Some signifi cant optical telescopes

5.11 Some signifi cant optical telescopes

5.11.1 Gemini North and South telescopes

The Gemini Observatory comprises two 8.1-m telescopes; Gemini North (Figure 5.14) is located on Mauna Kea, Hawaii at a height of 4214 m whilst Gemini South is at a height of 2737m on Cerro Pachón, Chile. Together, the twin telescopes can give full sky coverage with both sites giving a high percentage of clear weather and excellent atmospheric conditions. They have been designed to operate especially well at infrared wavelengths and to this end, their mirrors are coated with silver which refl ects signifi - cantly more infrared radiation than the aluminium used to coat most other telescope mirrors. As atmospheric water vapour absorbs infrared radiation both telescopes are located on high mountain tops where the air has a very low water vapour content.

Figure 5.14 The Gemini North telescope. Note the open sides of the dome to allow the telescope to remain in thermal equilibrium with the outside air. Image: Neelon Crawford, Polar Fine Arts,

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5.11.2 The Keck telescopes

The twin Keck telescopes, located at a height of 4200 m at the top of Mauna Kea, Hawaii, are the world’s largest optical and infrared telescopes (Figure 5.15). They have primary mirrors of 10 m diameter each composed of 36 hexagonal segments whose positions are adjusted (using active optics) to act as a single mirror. Tele- scopes can take time to thermally stabilize to the night-time temperatures when the domes are opened, so to minimize these effects, the interior of the Keck domes are chilled close to freezing point during the day.

When observing, twice a second when observing, the active optics system con- trols the positions of each mirror segment to a precision of 4 nm to compensate for thermal and gravitational deformations. The Keck telescopes also use an adap- tive optics system using 15 cm diameter deformable mirrors that changes their shape up to 670 times per second to cancel out atmospheric distortion, improving the image quality by a fact of ten.

The Keck telescopes have made a notable contribution to the detection of extra- solar planets by the ‘radial velocity’ method.

5.11.3 The South Africa Large Telescope (SALT)

With a spherical 10 m mirror made up of 91 hexagonal segments, SALT is the largest telescope in the southern hemisphere and its innovative design is based on the Hobby-Ebberly Telescope at the McDonald Observatory in Texas (Figure 5.16). The telescope is tilted at a fi xed angle of 37° from the zenith and moves only in

azimuth, rotating on air bearings to point to the region of sky that is to be observed.

Introduction to Astronomy and Cosmology

Figure 5.16 The South Africa Large Telescope. Image: SALT Consortium.

During the observation the telescope structure remains stationary whilst an opti- cal corrector assembly (to correct for the spherical aberration of the mirror) and instrument payload move across the top of the telescope tube to track the object being observed. The design allowed SALT to be built at less than a fi fth of the cost of a conventional 10-m telescope.

5.11.4 The Very Large Telescope (VLT)

The VLT is operated by the European Southern Observatory (ESO) and consists of four 8.2-m telescopes which can either work independently or in a combined mode when it is equivalent to a single 16-m telescope – making it the largest opti- cal telescope in the world (Figure 5.17). The light from the four auxiliary 1.8-m telescopes may also be combined with that from the 8.2-m telescopes to give high angular resolution imaging. It can observe over a wavelength range from the near ultraviolet up to 25 µm in the infrared.

The VLT is located at the Paranal Observatory on Cerro Paranal in the Atac- ama Desert, northern Chile, at a height of 2600 m (one of the best observing sites in the world). The four main telescopes have been given the names of objects in

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Figure 5.17 The four 8.2-m telescopes of the VLT, with the four 1.8-m auxiliary telescopes in the foreground. Image: Courtesy of the European Southern Observatory.

(the Southern Cross) and Yepun (the star Sirius). One notable achievement was the fi rst visual image of a planet, albeit around a brown dwarf rather than a normal star, which was observed in the infrared when using an adaptive optics system.

5.11.5 The Hubble Space Telescope (HST)

The HST (Figure 5.18) was launched in April 1990 to observe the universe over

a wavelength range that extends from the ultraviolet, through the visible to the near infrared; that is, from 0.12 µm in the ultraviolet to 2.4 µm in the near infrared (1 µm is 10 ⫺6 m).

The HST’s primary mirror is 2.4 m across so that in green light its angular resolution is given by:

∆θ ⫽ 1.22 λ/D

⫽ 1.22 ⫻ 5.1 ⫻ 10 ⫺7 /2.4 rad ⫽ 2.5 ⫻ 10 ⫺7 rad

⫽ 3.4 ⫻ 10 ⫺3 ⫻ 57.3 ⫻ 3600 arcsec ⫽ 0.053 arcsec ⫽ ∼1/20th of an arcsecond

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Figure 5.18 The Hubble Space Telescope. Image: Space Telescope Science Institute, NASA.

However, this resolution would have only been met if the mirror was ground to such a precision that it would be diffraction limited and to achieve this, the mirror was one of the smoothest ever made.

When placed in its orbit some 600 km above the Earth, the astronomers commissioning the HST were mortifi ed to fi nd that it could not be focused. The full optical system had not been tested on the ground and a problem in the test rig that controlled the shape of the primary mirror meant that the mirror suf- fered signifi cant spherical aberration. The resultant image quality was very poor. The centre of the mirror was just 2 µm too shallow and, as a result, rays from the edge of the mirror came to a focus 4 cm behind that from the centre. An ideal mirror will put 84% of the light into the central disc, but the HST mirror put only ∼ 15% of the light in the central disc with most of the light spread over a region

1.5 arcsec across, comparable with ground based telescopes! However, rather than being a badly made mirror of the correct shape, it was

a perfectly made mirror of the wrong shape! This meant that it was possible to correct the spherical aberration by introducing a correcting lens or mirror into the optical path, and this was achieved in the fi rst servicing mission. It is now a virtually perfect telescope and its contribution to our understanding of the uni- verse has been immense. One of its main legacies to astronomy will be the images of the distant universe in what are called the Hubble Deep Field and the Hubble Ultra Deep Field showing galaxies close to the time of their formation just a billion years after the origin of the universe.

5.11.6 The future of optical astronomy

Plans are being made for a number of very large optical telescopes to be

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Figure 5.19 Artist’s impression of the 100-m optical telescope concept. Image: European Southern Observatory.

telescope that would have the ability to observe details down to 1 milliarcsec. It would observe in the open air and be covered by a dome during daylight hours as shown in Figure 5.19. Due to the very high cost envisaged it is perhaps more likely that an optical telescope in the size range 30–60 m would be built in the coming decades. There is also the long term prospect of building and operating an optical telescope on the Moon – where there is no atmosphere to degrade image quality and no light pollution!