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An assortment of astronomical telescopes
Source: Business Channel
Astronomical telescopes are a bridge for scientists to know and understand the universe. As the saying goes, all roads lead to Rome, but each road can be different. So what are the classic designs of telescopes for observing the universe?
The Rosette Nebula (left) and the Great Nebula in Orion taken by a refracting telescope (Photo by Anhui University Student Astronomical Association).
The oldest design is the refractor telescope. Its core component is a convex lens, not unlike an ordinary magnifying glass. In fact, the human eye is a refracting telescope, in which the lens serves as the convex lens, the retina as the receiver, and other structures take on auxiliary functions such as focusing and tracking. However, because refraction is accompanied by scattering, such telescopes generally have chromatic aberration problems. If chromatic aberration needs to be corrected, a series of specially designed correction mirrors must be added to the optical path. However, chromatic aberration cannot be fully corrected, so in demanding scientific grade applications there is a tendency to avoid refracting telescopes in order to solve the chromatic aberration problem.
Another common architecture is the reflecting telescope. The prevailing view is that it was invented by Isaac Newton, hence the name Newtonian telescope. The principle is simple: parallel light is reflected off a paraboloid and converges to a point, acting similarly to a convex lens. The advantages of these telescopes are that they are inexpensive, they are not refractive, and therefore there is no problem of chromatic aberration. The disadvantage is that the use of parabolic reflection can not focus off-axis light well, although the use of spherical mirrors to improve this point, but the use of spherical mirrors on the front of the parallel light incident on the quality of the image is not as good as parabolic mirrors, it is difficult to achieve the best of both worlds.
California Nebula (left) and Heart Nebula (Photo by Anhui University Student Astronomical Association).
One of the more promising architectures outside of refracting and classical reflecting telescopes is the Cassegrain telescope. It is a modification of the reflector telescope, in which the secondary mirror has been changed from a flat mirror to a convex mirror, and its reflection has been changed from a 90 degree reflection to a 180 degree reflection, so that the imaging position has been moved from the side of the tube to the rear of the tube. Since the curved surface types of both the primary and secondary mirrors can be freely designed, the imaging potential has been greatly enhanced. In particular, the use of hyperbolic surfaces for both primary and secondary mirrors effectively eliminates primary aberrations and results in very good image quality. This high-quality optical architecture was invented in the early 1910s by the American astronomer George Willis Ritchie and the French astronomer Henri-Jacques Chrétien, known as the RC system. It is used in the Hubble Telescope, the James Webb Space Telescope and several astronomical telescopes at our Cold Lake Astronomical Observatory.
Jupiter (left) and Saturn photographed by the Schmidt-Cassegrain telescope (Photo by Anhui University Students’ Astronomical Association).
But nothing is perfect, and the RC architecture, despite its high performance, is very expensive. The reasons for this are twofold: firstly, hyperbolic surfaces are more difficult to process, and secondly, the excellent image quality places higher demands on processing precision, which further leads to soaring costs. On the other hand, spherical mirror on the one hand, easy to process, on the other hand, even if the processing is perfect, the image quality is not high, so the process requirements are much lower. Therefore, although the performance of the Cassegrain telescope based on spherical mirrors is limited, it is very suitable for popular science applications.
It is worth mentioning that the primary mirror of the James Webb Space Telescope consists of 18 hexagonal lenses, which together form a hyperbolic primary mirror, constituting the main part of the RC optical system. Due to the presence of the secondary mirrors and mounts, the incident light is partially cut off, and the remaining light is diffracted to form the distinctive octagonal asterisk. Interestingly, although refracting telescopes are not usually used in scientific applications due to chromatic aberration, they are significantly better than reflecting telescopes and Cassegrain telescopes in terms of asterisms due to the absence of a light-shielding part of their architecture. This proves once again that there is no such thing as the best telescope, only the most suitable telescope.
Contributed by the Science Popularisation Base of the Chinese Physical Society and the Student Astronomical Association of Anhui University.