Why do telescopes need to be big

This is particularly useful for astronomical instruments that are too bulky or heavy to be directly mounted on the telescope. The Nasmyth focus allows the them to be installed on a platform that rotates with the telescope, rather than having to swing round actually anchored to the telescope tube.

An artefact that is introduced by the incorporation of a secondary mirror is the diffraction spikes that radiate as crosses from bright stars in beautiful astronomical images. The secondary has to be suspended in place above the central axis of the primary by struts within the tube.

These cross the opening of the telescope, and light entering the telescope is diffracted ie spread around them. This means that light is slightly deflected when passing near the support vanes, and some of the light from the star is scattered away from its original destination point to end up elsewhere to appear as diffraction spikes.

Although aesthetically pleasing, these can be an irritation for astronomers as they can sometimes obscure light from fainter, maybe more interesting objects behind. There are of course, many refinements of these main designs of refractor and reflector. The exact shape of the primary mirror is important. The resulting blurred image can be corrected either by adding a correcting lens to the front of the mirror or by using a parabolic concave shape instead.

Of course, a parabolic surface is harder to create than a spherical one, as the deeper central depression it requires more specialised grinding and polishing. This was true not only in the 17 th century, but proved difficult to get right even in the late 20 th century! The Hubble Space Telescope originally suffered from severe spherical aberration, caused by a minute mistake in the exact shape of what was supposed to be a hyperbolic primary mirror. Two microns too flat on a mirror 2.

It took a long while between the invention of reflecting telescopes and them becoming the norm for the design of modern reflecting telescopes.

The main difficulty was in grinding a mirror to the required shape, and the resulting image distortions made them seem a poor companion to the refractor. The English astronomer Sir William Herschel — discoverer of Uranus and infrared radiation — constructed his Great Forty-Foottelescope between and Here as was the norm at the time the name refers to the 12m focal length of the primary mirror rather than its aperture, which was cm inch in diameter.

The m-long iron tube was mounted on a fully rotatable mount within a wooden frame. The whole tube and mirror weighed about 12 tons and had to be supported from walls flanking the east and west sides of the telescope; so although the telescope could be inclined through all altitudes, it only had a limited range of azimuth. Lord Rosse used the telescope to observe the nebulae that had been discovered by both Charles Messierand John Herschelin far more detail than had been possible previously.

He discovered that several of the nebulae had a spiral structure, some containing individual point sourcesand made some amazingly good drawings of these spiral nebulae. The biggest drawback in the construction of reflecting telescopes for many years was the manufacture of a good enough mirror. This alloycan be polished to make a highly reflective surface, although it becomes brittle and difficult to work with. For about years it was the only known reflecting surface that could be ground into a precise shape.

Speculum mirrors could be large, although Rosse could only build such a large mirror after he had developed steam-powered grinding machines, and new techniques for the casting and polishing.

His mirror was 13cm thick and weighed about 3 tons, and required support from below to prevent it deforming under its own weight. They had to be constantly repolished, as they tarnished quickly when exposed to the air.

In practice two or more mirrors were required for the running of a telescope, so one could be being polished in preparation to replace that in the telescope. The mirrors were sensitive to changes in temperature such as might be caused by a rapidly cooling night, which would distort the shape and reduce the quality of the images.

The breakthrough for reflectors came only in the mid 19 th century, with the concept of chemically depositing an ultra-thin film of metallic silver on the front surface of a glass blank. And even though large glass mirrors will be just as heavy as the large lenses they can be supported from underneath by a brace.

Built in , the inch cm Hale telescope was the first of the new era of large reflectors built using modern glass mirrors, and it was closely followed by the inch 2. Both telescopes were at Mount Wilson Observatory, northeast of Los Angeles, and were very successful, becoming amongst the most productive in astronomical history. The two telescopes at Mount Wilson were not surpassed until , until a telescope came along that was double the size. The inch Hale telescope was built on Palomar mountain in California, and remained the world's largest telescope for the next 45 years.

The Gemini Telescopes are twins, one in the Northern hemisphere in Hawaii, and the Southern in Chile, and each has an 8. The four individual telescopes can be linked together to form a giant virtual telescope.

Like the VLT, the two telescopes can be used either individually, or operated together to give greater detail. It saw first light in Summer , and it is located on the island of La Palma, in the Canary Islands. Large binocular telescope.

The light from two side-by-side 8. By the time a glass mirror has a diameter beyond inches, gravity causes the glass underneath to sag under its own weight.

The resulting distortion in the required precise shape of the mirror varies in amount according to where the telescope is pointing in the sky; although tiny, it can create blurring in the shape of the image. Up to the s, all large mirrors were made as single, rigid pieces of glass. To make the current 8-m class aperture telescopes, a different technique had to be employed. The mirrors are made thinner to make them lighter.

This not only reduces its weight to a quarter of what it would otherwise be, but makes it flexible. Active optics in the form of a computer controlled system of fast-moving mechanical actuators embedded in the mirror support can continually push and pull the mirror, making minor corrections to its shape on the timescales of tens of seconds. Thus the scientists can compensate for the predictable response of the mirror to the position of the telescope, the air temperature etc. Alternatively, one giant mirror can be mimicked by a close mosaic of smaller hexagonal segments that are combined to form a parabolic or hyperbolic surface, For example, the m diameter Keck mirrors are each composed from 36 smaller mirror segments.

The combined shape can again be continually adjusted by the use of actuators under each segment. Although cheaper than the thin mirrors, individual segments require a very precise shaping, and diffraction effects are introduced by the gaps of a few mm between the mirror segments. All these enormous telescopes have vast gathering power, enabling us to increasingly view fainter and further cosmic objects.

Astronomers quantify this requirement as angular resolution. This measures how far apart two stars must be before you can observe them as individual sources rather than one blobby object; the smaller the angular resolution, the finer the details that can be made out in an image.

The angular resolution of a telescope is absolutely limited by diffraction, the tendency of light waves to spread out from a point of origin. For any wavelength of light, the wider the diameter of the objective, the less diffraction occurs giving better angular resolution. Yet again, larger telescopes win as they yield images with more detail. But only the corrected Hubble Space Telescope can achieve this theoretical best resolution.

Even the very best large-aperture ground-based telescopes have their angular resolution comprised by the fact that light has to travel through our atmosphere. Pockets of micro-turbulence in the air cause continual refraction of the light making it seem as if the images dance around — twinkle.

Light collected over time is smeared out into a blob rather than the pinpoint of light it should be. The best locations for telescopes are high on mountaintops where the starlight has to travel through only the less dense and colder part of the atmosphere, suffering less refraction.

Seeing can vary between sites, between nights, and even during a night. Telescopes can be made to compensate for the turbulence within the atmosphere by real-time changes responding to the conditions, in a variety of methods that come under the guise of adaptive optics which work alongside the active optics.

The aim is to decrease the angular resolution so that it begins to approach the theoretical diffraction limit of the telescope. The simplest method is to continually monitor the shape of a bright star up to a hundred times a second while observing.

A computer calculates the way in which the star is blurred, and changes the mirror shape using the actuators underneath to correct for the seeing. In this way a dramatic improvement in the angular resolution can be achieved. However, the technique is limited by the need for a bright star so that there is sufficient signal to carry out the modelling of the blurring accurately; this bright star has to be in the close vicinity of the target of the observations. Ideally it has to be in the same field of view, as the seeing differs across the sky.

When it comes to telescopes, unless you're a pirate trying to fit your spyglass back in your shirt after spotting land, bigger is always better. A larger telescope means more collected light, which means better resolution and the ability to see fainter and further objects.

For most telescopes, though, even a slightly bigger size means an exponentially larger cost. But that's not the case with radio telescopes, which collect radio light waves instead of visible light. That's why astronomers are proposing to build a new radio telescope roughly the size of Nebraska. Radio telescopes can scale so easily because radio waves are long enough that several separate antennas can add up to one telescope. Many of the largest radio telescopes, such as the Very Large Array in New Mexico and the Atacama Large Millimeter Array in Chile, are made of dozens of smaller dishes and antennas that combine to create a very large effective telescope size.

GRAND's large scale will allow it to hunt for high-energy cosmic particles, which, if found, could help us learn more about the biggest galaxies in the universe—and the early stages of the universe itself. GRAND is in search of neutrinos, exotic particles emitted by stars like our sun and the black holes in the center of galaxies.

These neutrinos will help astronomers find what they're really after: the source of other energetic particles called ultra-high-energy cosmic rays. The most energetic particles likely originated in the most powerful galaxies in the early universe, where "blazars" emitted cosmic rays millions of times stronger than those produced by our sun.

Watch : Mining the Moon for rocket fuel. Queen guitarist Brian May and David Eicher launch new astronomy book. Last chance to join our Costa Rica Star Party! Learn about the Moon in a great new book New book chronicles the space program. Dave's Universe Year of Pluto. Groups Why Join? Astronomy Day. The Complete Star Atlas.

Using a large, high-quality telescope is one of the joys of amateur astronomy. By Michael E. Bakich Published: Thursday, July 1, Click the image start your search. W hen all is said and done, aperture rules. If you have the space to set up a nearby observatory so you can leave your large telescope assembled — or the wherewithal to transport a large telescope to a remote site — you will be rewarded by views unmatched by smaller telescopes.

Galaxies are a great example of objects that are better viewed through large scopes. You can be the greatest observer on the planet, but if you're trying to observe galaxies with a 4-inch telescope, your observing log is going to be filled with reports of rough shapes, central condensations, and descriptors like "hinted at" and "small and faint. If you want to observe galaxies — and I mean really get something out of the time you put in at the eyepiece — you're going to have to use a large telescope.

Advantages Large telescopes have several advantages over their smaller counterparts, and "resolution" — how much detail we see in a celestial object — is the first. Resolution depends on two things: the stability of the atmosphere and the aperture of your telescope.

You only can control one of these factors. According to an optical law called Dawes Limit, a 4-inch telescope can resolve two double stars with an angular separation of 1. Compare this with the resolution of a inch telescope: 0. Quality telescopes under an excellent sky often surpass the number given by Dawes Limit, but you get the idea. It's much easier to resolve small storms in Jupiter's atmosphere, bright knots in distant galaxies, or tiny lunar craterlets through a large telescope than through a small one.

The other major advantage of a large telescope is light-gathering power. This advantage is evident in two practical ways: First, if you compare two telescopes of different sizes, a celestial object will look brighter through the larger one.

And, as discussed in the previous paragraph, you'll also see more detail. Second, you'll be able to see fainter objects through the larger telescope. When stars which are point sources are viewed, the larger scope will have a higher limiting magnitude. The limiting magnitude is the faintest magnitude that can be detected by an instrument.

This quantity doesn't go up as much as light-gathering power, but it's still important. Disadvantages Large telescopes can be unwieldy and heavy to transport. Purchase a large scope only after you compare what it weighs to what you can lift and carry easily whether it be from your house to your yard or from your vehicle to your observing site. A inch Schmidt-Cassegrain telescope is a wonderful instrument, but if you can't lift it from its storage case to the top of its tripod, it's useless to you.

An observing partner can help in such cases, but then you are dependent on your partner's availability. New Dobsonian reflectors often are designed as "truss-tube" telescopes. Such a scope's heaviest components are the mirror and rocker box assemblies. The "tube" is constructed out of lightweight aluminum poles, which are attached to a light secondary mirror assembly.

Because large telescopes generally have thick mirrors, they take longer than smaller telescopes to adjust to ambient temperature. Small whisper fans can speed up the cool-down time, but the best way to cool down a telescope is never to let it warm up in the first place.