At optical wavelengths it is convenient to think of the radiation as consisting of a stream of photons. All optical detectors function by transferring some or all of the energy of an incident photon to one or more electrons. The excited electrons arc then detected in a wide variety of ways. A number of basic […]
The basic form of radio-wave detector is the simple dipole or antenna. It consists of two vertical metal rods, one above the other and a quarter of a wavelength long. If, in the vicinity of the dipole’ there is an electromagnetic wave which has its electric field parallel to the length of the dipole then […]
The bewildering variety of devices used as radiation detectors could easily give the impression that this is a difficult subject, but this is not so. Electromagnetic radiation can only interact with charged particles. At wavelengths longer than those of X-rays the energy of a single photon is so small that only the lightest charged particles […]
Before considering the variety of techniques used to detect the radiation collected by telescopes and analysed with spectrographs or polarimeters, we need to establish what we must do to detect radiation. Most astronomical objects are very faint; they are ob¬served in the presence of a considerable amount of unwanted radiation, such as that from the […]
Electromagnetic waves consist of rapidly alternating crossed electric and magnetic fields. The radiation from most astronomical objects is such that the direction of the electric or the magnetic fields in the waves varies in a random manner. We say that the radiation from these objects is unpolarized. Some objects, however, emit waves which have these […]
Perhaps you are a competent radio spectroscopist, probably without realising it! Every time you turn the tuning knob on a radio or television receiver you are looking for peaks in the radio-frequency spectrum present at the antenna (or aerial) of your receiver. The peaks are radio and television broadcast stations. The same technique is the […]
Instruments that give the spectrum of the radial ion from an object are called SPECTROGRAPHS. The most important part of an optical spectrograph is the element that breaks up the light into its constituent colours. This can be done simply by use of a series of colour filters, hut such an approach is not an […]
The angular diameters of stars are much smaller than the limit imposed by seeing conditions in the atmosphere. It may therefore seem that there is no point in building an optical analogue of the radio interferometer in order to improve the resolution of an optical telescope. However, there are moments when the image stabilizes for […]
The basic purpose of a telescope is to deflect the radiation incident on every part of its surface towards a focus, and to arrange that all the signals reaching this focus are of the same phase. Most astronomical objects only vary their sizes and brightnesses very slowly. If the emitting object remains unchanged for a […]
The largest fully-steerable parabolic telescope is the 100-m radio dish near Bonn, West Germany, At a wavelength of 6cm it has a resolution of about 2 arc min. To achieve a resolution at radio wavelengths comparable with that set by seeing limitations at optical wavelengths would require a telescope at least fifty times larger. The […]