The commonest shape for the primary mirror of a reflecting telescope is a simple paraboloid. It gives excellent images at the centre of the fiat focal plane although the area over which acceptable images are produced can be small. Much of the variety seen in telescope design is a consequence of getting an observer or […]
One of the simplest image-forming devices is a concave mirror. If the mirror is parabolic, on-axis rays which makeup a parallel beam are deflected and converge at a single point. The radiation from different parts of an extended object, such as a galaxy, reach the mirror from slightly different directions . They are then deflected […]
Once the radiation from an astronomical object has passed through Berth’s atmosphere, it must be gathered into a form that enables us to detect it fairly easily. Only after this detection has taken place and the signal from the object recorded can any analysis of the data take place. Astronomical objects are so far away […]
No terrestrial observing site is perfect and any one site can only be a compromise between the many, often conflicting requirements of a so-called site. What makes a good site for an optical or infrared observatory? Firstly, it must have many good, clear nights. Then it must be as dark as possible. This means that […]
On a hot day we can look along a surfaced road at a distant object and see that it seems to shake and shiver as we look at it. This happens because ‘bubbles’ of hot air rise from the surface of the road. The fluctuations in air density caused by these bubbles of hot air […]
Outside the radio and optical windows, the main source of night-sky emission is the re-radiation which accompanies absorption. In the radio window the sky is very dark, by day and by night, since it radiates very little indeed. Radio observations at wavelengths longer than a few centimetres are therefore just as easy by day as […]
The surface of the Earth is covered in a thin layer of gas called the ATMOSPHERE of our planet. It is made up of a large number of constituent gases of which nitrogen, oxygen, argon, carbon dioxide and water vapour are the most abundant, with ozone being important at high altitudes. Much of the radiation […]
In the midst of the early turmoil over the distance scale and the internal structure of stars, a few vague attempts were made to examine a whole new aspect of astronomy. Until this time, astronomers had been observing only in the optical part of the spectrum, that part ranging from about 300 to 1000 nm. […]
Bubble had shown definitively that spiral nebulae were external galaxies. In the 1930s he continued work on these objects with the Mount Wilson 2.5-m reflector, which was then the world’s largest telescope. Studies over the next decade revealed that the extra-galactic nebulae could be divided into four main classes: irregular, elliptical, normal spiral and barred […]
Progress on the nebulae in the early twentieth century was stim lated by the construction of large telescopes in good climates and the regular use of permanent records such as photographic plates and spectrograms. The Harvard telescope in Peru, the Crossley 1-m reflector at the Lick Observatory and the Mount Wilson 1.5-m played an important […]