![]() ![]() Surface it will present an angled surface to each ray. Side of this material its angle will be changed again back to its originalĪre produced with a curved surface so when parallel rays of light meet the As the light re-emerges through the opposite Refraction occurs when light passes between two different transparentĪn angle, through the surface of a block of glass the angle is changed. Property called REFRACTION to change the direction of rays of light and direct More lens elements to produce a clearer image and reduce colour distortions This lens is normally made up from two or Telescopes use a glass lens as their primary focusing unit. Telescope should have a focal length of around 1000mm to 1200mm. A shorter focal length telescope will be more suited to wide field views Long effective focal length should be sought. The cloud bands on Jupiter are to be studied then a telescope with a relatively To summarise, if the craters on the Moon or Used on a 1500mm focal length telescope (that naturally produces a larger A short focal length 10mm (high power)Įyepiece used on the same 1000mm telescope will produce a magnification of 1000 Larger image to start with will allow the eyepiece to produce a higherĪ long focal length 25mm (low power) eyepiece used on aġ000mm telescope will produce a magnification of 1000 ÷ 25 = 40x. Image and a shorter focal length will produce a smaller image. A longer focal length will produce a larger Length may produce an image of the full moon 10mm in diameter. For example a telescope of a certain focal Produce an image of a specific size and this cannot be changed. A telescope of a specific focal length will The telescope it mainly depends on the eyepiece being used. Magnification, strictly speaking, is not an attribute of Short focal lengths are best for looking atįocal lengths are most suitable for small objects and studying fine detail, for Of view (small area of sky) but the objects in view will appear larger. A long focal length will give a narrow field Of view but the objects in the field of view will appear small. A short focal length will give a wide field The FOCAL LENGTH is effectively the length ofĭistance from the main optic to the point where the image is formed. The observer to see objects about 200 times fainter than could be seen with the A 100mm diameter telescope has an apertureĬapable of directing up to 206 times as much light into the eye of the This is equivalent to 38 square millimetres (38mm²). In diameter when fully adapted to the dark. The pupil of a young human eye is about 7mm One of the most important attributes for an astronomicalĪn expression used by astronomers to describe the process of the main opticĭirecting light from a distant object into the eye. Grasp and permit a high enough magnification to see detail on the brighter This combination will provide enough light Telescope should be: a minimum aperture of at least 90mm for a refractor or 130mmįor a reflector and 1,000mm focal length for each. The main specifications for a really useful first The first telescope should be easy to set up,Įasy to use yet give impressive views of the sky.įirst we must consider some of the physicalĬharacteristics of a telescope and how they affect our requirements. Spend its time at the back of a shed or garage and never be used. Before looking at the range of telescopes that isĪvailable, consideration should be given as to wh at the telescope is to be usedĬhoice is the one that never gets used. The telescope that is too complicated or too cumbersome to set up will
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