Telescopes
Introduction
There are two main types of telescope - refractors and reflectors. In both cases the image will be inverted. A quoted size of a telescope refers to the diameter of the objective, which is either the light-gathering lens (refractors) or the mirror (reflectors).
Nowadays, performance can be improved by new technologies, e.g. CCDs.
Refractors
Refractors used for astronomy will produce an inverted image. No attempts are made to correct this because this would involve introducing an additional lens system which would reduce the light level, and this is obviously not something we want from an astronomical telescope.
Professionally, they suffer from the disadvantage that there is a limit to the size of the objective lens. Amateurs like them for observing the Moon and planets, and the Sun. It also has to be longer, since a long focal length is required to reduce chromatic aberration.
Reflectors
The primary mirror is the objective, and since this mirror can be firmly supported from behind, the objective can be made bigger than the objective of a refractor. Additionally, chromatic aberration does not occur.
The primary mirror is a paraboloid which reflects the light to a secondary mirror, as shown in the diagrams.
The length can be reduced in comparison with a refractor.
Reflectors come in two main types - Cassegrain and Newtonian.
In a Newtonian the secondary mirror is a flat plate which reflects the light 'sideways'
to the eyepiece.
The original Newtonian was primitive. John Hadley built the first serviceable reflecting telescope.
The Cassegrain was invented in 1672 by the Frenchman Laurent Cassegrain. In a Cassegrain, the secondary mirror is convex and placed close to the prime-focus position. It reflects the light thru a hole in the center of the mirror, into the eyepiece.
Instead of using a secondary mirror, professional telescopes often place a camera at the prime-focus position at the top of the telescope tube.
Generally speaking a reflector with a shorter focal length than a refractor produces a wider field of view which is ideal for observing nebulae and deep sky objects. The longer focal lengths of refractors tend to make them useful for planetary and lunar observations.
Magnification
Magnification is not the be all and end all of a telescope. Magnifying an image makes it fainter, so you need to be able to collect enough light in the first place. This requires a decent objective. The amount of light collected by the objective is proportional to the square of the radius, as you could guess from the formula for a circle - πr2.
Magnification can be calculated from the formula
M = F/f
where M is magnification, F is is the focal length of the Objective and f is the focal length of the eyepiece
Telescopes generally come equipped with a range of eyepieces giving the different magnifications that are required for different observations.
high magnification - giving a limited field of view, to study detail (on the moon, for example).
low magnification - with a wide field of view, to study a star cluster
The maximum maginification used would per 2.4 times the objective diameter in millimeters.
a) If a telescope has an objective of 20 cm diameter, how much more light will it collect than a telescope with a diameter of only 5 cm ?
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Resolution
Resolution is the ability to distinguish between objects. Resolution can be reduced by diffraction and thus a larger aperture will improve resolution.
Mountings
Equatorial mountings allow the telescope to follow the stars more easily as they move across the sky.
There are several types - German, English, Fork.
It is common to couple an equatorial mounting with a drive mechanism, allowing the telescope to remain fixed on a certain portion of the sky. However nowadays, the use of computers allows the use of altazimuth telescopes which is indeed the method now being used for professional telescopes and increasingly for amateurs as well.
Prior to computerization, an altazimuth mounting was considered the most basic form of mounting with a straight up-and-down, right-to-left movement. It was, and still remains, a common mounting - probably the most popular is the Dobsonian (see diagram below).
Binoculars
Binoculars are effectively a pair of refractors, except that the image can be made to be the right way up.
Binoculars producing an inverted image are available but the 'traditional' right-way-up type appears to be the most popular type for astronomy.
Binoculars are used for wide-field viewing and are found to be more relaxing than a telescope. The effect produced by two eyes is also claimed to produce a different effect of seeing, i.e. the image you can 'process' thru two eyes is better than just thru the one.
A 7 X 50 would be a basic type for astronomy, where the first number refers to the magnification and the second figure gives the diameter of the objective in millimeters. A 25 X 150 is at the top end of the 'serious' type. For any serious use of binoculars, a tripod will be needed.
Photographs
A basic question often asked by 'lay' people and beginners is: 'Are the colors in astronomical photos (as seen in books and magazines etc) actually genuine colors?'. An appropriate answer would be an English equivalent of the good German word 'Jein', if such an English word actually existed.
Colorful photographs could be genuine colors - if this is so then for the most part these would be colors that the human eye would not be able to detect. They are colors that would only be detected by long-exposure photographs.
Nowadays there is camera film that you can use during the night and which will produce photos showing green grass and so on. This would be a good example of the type of effect mentioned in the previous paragraph - the camera is picking up a color which we do actually know to be genuine but which the human eye would never detect during night-time however long it looked at a particular object or a particular scene.
CCD
CCD is short for Charged Couple Device. These are semiconductor devices, typically about two centimeters square (or almost square). They are extremely efficient, usually recording about 70% of the radiation that hits them (photographs only record about a few percent).
Needless to say, there are problems - to briefly summarize a few
- In any CCD, some of the detector elements (detector pixels) will be faulty. In a similar vein, the sensitivity to light of the many pixels will vary slightly.
- 'Noise' will be recorded from ionizing radiation such as cosmic rays
- Signals could be generated due to motion of electrons arising from the thermal energy of the CCD (this is called dark current). This effect is often countered by cooling the CCD (using liquid nitrogen, for example). The effect is often unimportant for visual light detection, but is more important for working in the infra-red.
- Problems with faulty pixels and noise could be countered using computer imaging, by comparing with adjacent pixels.
- Different sensitivities to light could be countered by 'flat-fielding', i.e. taking an image of a uniformly-illuminated light source and using this information to adjust the astronomical image.
- Dark current could be counterd by taking exposures with the shutter closed, and subtracting this exposure from a 'proper' image.
