REFRACTOR TELECSOPES:

There are two 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 adjusted 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.

REFLECTOR TELECSOPES:

The second method of focusing 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 f/9 or higher. However, with large reflectors and those with focal ratios of f/8 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 focused 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 focuses all of the light rays to the same point.

MAKSUTOV

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.

 

Telescope Limiting Magnitude

The dimmest star that can be seen without optical aid in a dark sky is around 6 magnitude depending upon the observer's eyesight and sky conditions. Telescope Limiting Magnitude table gives a rough idea of the faintest star magnitude that can been seen through a different aperture telescopes. The magnitude values are not precise because many factors affect the magnitude values such as optics, sky conditions, etc.