Telescopes

The telescopes although does a very complicated job to look at an object millions and billions of kilometres away and still collect enough light to show us details, it is safe to say the science behind its working is very simple. The telescope used the the phenomena of refraction or reflection depending upon the design of the telescope to show you details.

Refraction: Refraction is a phenomena observed in certain transparent/translucent object when the light passes through it. The light bends towards the normal to the surface when entering a denser medium and away from the normal when leaver the denser medium. The phenomena of bending of light is called refraction. The most common example to observe this phenomena is to place a spoon in a glass container filled half way with water and see how the shape of spoon deforms.

Reflection: Reflection is a phenomena observed on certain surfaces when one can see the image of something else on the surface. A plain glass mirror is the most common example. The surface is polished in such a way that it does not allow the light to pass through that rather the polish on the surface allows the light to bounce back and you see your own image. Another example is the mirrors we use in our vehicles to keep an eye on the traffic behind us.

Other important terms one should understand before they can understand the telescopes:

Primary Surface: The ‘Primary’ here means the first optical surface on which the light falls. In case of reflector telescopes, the primary optical surface is the concave mirror whereas in refractor telescopes, the primary surface is the convex lens. Sometimes primary lenses are also referred as objective lens.

Secondary Surfaces: The secondary surfaces in the telescopes is the second surface where the light from the stars touch. After refraction, the light reaches the secondary lens which in more common cases is an eyepiece. In reflective telescopes, the secondary surface is another mirror which further send the light towards the exit of the telescope from where once can see the image created by the optical system.

Eyepiece: Eyepiece is a small lens at the exit of the telescope where the observer put up their eyes to look at the image. The eyepiece is generally a convex lens with very less focal length so that the image can be magnified. In earlier telescopes, concave lens would be used however in all the modern telescopes, a convex lens is used more prominently.

Aperture: The aperture of a telescope is known as the size of the primary mirror or lens used in the telescope. The diameter of the primary surface is how the telescope is defined or identified. The aperture is either measured in millimetres, inches or meters. For simplification, millimetres is used for apertures upto 150mm or 6″ while inches is used for apertures between 6 inches and upto 40 inches or 1metre. All the telescopes above the aperture size of 1 metre are referred in metres only. Although this is not a stringent rule to follow all the time but this is more accepted norm in the international community and amateur astronomy fraternity.

Focal Length: The focal length is a term in physics applicable to both mirrors and lenses. It is basically the distance travelled by light after refracting through the lens or reflected from the mirror surface and before the bending light meet at the point. The distance travelled is called focal length and the point at which they meet is called focus.

Focus: Weather the light bends towards the normal during refraction in lenses or bends backward due to reflection in mirrors, either way they meet at a point where one can see the image sharper. This point is called focus point. The image so formed is by placing another lens whose focus meets at the some point where the focus of other lens/mirror is matched. The image appears shaper if the focus is matched at the same point. If not, then the image will be blurred.

Image: The image so formed will be inverted or upright depending upon the type of lens/ mirror used. In lenses, convex lens inverts and image as it produces real images which are magnified. Whereas the concave lens produces an imaginary image (the image is real but the focus is before the light passes the lens and hence the image is imaginary). The image is diminished or unmagnified, and the image is upright.

In case of mirrors, convex mirrors, just like concave lens produce imaginary, upright and unmagnified images. The concave mirrors however produce real, inverted and magnified images and hence find their application in the field of telescopes.

Magnification: A telescope’s primary job is to collect the light and hence the only important aspect in choosing a telescope is the aperture of the telescope. But one of the critical job of a telescope is to magnify the object you are seeing as the object appear to be very small in the telescope due to its distance. The magnification does not depend on the aperture but the ration of focal length of primary surface and eyepiece. It means the magnification can be changed according to the need by changing the eyepiece.

Focal Ratio: Focal ration is another important aspect of telescope which is essential in deciding upon which telescope to buy or to identify its usage/preferences. Focal ration is the ration of the aperture of the telescope to that of focal length of the primary surface. The smaller ration ensures that the telescope will capture more light and the surface brightness of the object will remain high. That enables and ensures the visibility of fainter objects with more details but less magnification. If the ration is higher, the object will be less bright and hence the fainter objects will be difficult to spot. The objects will be highly magnified which means you can see small objects however only if they are bright.