INTRODUCTION TO ASTRONOMY
This article
featured in the January 2003 Beginners Magazine
TALKING ABOUT TELESCOPES
In the first article we considered the first step for the astronomy beginner to take, this was to buy a book with sky maps and become familiar with the night sky. The second was a look at binoculars as the first instrument needed to improve the view of the night sky.To most people the first question to ask about a telescope is "What magnification will it give?" but the important properties of an astronomical telescope are 'the light gathering potential' and 'the accuracy of the optics'. This means the bigger and more accurately configured the optics are, the more powerful the telescope.There are two basic forms of telescope, namely REFRACTOR or REFLECTOR.
The refractor uses a lens as its main optic to focus light into an image and a reflector uses a concaved mirror to focus the image. In both types the image may then be magnified and studied using an eyepiece rather like using a microscope.For any given size of primary optic, lens or mirror, the image is always the same size and cannot be changed without reconfiguring the main optic. The size of the image is however dictated by the focal length of the optics. (Focal length is the distance from the optic at which the image is formed.) The longer the focal length the larger the image formed.
The FOCAL RATIO is the relationship between the diameter of the main optic and its focal length. Focal lengths are usually identified as a 'f'' number and is obtained from the formula :-
FOCAL RATIO (f number) = FOCAL LENGTH (divided by) APERTURE
So as an example take a lens or mirror of diameter 150mm and with a focal length of 900mm, this would have would have a focal ratio of :-
FOCAL RATIO = 900/150 = f 6
High focal ratio primary optics are selected where the need is for high magnification and low focal ratios used where a wider field of view at a lower magnification is desired. Of course an instrument with a long focal length requires a long mounting tube and would be very cumbersome and difficult to mount. A focal ratio of f6 is a good 'general purpose' instrument. Therefore careful consideration must be given when selection the focal length of a telescope which is to be bought or built.
So, what is a telescope ?. Of course everyone knows what a telescope is. We have all seen pictures of a sea captain looking through one and almost everybody has looked through a pair of binoculars, these are just two telescopes joined together. Patrick Moore often talks about his telescopes or about some of the big astronomical telescopes used by scientists all around the world but what is inside and how do they work, that is what we want to know here.All telescopes including binoculars and even radio telescopes such as the one a Joderall Bank consist of two vital main components. A primary focusing unit and a secondary magnifying unit (or amplifier in a radio telescope). All the other parts would be too many to list and discuss here in detail but are either used to support these two components or to improve or manipulate the images formed in the instrument.
THE PRIMARY OPTIC
This is the part of the telescope that gathers light and focuses it into an image inside the instrument. The human eye when exposed to complete darkness will open its pupil (the black part at the centre) to its maximum diameter to gather as much light as possible. The maximum diameter the pupil can open is about 8mm giving an area of 0.5 square centimetre. This is the maximum area through which your eye is able to capture light and convert it into a signal to the brain. The Primary Focusing Unit has the job of gathering a larger area of light and converging it into a smaller area where a picture is formed this is called the primary focus image which we shall call the 'image'. A primary focusing lens or mirror of 150mm (6") diameter will gather 182 square centimetres of light which can be seen is 364 times as much as the human eye. This means that whatever object or light source this primary focusing unit is pointed at will appear 364 times brighter than it would when viewed only with the naked human eye.
SECONDARY FOCUSING UNIT
The image can be seen if brought to focus on a piece of white card but cannot be viewed directly by the unaided eye. A special lens unit is used to view the image, this is known as the SECONDARY FOCUSING UNIT or 'Eyepiece'. The Eyepiece is, in effect, a microscope used to focus and magnify the image formed by the primary optic. The bigger and longer the telescope the bigger will be the image formed. By using different lenses in the eyepiece, all or part of the image can be magnified and examined. By using a large, shallow curved eyepiece lens, (long focus) a wide angle, low magnification view of all or most of the image can be seen. Using a small deep curved eyepiece lens, (short focus) a high magnification view of part of the image can be seen. This is a simplified explanation of the eyepiece, in fact almost all eyepieces have at least two lenses some have more to improve the quality of the viewed image.
REFRACTING TELESCOPES
All refracting telescopes use a glass lens as their primary focusing unit. This lens is normally made up from two or more lens components to produce a clearer image and reduce colour distortions caused in the refracting process as explained below. Lenses use the property called REFRACTION to change the direction of rays of light and direct them towards a desired position. Refraction occurs when light passes between two different transparent materials such as glass, air and water. This effect can be seen when we put the straight pole of a fishing net into the river, the pole seems to bend at the surface. When light passes, at an angle, through the surface of a block of glass the angle is changed. As the light re-emerges through the opposite side of this material its angle will be changed again back to its original angle. To utilise this phenomena lenses are produced with a curved surface so when parallel rays of light meet the surface it will present an angled surface to each ray. The paths of all the rays hitting the lens will be bent towards the centre line of the lens. As the light emerges from the back face of the lens it is again bent. If the back surface is convex the same as the front surface then the light will be bent even more towards the centre line of the lens. .
Return to Introduction to Astronomy
