The beauty of an equatorial mount is the one-to-one relationship between RA and Dec and the axes of the mount. Not only does this make it easy to point at an object, this also makes it simple to track the motion of an object due to the Earth's rotation by driving just the polar axis at an equal angular velocity, but in the opposite direction, to the angular velocity of the Earth on its rotation axis.
There are many different types of equatorial mount, described in detail for the curious in Vik Dhillon's notes. However, all share the same flaw - the polar axis must be aligned with the Earth's rotation axis. This means that unless the telescope is situated at the Earth's poles, the polar axis will be inclined relative to the ground, thereby creating stresses on the axes which vary as the telescope moves. For large research telescopes, such as the 3.9m Anglo-Australian Telescope, this makes equatorial mounts incredibly hard to build, since the amount of material needed to make them rigid becomes prohibitive.
The solution to this is use an Alt-Az Mount, short for Altitude-Azimuth. As the name implies, the two axes of this mount are aligned with the altitude and azimuth axes of the horizontal coordinate system. The telescope is mounted on a short fork, as shown in figure 33. The great advantage of the Alt-Az mount is the telescope's center of mass is fixed above the mounting point, meaning that very stable mounts can be built, even for the largest telescopes.
Alt-Az mounts are the only choice for the largest telescopes in the world, but they are not without disadvantages. For a start, tracking an object requires moving both axes of the mount, unlike an equatorial mount. Moreover, the speed at which each axis must move depends on the location of the object. The calculations required to move the two axes are quite complex (a coordinate conversion between the equatorial and horizontal systems) and need to be performed many times a second. This requires the use of a computer, but computers only became readily available in the 1970's. It is for this reason that every major research telescope made since the 1980's uses an alt-azimuth mount, but every telescope made before this time was mounted equatorially.
Another big disadvantage of Alt-Az mounts is that the orientation of the telescope remains fixed with respect to the horizon. Since the night sky rotates around an axis which is inclined to the horizon, this means the orientation of the night sky, with respect to the telescope, changes over time. This effect, known as field rotation, is shown in the video below. Field rotation of Alt-Az mounts means that they are unsuitable for amateur astro-photograpy. Professional observatories get round this problem by having a field rotator, which mimics the rotation of the sky, keeping the camera orientation fixed with respect to the stars.
Figure 34: A video showing the effects of field rotation. Note that the equatorial mount does not suffer from this effect, making it suitable for amateur astrophotography.
There no world-class optical telescopes in the UK. The reason for this is cost. The world's largest telescopes cost hundreds of millions of pounds. Construction has started on the 32-m diameter E-ELT, with an estimated cost of €1 billion. It makes sense to build these telescopes in the best locations on Earth. None of these are in continental Europe. So what makes a site suitable for a billion Euro telescope?
Figure 35: Monthly average cloud cover, as revealed by NASA's Terra satellite. Colours range from blue (no clouds) to white (totally cloudy).
Radio telescopes can see through cloud. Optical telescopes can't! This is one of the primary reasons there are no major research telescopes in the UK. Figure 35 shows monthly average cloud cover. Desert regions including Antarctica, Australia, parts of Africa and the west coast of the Americas provide some of the best cloud-free skies.
Light from street lights and industry can greatly increase the sky background. The increased background light causes increased noise in astronomical images, making it harder to detect faint sources. Therefore, the largest telescopes are placed in regions of low light pollution. Figure 36 shows the artificial night sky brightness around the world. It can be seen that some of the regions which show low cloud cover in figure 35, such as the sparsely populated western coasts of South America and Africa, are also largely free of light pollution. Figure 36 also clearly shows why no major telescopes are now built in mainland Europe.
Selecting a site with good seeing is one of the most important criteria. Lower seeing improves the spatial resolution of images. Since the light from the stars is also spread out over fewer pixels, good seeing also improves the signal-to-noise ratio. Seeing is caused by turbulence in the atmosphere and this has two causes: heat from the ground rising through the atmosphere and strong winds higher in the atmosphere (e.g the jet stream).
The atmosphere over the sea tends to be much less turbulent than the atmosphere over land, as the sea exhibits an essentially smooth, constant-temperature surface compared to the land. Some of the best astronomical sites are therefore located on small islands in the middle of oceans, such as Hawaii and the Canaries, as these small land masses cause little additional turbulence. For the same reason, coastal regions that receive winds predominantly from the direction of the ocean, such as the western coasts of the Americas and Africa, also exhibit excellent seeing.
height above sea level
The higher the site of an astronomical observatory, the thinner the atmosphere above the telescope. This reduces atmospheric extinction, and can also help reduce the seeing if the observatory lies above a turbulent layer in the atmosphere. High-altitude telescopes are also often above the local inversion layer in the atmosphere, meaning that local cloud formation occurs below the telescope, significantly increasing the number of usable nights at the observatory compared to a telescope sited below the inversion level.
Additional factors, such as political stability, accessibility, humidity and wind speed all play a lesser role in locating professional observatories. For a detailed discussion of many of these factors, see Vik Dhillon's old notes.
Figure 37 shows the sites of the worlds largest telescopes. For the reasons discussed above these are clustered around a few major sites, namely Chile, Hawaii and the Canary Islands. The gallery below shows photos of these observatories. These sites have low cloud cover, low light pollution, good seeing and low humidity. They are all at high altitude and are politically stable.
Figure 37: A map showing the location of the world's largest optical telescopes. The overlying heat map shows the density of big telescopes around the world. The worlds largest optical telescopes are concentrated on Mauna Kea, Hawaii and the Atacama desert in Chile.
Figure 38: Photographs of arguably the world's best astronomical observatories.
The factors which degrade astronomical images, such as seeing, sky background, transparency variations and extinction are all atmospheric-induced phenomena. These effects can all be removed at a stroke by siting optical telescopes in space. The best known example of this is the 2.4 m Hubble Space Telescope (HST), which has helped to revolutionise astronomy since 1990 with its diffraction-limited imaging capability. Because space-based telescopes don't suffer from atmospheric extinction, HST also allows astronomical observations in the UV - impossible from the ground! The main drawback with siting telescopes in space is the cost: the HST cost many billions of dollars to build and operate, approximately ten times the sum required for the largest ground-based telescopes.
Another drawback with space is the risk involved in the launch, and the great difficulty of fixing problems, servicing the telescope and upgrading the instrumentation once the telescope has been deployed. For the HST, servicing missions, which cost nearly $1 billion each, were of limited number and were very risky for the astronauts involved. More recent telescopes, such as the infrared Herschel telescope, have been located much further from Earth and the Sun to keep them cool for as long as possible; servicing missions for these telescopes are impossible, so they had better work!
We have already seen that telescopes use mounts to track the motion of stars in the sky. In practice, it isn't possible to do this accurately enough for professional work, or even for amateur astrophotography. This is due to mechanical imperfections in the drive systems of the mount, flexure of the mount, and misalignment of equatorial mounts with regard to the celestial pole.
The result is that stars drift slowly across the field of view, which would cause smearing in long exposures. To get round this, it is necessary to guide telescopes. This is done using an autoguider, a system which monitors the position of a star somewhere in the field of view, and nudges the telescope to keep the guide star locked onto the same pixel on the autoguider's detector.
There are two main types of autoguiders in use. Guidescopes use a separate, smaller telescope to image a wider field of view. The wide field of view means that guide scopes are likely to find a bright star to guide on, but because they are mechanically separate from the main telescope, they can flex relative to the main scope. This means that, even though the guide star is held fixed, the main telescope's view of the sky can drift slowly. Off-axis guiders use a mirror on a moveable stage to pick off some light from the main telescope, and focus it on a separate autoguiding camera. This exhibits less flexure than the guidescope method, and is the one most often used for large telescopes. Both designs are shown below in figure 39.
Figure 39 - Top: an off-axis guider using pick-off optics. The pick off optics can be moved in order to send the light from a guide star to the guide camera. Bottom: A photograph of ROSA, the 10" robotic telescope on the roof of the Department of Physics and Astronomy at Sheffield, with the dome removed. The piggy-back mounted guide scope, a 3-inch refractor, can be seen on top of the main telescope. A CCD autoguider has been inserted in place of the eyepiece.