Telescopes, Detectors and Space Probes

From Earth-based observatories astronomers can detect predominantly optical and radio waves. Using telescopes for optical astronomy began with Galileo in 1609. He applied the design of Hans Lippershey, a Dutch spectacle maker, and improved it.

His telescope allowed him to make the most remarkable discoveries in his day and age, including observations of the moons of Jupiter which strengthened the Copernican view of a heliocentric Solar System. It also encouraged those who followed him to build bigger and better telescopes in an effort to discover more and more. Newton also designed a style of telescope which is still widely used today by amateur astronomers and, indeed, is known as the Newtonian.

Today’s professional astronomy telescopes are enormous in size compared with those original ones. Many existing telescopes have mirrors which are 4 metres in diameter, whilst a new generation of telescopes is currently under construction which possess 8-metre mirrors.

The largest telescope in the world is the Keck Telescope on the extinct volcano, Mauna Kea, Hawaii, which possesses a primary mirror 10 metres across. It is so large that it was unrealistic to build a single, continuous mirror of the size. Instead, it is constructed like an insect’s compound eye, with 36 hexagonal mirror segments held in place by electronic support arms to create the giant mirror.

The Keck Telescope has proved to be so successful that a second, identical telescope is currently undergoing construction on the summit of Mauna Kea. This siting of the telescope is not unusual for modem observatories despite the sometimes difficult conditions which working at altitude can create. Mountain tops are an ideal situation because the telescopes are elevated above most of the weather and turbulent air, which obstruct observations from sea level.

Atmospheric turbulence can also be combated in other ways. For instance, a technology known as active and adaptive optics is just proving itself viable. Active and adaptive optics form a system which allows a reflecting surface to be controlled by mechanical actuators. The quality of the image being produced by the telescope is continuously monitored and any deviation from perfection is registered so that steps can be taken to make corrections.

A computer determines how to move the mirror to make the image perfect again (see Fig. 2. 8). This system has two advantages. The first is that a large mirror can be manufactured and supported in many places. Formerly, the rigid construction of a mirror to a level which prevented flexure made it prohibitively heavy.

Fig. 2. 8. Schematic diagram of an adaptive optics system. The key component is the wave front detector, which senses the distortion in the incoming wave front, allowing the computer to adjust the mirror to compensate

The second advantage is that, providing the system can react fast enough, it can detect imperfections in the image caused by atmospheric turbulence and compensate for them by manipulating the mirror. This type of technology looks certain to be installed in more and more observatories over the coming decades.

All a telescope primarily does is collect light. It is up to the astronomer what is done with that light once it has been collected. Originally, visual observations were sufficient, but contemporary science demands hard facts and data to back up any assertions which are made. A plethora of techniques and detectors has sprung up to help astronomers record their findings.

Two basic techniques of analysing the light collected by telescopes are those of spectroscopy and photometry. A spectroscope splits the incoming light into a spectrum so that its spectral lines can be studied. As discussed earlier, the pattern of spectral lines can indicate the chemicals present in the object under investigation, and the size and shape of the spectral lines can lead to conclusions about the motion of that object.

Above and below the spectrum, the spectroscope superimposes reference emission lines, so that accurate wavelengths for the observed spectral lines can be measured. Devices known as microdensitometers can be used to study the intensity of radiation along the spectrum, and it is from these traces that the shapes of the spectral lines can be found (see Fig. 2. 9).

Fig. 2. 9. Absorption and emission lines are characterised by a specific shape known as a Gaussian profile, which can be obtained by tracing with a microdensitometer across a spectral line. (Adapted from Kitchin, C, Stars, Nebulae and the Interstellar Medium, Adam Hilger, 1987.)

It is fair to say that the vast majority of astronomers use spectroscopy to study their chosen objects because the level of information return from a single observation can be very high. However, spectra can be time-consuming to record because, in traditional spectroscopes, the light from an object is passed through a slit aperture and then expanded into its constituent wavelengths. This can dramatically decrease its brightness and thus increase the time necessary for the exposure.

Photometry is a technique whereby the brightness of a celestial object is taken at various different wavelengths. Those wavelengths are known as photometric bands and have been standardised to specific wavelengths. They have also been given letter names: for instance, the К photometric band refers to the wavelength of 2.22 microns which is in the infrared region of the spectrum.

The technique is practised by placing filters between the telescope and some form of photon counting device. The filters only allow specific wavelengths of light through to the counter, which records the intensity of light at that wavelength (see Fig. 2.10).

Fig. 2. 10. The spectral response of the dark-adapted eye and of the U (ultraviolet), centred on 365 nm, В (blue), centred on 440 nm, and V (visual, i.e. yellow-green), centred on 550 nm, pass-band filters used for photometry in the UBV system

Photometry can be used to classify stars because it can reveal their black body temperatures. It is very useful at tracking variations in the amount of radiation which is output by celestial objects over a period of time. A good example of where photometry is particularly useful is in the classification of variable stars. Photometry shows just how they vary their light output over the course of time and allows light curves to be constructed (see Fig. 2. 11).

Fig. 2. 11. The light curve of a variable star such as a Cepheid (a) or of a supernova explosion (b) can provide excellent diagnostic information for the astronomers studying these objects

Sophisticated analysis of photometric data covering a wide range of wavelengths can yield many of the same conclusions which can be derived from spectroscopy but, unfortunately, the collection of photometric data is even more time-consuming than the collection of spectroscopic data. Thus, the construction of highly complicated spectroscopes is still favoured over the use of the simpler photometers.

A third, less popular though no less valid, technique is that of polarimetry. This method of observing celestial objects collects information about the orientation of the electric vector in the electromagnetic radiation being collected by the telescope. It can be a powerful tool in probing regions where radiation is being scattered by dust or electrons. It can also help to show up regions of space containing magnetic fields which have aligned dust grains. Thus, the technique is excellent for probing the interstellar medium in this and other galaxies.

Visual observations are now insufficient for the professional astronomer. The light captured by large telescopes and processed via one of the above techniques must now be recorded for later analysis and publication to sceptical colleagues.

Photography is still used by some, but by far the most rapidly developing technology is in a type of detector known as a CCD, or ‘charge coupled device’, an instrument which is so sensitive that it records almost every photon which strikes it. It is a computer chip which uses a detection mechanism, similar to the photoelectric effect, to record the number of photons striking it. These are instantly turned into an electrical signal which can be read out into a computer memory for later analysis and display.

Another way of saying that a CCD has the ability to count almost every photon which strikes it is to say that it has a high quantum efficiency. This is a ratio which can be easily understood as the number of detected photons divided by the actual number of incident photons. In other words it gives the percentage of photons which are actually detected when they strike the detector. A CCD will typically have a quantum efficiency in excess of 75 per cent, whereas a photographic system will rarely be more than 1 per cent.

CCD chips are made of semiconductors, the choice of which determines which part of the electromagnetic spectrum will be detected. Ultraviolet, optical and infrared can now all be observed with the correct CCD. The collection of radio wavelengths, however, takes place with totally different types of telescopes and detectors.

A radio dish works in exactly the same way as an optical telescope. The reflector still needs to be parabolic but, because the wavelengths of radio radiation are so much longer, it no longer needs to be silvered. The detector is placed in the prime focus position on the radio dish, where a secondary mirror is usually, but not always, found on an optical telescope.

Radio astronomy has pioneered a technique known as interferometry, which effectively increases the resolving power of a telescope by linking it in tandem with another. The resolving power of a telescope is simply a measure of how apparently close two celestial objects can be to each other before the telescope is incapable of showing them as two distinct objects. For optical telescopes, the resolving power can often be so good that the distortion of the images through atmospheric turbulence is the biggest source of image degradation. For radio telescopes this is not the case because the wavelength of radiation is so much larger.

The resolving power of a telescope is proportional to the wavelength of radiation divided by the aperture of the telescope. Thus with an increase in the wavelength, the resolving power of the telescope can be maintained only if a commensurate rise in the aperture of the telescope is also achieved. The radio waves given out by neutral hydrogen are six orders of magnitude longer than visible light so, in order to get the same resolution from radio telescopes the aperture would need to be one million times the size of optical telescopes! Obviously, another way had to be found.

Interferometry was that other way. It was pioneered at Cambridge University in the 1940s. It allows an object to be observed simultaneously by two or more radio telescopes. The signals collected by the dishes are then combined and interference patterns are obtained. Using Fourier analysis those interference patterns can be used to construct detailed images with much greater resolution than a single dish on its own could achieve.

Interferometry in the radio region of the spectrum has been so successful that the method has now been applied to optical telescopes too. A pioneering team of astronomers at Cambridge has applied the technology to produce an image of the star Capella using four small telescopes together. Now, a number of much larger observatories are under construction around the world which all hope to exploit optical interferometry.

The impenetrability of the Earth’s atmosphere makes it necessary to send space probes and satellites into orbit in order to collect those wavelengths from which we are shielded. High energy radiation is almost totally blocked out by the atmosphere. Apart from the small amount of ultraviolet radiation which penetrates the atmosphere and causes suntans, the vast majority of ultraviolet radiation has to be collected by space probes such as the International Ultraviolet Explorer (IUE).

At even shorter wavelengths, the Einstein X-ray Observatory has surveyed that region of the electromagnetic spectrum. These space probes require a slightly different type of mirror system known as a grazing incidence mirror. This type of mirror is an annulus taken from the cylindrical wall of a paraboloid rather than from the bowl as in optical telescopes. In order to maximise the amount of high energy photons collected, annuli of successively smaller radii are nested within one another.

Molecular absorption blocks a lot of infrared radiation, but this too has now been sampled from space by the Infra-Red Astronomical Satellite (IRAS) and the Infrared Space Observatory (ISO). Even wavelengths which can be studied from Earth have been collected in space. High above the Earth’s atmosphere, the stars remain steady and unwaveringly sharp. The celebrated Hubble Space Telescope (HST) is currently performing magnificently by collecting unprecedented images of the cosmos.