Activities such as astronomy

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 to increase our visual range. A telescope is an optical device which makes far 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. The diameter of the observed circle depends on the field of view of the eyepiece.

Some telescope advertisements include phrases about the very high magnification or power that their instruments can achieve. These telescopes usually have about 60mm (2.4") diameter apertures, and claim magnifications of 600x or more. It is true that their images can be magnified that much but what they end up magnifying is all the turbulence in the air between the telescope and the subject. When you are looking at astronomical objects, you are looking through a column of air that reaches to the edge of space and that column seldom stays still. Similarly, when viewing over land you are often looking through waves of heated air radiating from the ground, houses, buildings, etc. A good rule of thumb is that the usable magnification of a telescope is about 50x per inch (2x per mm) of aperture under good conditions. Values of 3x per millimeter or higher are often quoted for ideal conditions, but these conditions are usually very rare. The final resolution that an astronomical telescope can achieve depends on the amount of light that it can capture. The bigger the aperture, the higher the resolution and therefore the better the image. However, there are times when the earth's atmosphere is so unsettled that a smaller aperture will give better results because it sees fewer turbulent zones. A telescope cap with a smaller opening which acts as a mask, can prove to be a useful accessory under these conditions. Sky conditions are usually defined by two atmospheric characteristics, seeing, or the steadiness of the air, and transparency, the clarity of the air due to the amount of water vapour and particulate material present.

Blurry moon under magnification above the limit

When light is focussed by passing it through a lens made from ordinary glass, such as crown glass, each wavelength of light bends a different amount. This is the reason, we are able to see light separated into its spectrum when it passes through a glass prism. This different bending leads to a problem, because each wavelength focusses at a different point. The result is a focal zone rather than a focal point. When a bright object is viewed through such a lens, it is blurry and has a fringe of false colour. Technically, this is referred to as chromatic aberration. Reflectors don't suffer from this effect because their light rays don't pass through any glass. A second problem, called spherical aberration, occurs when optical surfaces of lenses or mirrors are not properly figured or shaped. As with chromatic aberration, the focal point becomes a focal zone.

The first way telescope makers tried to correct these problems was to make telescopes longer. This results in a higher focal ratio and the aberrations become less pronounced. The focal ratio is the focal length, (the distance from the primary lens or mirror to the focal point), divided by the aperture (the diameter of the primary). Small focal ratio telescopes, often referred to as fast telescopes, are more subject to chromatic aberration. Making telescopes longer is fine for small apertures, but with large apertures they quickly become unwieldy.

A second approach is to add another matching lens of a glass having a different refractive index. For example, when positive, low-index, BK7 crown glass is matched with negative, high-index, F2 flint glass, the light rays are bent again so that all wavelengths focus near the same point. The result is called an achromatic refractor and the matched lenses may either be cemented together, or air-spaced by mounting them in a cell which holds them in their correct positions. The two-element lenses used in today's achromats greatly reduce the chromatic aberration. .

In the ongoing search for the perfect telescope, lens makers produced other lens element combinations and special types of glass, in order to remove all of the false colour. These developments have resulted in semi-apochromatic (almost without colour) and apochromatic (corrected in three colours) refractors but these are very expensive compared to achromatic refractors.

When light enters or leaves a lens, there is a loss of some transmitted light due to reflection. By applying a surface coating of an antireflective material such as magnesium fluoride, the transmission can be greatly increased and internal flare can be reduced. When all lens surfaces have been coated they are said to be fully-coated and when the surfaces are coated with multiple layers to maximize transmission, the optics are said to be multi-coated.

Coatings also play a big part in the performance of reflectors because not all of the light is reflected; there is a small loss at each mirror surface. Today's reflectors usually have a thin coat of aluminum as the mirror and then an overcoat of silicon monoxide or silicon dioxide to protect it. Silicon dioxide produces a more durable coat than silicon monoxide but requires specialized equipment to apply it and is therefore more expensive. Protection is needed because, in most reflectors, the mirror is open to the elements and deterioration of the reflective layer reduces the resolution of the telescope.

Resolution can be defined as how much detail a particular telescope can see. It is dependent upon the size of the aperture and the quality of the optical surfaces, assuming that the optical system is correctly collimated. If the diameter of the aperture is twice as big then the resolving power should be twice as good. What it really comes down to is that the more light that a telescope can gather, the more detail that it can provide. Resolution is generally stated in arc-seconds and there are sixty arc-seconds in an arc-minute and sixty arc-minutes in a degree.

The second factor that affects resolution is the quality of the lens or mirror surface. Optics which are badly figured, poorly mounted or which have surface imperfections can present many different aberrations. Well made optics where everything snaps into focus are a joy to use.

When we look at the moon or a planet through a telescope, we are looking at an extended object and as we increase magnification under good conditions, we see more detail. However, when we look at a star, we are looking at a point source and no matter how much we magnify, it is so far away that all we get is a point of light. In fact, due to diffraction, we don't even see a point of light through a telescope, we see a circle of light called an Airy disk. The arc-second diameter of this disk decreases as the aperture of the telescope increases. The Airy disk is surrounded by increasingly faint concentric rings of light and the whole grouping is called a diffraction image.

A method that is often used to measure resolution, is to split two very close stars. The ability to separate the two stars is actually the ability to separate their Airy disks. This is often called the resolving power of a telescope. A formula to estimate the distance apart that two equal brightness stars must be to separate them is 4.54 divided by the aperture in inches (or 116 divided by the aperture in mm). This is known as the Dawes' limit after the amateur astronomer who derived it in the Nineteenth century. It should be remembered that it is an empirical value, found by trial and error, for approximately magnitude six stars and using an unobstructed telescope. It is frequently exceeded by wellmade, modern telescopes.

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