Have you ever looked into a spoon and noticed your reflection is upside down? The curved mirror in a telescope is like a spoon: It flips the image. Luckily, the solution is simple. We just use other mirrors to flip it back. A simple reflecting telescope uses mirrors to help us see faraway objects.
Since they are much lighter than lenses, mirrors are a lot easier to launch into space. Space telescopes such as the Hubble Space Telescope and the Spitzer Space Telescope have allowed us to capture views of galaxies and nebulas far away from our own solar system. Set to launch in December , the James Webb Space Telescope is the largest, most powerful space telescope ever built.
It will allow scientists to look at what our universe was like about million years after the Big Bang. Dubner University of Buenos Aires. How Do Telescopes Work? The Short Answer:. Early telescopes focused light using pieces of curved, clear glass, called lenses. It decreases as the two sides get closer together at a narrow angle. At the extreme, if the two sides of a "prism" were parallel, there would be no rainbow - but then it's not a prism anymore, it's just a glass plate, completely flat.
A "fat" lens at the edge is like a fat, blunt prism. A "thin" lens at the edge is like a sharp, slender prism. The former makes a big spread of color. The latter makes a much more tight rainbow. So that's what the old telescope makers did, back in the day Hevelius, late s.
They just pumped up the focal ratio, made these super-long refractors with weakly curved lenses to combat the rainbow edge images which are so annoying and ultimately can falsify your observations. Now, if you study the properties of lenses, you notice that divergent lenses the kind that make the image smaller have a chromatic aberration opposite to convergent lenses lenses that magnify, and that were used as primary, or objective lenses in telescopes back then.
It could be argued that by combining a convergent and a divergent lens, chromatic aberration might be greatly reduced; as long as the convergent lens is stronger than the divergent companion, the combination should remain convergent.
Also, different glasses need to be used, so that the convergent lens is strongly convergent, but weakly dispersing, while the divergent lens is weakly divergent, but strongly dispersing. That's how you get near zero dispersion, but still pretty decent convergence.
In the early s this was accomplished by British opticians. Chester Moore Hall is credited with the making of the first achromatic objective lens using the scheme shown above.
Hall failed to recognize the practical applications of the achromats, neglected his own invention, and years later Bass mentioned the achromat scheme to John Dollond, who understood its value and started to popularize the new design.
The achromatic combo guarantees zero chromatic aberration at two wavelengths. So you basically correct the aberration in the red and blue, or something like that, and the rest is "good enough". Things can be improved further by adding another lens - the apochromatic objective, or the "apo", or the "triplet". This was invented by Peter Dollond, John's son, in the mids.
The apochromat guarantees zero chromatic aberration at three wavelengths - usually in the middle and then closer to the two extremes of the visual spectrum. So that's overall better than the previous scheme. The best apo refractors have extremely low chromatic aberration, basically invisible to the eye in most situations. The higher the refractive index, the more light is slowed down by the substance. Substance Index of Refraction Vacuum 1 Air 1. If light enters the new medium at a right angle to the surface, it will change speed, but not direction.
If it enters at an angle, its speed and its direction will change. The direction the light takes depends on whether it travels faster or slower in the new medium. Imagine driving a car from smooth pavement onto a sandy beach. If you approach the beach straight on, the car will slow down, but not change direction. If the you approach the beach at an angle, one of the tires will be slowed down by the sand before the other is, and the car will turn in the direction of the tire that touched the sand first.
Light follows the same same principle and bends towards the normal when traveling into a medium with a higher index of refraction, and away from the normal when traveling into a medium where it can go faster.
In the diagram below, light is leaving air and entering glass, so it bends towards the normal on the way in, and away on the way out of the glass. Lenses form images by refraction and are typically made of either glass or plastic. They are ground so that their surfaces are either segments of spheres or planes. If a lens is convex or converging, it takes parallel light rays from a distant object and bends them so that they converge to a single point called the focal point. The distance from the lens to the focal point is called the focal length of the lens.
If a lens is concave or diverging, it takes parallel rays and bends them so that they spread out. To begin with, there are serious technological problems in producing very large lenses. To ensure that the initial block of glass, from which the lens is to be made, is perfectly transparent and optically homogeneous throughout, the molten glass may need several years! Next comes the problem of grinding and polishing — it is not easy to sustain a perfect spherical curvature for a very large focal length lens over the whole of its surface area.
And when you have a large lens, it is inevitably a thick lens, which therefore absorbs light, preferentially in the blue and violet part of the spectrum. It is also a very heavy lens, which means that it would have a tendency to sag under its own weight.
In practice, usable objective lenses with a diameter much larger than 1 metre cannot be made. Figure 4 shows a photograph of one of the largest refracting telescopes in the world, the 36 inch refractor at the Lick Observatory, California. Note the extremely long body of the telescope in relation to its diameter.
Achieving high magnification with a telescope requires a long focal length f o , but limits on the maximum possible value of f o are set by the need to make the whole instrument movable.
It is clear from Figure 3 that the physical length of a Keplerian refracting telescope cannot be less than f o. Hence, it would hardly be realistic to plan a telescope with a focal length of metres using this design! However, it is important to remember that achieving high magnification is not necessarily always useful, and sometimes it is better to have very short focal lengths. This will increase the field-of-view of the telescope and make the images appear brighter, as the light is less spread out.
Designing optics with very short focal lengths leads to some optical aberrations, which we discuss briefly. Optical aberrations are not errors of manufacture, but are undesirable physical characteristics of refracting and reflecting surfaces. For example, parallel rays of light passing through different parts of a lens are not focused to the same point by spherical surfaces; this is known as spherical aberration.
This wouldn't be a problem except for the fact that spherical surfaces are relatively easy to produce, whereas parabolic surfaces, which give a perfect focus, are much more difficult to produce. Even from the same part of the lens though, waves of different frequency i. By combining several lenses of different optical strengths and different materials, chromatic aberration can be reduced, but the problems are formidable and increase with the increasing size of the lenses and with the angle of the rays with respect to the optical axis.
Thus, in practice, refracting telescopes have only a relatively narrow field-of-view within which the resolution is good.
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