The telescope

The telescope is the most obvious tool of the astronomer, associated with every wild haired practitioner in fiction, many in real life too. This year marks the four hundredth anniversary of the first recorded use of telescopes for astronomy. In celebration of this and as part of the International Year of Astronomy 2009, the Society for Popular Astronomy are distributing one thousand telescopes to schools. The cheap Galileoscope has also been brought out in America, with an interesting guide to observing and recording observations included in their download page. With all this going on I thought I’d give a quick overview of telescopes in all their forms.

Astronomers in Hyde park, London watch an eclipse

Astronomers in Hyde park, London watch an eclipse

History

Although lenses for viewing and magnifying had long been used in the east, the first recorded use of a tube and lens assembly are thought to be around the turn of the 17th Century. Three men in the Netherlands submitted patent applications for telescopes in October 1608; Hans Lipperhey, Jacob Metius and Zacharias Janssen. Three men in one area independently inventing the same object in the same month has led to suspicion that they had gathered information about other, perhaps pre-existing, telescopes outside of the Netherlands and had rushed to submit for the national patent themselves. However it occurred, the telescope was now becoming more well known around Europe.

The following year, 1609, saw the first observations for astronomy. Thomas Harriot is said to have begun making observations of the Moon at the end of July, beating even Galileo to the feat. Harriot later became a lunar Cartographer of great skill and also made observations of Sunspots before Galileo, making him (with Johanes Kepler, who unwittingly saw a sunspot, mistaking it for a transit of Mercury) one of the earliest Europeans to have studied the phenomenon. Chinese observations date back to even pre-Roman times.

In the same year, Galileo also started his observations, though a little later on. He was to observe the rings of Saturn (mistaking them for two smaller orbiting planets), the four Galilean satellites of Jupiter (recognising them as being in orbit of the planet and so getting their orbital periods), the Moon and sunspots. His observations of the phases of Venus disproved the Geocentric model of Ptolemy, which had predicted a different set of Venusian phases. He also worked on telescope design, creating scopes with up to 30x magnification. At a banquet, Galileo’s devices, known as perspicilla, were renamed ‘telescopes’ by Giovanni Demisiani, combining the Greek ‘tele’ – far and ‘skopein’ – to look or see.

A contemporary of Galileo, Nicolo Zucchi, also constructed the first recorded ‘reflecting’ telescope in 1616. Others such as Cesare Caravaggi and James Gregory built or designed telescopes using mirrors rather than lenses, however, it wasn’t until 1669 that Isaac Newton constructed one that worked well enough to be used for astronomy. The spur for such a design was the knowledge that lenses, like any prisms, split the light entering them into a spectrum. This leads to ‘chromatic aberration’, where the object being looked at seems a little redder on one side and a little bluer on the other.

It took a further century for the mirror making process to produce good enough mirrors for the reflecting telescope to start challenging the refracting telescope for optical quality, then the stage was set for a battle of adaption, with both types of telescopes being reinvented to challenge the other.

Refracting Telescopes

Galileo’s original astronomical telescope used a convex lens to gather light and a concave one to focus the light onto the eye. This gave an upright image, but poor eye-relief – it is more stressful on the eye to look through a lens arrangement like this than one with two convex lenses. That latter arrangement was put together by Kepler, who recognised a large distance between the two lenses was required to keep down chromatic aberration and spherical aberration – where an imperfect curvature of the lens causes light rays coming through different bits of it to focus in slightly different places.

Chester Moore Hall in 1733 and John Dolland in 1758 both decided to place new lenses into the refracting design to combat the aberrations. The Achromatic lens is constructed of two different types of glass, with two different refractive properties. The idea is to actually cause chromatic aberration, but in reverse. The result is two different wavelengths, eg red and blue, can have their focal points put together, rather than the original lens just being optimised for a single wavelength. Achromatic refractors have since been built with three, four or more lenses put together in order to further remove chromatic aberration as refractors have increased in size and so small aberrations have become bigger abbreviations at the eyepiece.

Apochromatic lenses are the latest design of refracting telescopes using modern materials specifically designed to focus all colours at the same point. Low dispersion glass is used in high end amateur telescopes as well as cameras. Superachromatic telescopes also exist, which combine the two methods.

Nevertheless, whatever lenses sit in the scope, the tubes of refractors are generally kept long compared to the size of the lenses. As a result, increasing lens size leads to ever longer tubes. When I studied for my undergraduate degree at University College London, one of the toys we got to play with was the Radcliffe Telescope (shown below). This is a 24-inch 18-inch doublet – that is there are two telescopes held together like binoculars, one with a 24-inch lens, one an 18-inch lens. The telescope came from the great age of late Victorian refracting telescopes and was designed as a survey telescope, measuring the parallax shifts of stars to determine their distances. The 18 inch telescope was (and is) used for visual observations whilst a photographic plate holder (which still exists) was placed under the 24-inch telescope (now replaced by a CCD). These massive lenses, perhaps ten or twenty times the size of lenses in standard spyglasses, were held in a tube that was in the same proportion to the lens as the tubes of standard spyglasses are to their lenses. The result is a tube so long the floor of the telescope dome has to mechanically rise and fall so observers can get to the eyepiece. This telescope is technically an ‘Astrograph’ with an optical guide as the 24-inch telescope isn’t designed for the human eye’s focusing abilities, but instead those of photographic plates.

The 20-inch, 18-inch doublet refractor telescope at ULO

The 24-inch, 18-inch doublet refractor telescope at ULO

But the Radcliffe isn’t the largest refractor on the block. The very largest (used only for a couple of astronomical observations) was built in Paris for that city’s Great Exhibition in 1900. It was a fixed tube, not on a mount that could swivel, and ended up snapping (though the lenses still exist). The lenses for that were 78 inches in diameter. The largest operational refractor is the 40 inch refracting telescope at Yerkes Observatory, California.

Refracting telescopes of this kind of size suffer from a number of manufacturing problems. Asides from the ability to create a smooth lens of this size (and get it where it’s going without dropping it), air bubbles and other imperfections are more abundant with larger blocks of glass. The lens can sag slightly under gravity. Finally, another killer problem for refractors is the absorption of light by the glass of the lens, which is never fully transparent over the entire desired spectrum of light. It is often preferable to have a telescope with no light passing through any form of lens.

Reflecting Telescopes

UCL’s observatory also had a few reflecting telescopes (as do I). The general shape of a reflecting telescope works on a similar basis to a satellite dish. There is a dish shaped reflective surface, signals bounce to a small secondary surface held in front of the dish and are then reflected again to where they need to be. Unlike satellite dishes, the usual configuration (at least for amateur astronomers, not in the big observatories, which do use satellite dish style reflectors) is to have a tube, like a refracting telescope but shorter compared to the diameter, with the mirror at one end and the secondary mirror held on struts on the other end.

The open end faces the camera. A CCD has been placed at the eyepiece

The open end faces the camera. A CCD has been placed at the eyepiece

In the case of Newtonian reflectors, the secondary mirror shines the light out of an eyepiece on the side, close to the opening. Other ways of focusing have also been created for ease of use. These include the Cassegrain focus, where a hole in the primary mirror allows the secondary to beam light straight through and onto an eyepiece or detector; prime focus, where the detector replaces the secondary mirror altogether; Naysmith telescopes follow a similar principle to Cassegrains, but put the hole in the side, with a tertiary mirror reflecting the secondary mirror’s light out of that hole, usually coming out of the mount itself; finally a Coude focus extrapolates that, so more mirrors take the light down through the mount and onto a detector on a fixed place in the observatory.

A one time balloon mounted cassegrain reflector for infrared studies

A one time balloon mounted cassegrain reflector for infrared studies

The reason for all these weird and wonderful designs was the increasing size of detectors and mirrors. The last thing a highly controlled and slow moving telescope needs is some massive load of wires and detectors sticking out of the back, weighing it down. The largest single mirror telescope is the Subaru Japanese telescope on the Mauna Kea group of Observatories, Hawaii, at 327 inches across. Mirrors built from segments have also been constructed at up to 410 inches, and it is likely this is how telescopes of larger sizes will continue to be constructed. Telescopes with fluid mirrors or even lenses do exist, but don’t operate at the same level as solid state telescopes.

Of course, telescopes are no longer just ground based objects. In the sixties, many reflectors were seen hanging around on balloons, or flying on rockets. Hubble, the International Ultraviolet Explorer and COROT are three large reflectors in space. SOFIA is a reflector on a plane, which flies around looking for things to spot. This is because the reflector design has slowly been spreading out of the visual spectrum and into near-infrared and Ultraviolet parts that still respond to similar mirror types as optical light. Unfortunately, these are heavily absorbed by the atmosphere and require high altitude observations.

A final type of optical telescope is a Catadioptric telescope. These are reflecting telescopes with lenses on the front to allow for fast focal ratios (good for reducing photographic exposure times, bad for trying to look at something bright and getting delicate features out of it). One example of these is the Maksutov telescope, a common piece of amateur equipment.

Radio-waves and beyond

As we go to longer and longer wavelengths, the mirror design does still hold to some level. Satellite dishes looking at radio and microwaves take over as the usual mirrors that work up to infrared start to fail to reflect. Larger wavelengths mean lower possible resolutions, so larger telescopes need to be used to get a suitable image. This is normally done through aperture synthesis – synthesising a giant telescope. The most common way of doing this is Interferometry, where multiple telescopes work together as one. They are pointed and positioned very precisely so the image of large waves can be reconstructed back at base.

My rather unused satellite dish

My rather unused satellite dish

Grote Reber produced the first Dish, a nine metre design, in 1937, following the Bell Lab’s discovery of microwave sources in the sky. The Bell Telephone Laboratory hired Karl Jansk to search for any sources of radio interference that may affect it’s services. He noticed in 1931 several fixed sources of radio emission in the sky that seemed to rotate as the earth rotated. The war interrupted developments in radio astronomy, but since then it has come on in leaps and bounds.

As well as interferometry, some large single aperture dishes exist. The largest current one is the 305 metre Arecibo Observatory, that played a bit role (in which it was destroyed) in the Bond film Goldeneye.

For amateur astronomers wishing to get into radio astronomy, but without your own football pitch to convert into a dish, NASA has taken over the Radio Jove project, which produces radio telescope kits primarily for schools. These can be used to observe solar or Jovian (hence the name) radio emissions, though there’s nothing to stop you searching everywhere else in the sky. For those in my area (the northwest of the UK) Jodrell Bank is a radio observatory of good repute with many years and dishes in its belt. It also twitters, oddly enough…

X-ray telescopes

Moving the other way along the spectrum, past the ultraviolet and there starts to be a minor problem. The photons no longer stop at the mirror, but pass through it disdainfully. That’s no good when the few you do get to reflect off the primary mirror then end up passing through the secondary mirror, and those that actually reflect off that pose a risk to the detector. This is all assuming you are high enough to get the rays the atmosphere doesn’t absorb in the first place.

This third and final category of telescopes, sadly not available through any for schools projects, uses glancing or grazing reflection techniques, where the light is given a little nudge. Hans Wolter invented this type of telescope in 1952, and it has been applied to x-ray observatories most notably in XMM Newton, ROSAT and Chandra space based x-ray observatories. Balloons and sounding rockets have also been known to fly a telescope or two, but as the reflective surfaces are most effective when covered in heavy metals such as gold, not every funding agency will hang these things on balloons. By the time the most energetic gamma rays are reached, telescopes give up completely, and we rely on detectors.

This slightly offsets the gain in possible resolution from the low wavelengths in this region of the electromagnetic spectrum.

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