reduce the amount of light reaching the focal point. However, they are not seen in the focussed image presented through the eyepiece. Since the eyepiece is near the front of the tube, reflectors can be mounted lower to the ground giving more convenient viewing and greater stability. Only two surfaces need to be shaped, polished and coated and these can be tested separately. This makes them less expensive to produce than other telescope designs. On the negative side, a long optical tube Newtonian on a German equatorial mount can be more susceptible to wind vibrations than shorter designs. Collimation of both mirrors is part of the regular maintenance for reflectors.
|
| |
 |
REFRACTORS
There are three basic ways to bring light rays to a focal point. The earliest method used by telescope makers, was to bend the rays by passing them through one or more pieces of glass which had curved, polished surfaces. This method produces a type of telescope called a refractor.
Refractors have several advantages over other designs. They are enclosed so that dust and moisture doesn’t enter the optical tube.
|
They have fixed optics so that they don’t require routine collimation, which means that the optics don’t have to be aligned by the user.They do not have a central obstruction, which reduces the amount of light entering the tube and causes an alteration of the diffraction pattern. The resulting high-contrast, fine-resolution images produced are considered ideal for planetary viewing. A problem with refractors is that since many wavelengths of light are passing through glass, the uneven bending of the rays causes false colour, around bright objects. This must be counteracted with additional lenses and special glass. Since at least four lens surfaces usually have to be very accurately shaped, polished and coated, they are more expensive to produce than other telescope designs.
|
| |
 |
CASSEGRAIN
A third group of telescopes, called Cassegrain, are hybrids of the two previous methods. Cassegrain telescopes use a combination of both mirrors and lenses to manipulate and focus the light rays. Examples of these are the Schmidt-Cassegrain, and the Maksutov-Cassegrain.
|
Schmidt-Cassegrain telescopes use a thin aspherical corrector plate, which is a lens carefully matched to the primary concave mirror to correct for spherical aberration. Parallel light rays enter the telescope through the corrector plate and are then reflected by the primary mirror to a convex secondary mirror which is mounted inside the focal point and concentric with the corrector plate. The secondary mirror reflects the rays back down the tube and through a hole in the centre of the primary. The eyepiece can be placed directly behind the primary mirror or a diagonal can be used to change the angle at which the image is viewed. Focussing may be achieved by moving the primary mirror or by moving the eyepiece.
Maksutov-Cassegrain telescopes are similar to the Schmidt-Cassegrains. They also have a corrector plate to remove spherical aberration, but they use a thick, meniscus lens instead of a Schmidt lens. Light enters through the concave side of the corrector plate and the primary mirror reflects it back up the tube to the secondary which is often a mirrored spot on the convex side of the corrector plate. As with the Schmidt-Cassegrain, the light rays are reflected through a hole in the primary to reach the eyepiece This design is easier to produce than the Schmidt-Cassegrain, but the thicker corrector plate makes it heavier. The Maksutov-Cassegrain telescope was developed in the 1940’s by several different inventors of slightly varying designs. Most commercial Maksutov telescopes available have similar optical designs. The main advantage of this design is that, because the light path is folded back on itself, it provides a very portable, short physical length telescope with a long focal length. |
| |
Back to top
|
