One of the challenges is making the telescope is the high degree of smoothness needed for the mirror. By way of a rough analogy to stress the point, it is said that if a one meter mirror were to be expanded to the size of the Indian subcontinent, the height of Mount Everest or the depth of a deep lake should not theoretically exceed that of a cricket ball! As different telescopes were constructed, the nature of light, especially the limits in its practical use became obvious confirming many theories.
Lord Raleigh specified that the wave front (waves hitting the telescope ) should not be distorted by the atmosphere by more than a quarter of the wavelength of light, if a telescope were to function to its maximum capacity called diffraction-limited. This limit , through applied to a point like object , served as a guide for appreciating the limits of telescope , until technology found an answer in the from of adaptive optics.
In the 20th century, the discovery of the electron and its applications had a big impact an imaging. The quantum nature of matter and its links to light have led to new imaging techniques. Almost all telescopes these days are reflectors, which alter the polarization properties of incoming light from celestial sources.
What The Light Spectrum Reveals
Telescope yield two kinds of information; picture or images; and spectra, the colored bands that emerge when light is passed through a spectroscope and recorded.
Visible light is divided into its component wavelengths. A spectrometer can not only resolve colors better than our eyes but can also identify the atoms and molecules that emit radiation. A simple detector can distinguish visible light from ultraviolet and infrared radiation. But a high-resolution spectrometer can divide the seven visible colors of the rainbow into as many as a million discreet parts
Hence astronomers do spectral analysis to distinguish the chemical nature of the matter that emits the radiation at great distances. Increase spectral resolution can ‘decode’ sunlight or starlight and get information on the temperature, dynamics and motion as well as chemical elements and even the isotopes that emits the radiation. In 1869, A. J. Angstrom used a diffraction grating instead of a prism to study the spectrum.
Niels Bohr (1913) explained the transition of electrons from one energy level to another. Accordingly, an atom can absorb light, while it drops to a lower energy level. The transition is marked by specific wavelengths corresponding to the quantum of energy of the involved. It requires light of the correct wavelength from another source to make the atom jump to a higher energy level so that the photon can be absorbed. Different substances have different energy levels and produce different spectra.
Spectra of object may be seen in three flavors :
- A Continuous spectrum (A rainbow of light), when a source of white light emits across all visible wavelengths.
- As Absorption spectrum results when a absorbing medium such as cool gas , hides incoming white light and absorbs certain specific wavelengths, leaving gaps in the spectrum.
- An Emission spectrum reveals certain wavelength of light emitted by a hot tenuous gas.
A multi-channel spectrometer uses, for instance, 32 photo multipliers that cover the entire visible spectrum. Nowadays, a spectrograph automatically deletes background noise from the night sky. Linked with a computer, the data, collected by the telescope is directly sent to the database. Astronomers today see wave- like lines on the computer screen. In fact, it is even said that astronomers, who have access to computers connected to the processing of their data need no longer look through an eyepiece, especially as warm bodies are not encourage inside the cool dome at night!
A telescope’s ability to see fine details depends on light-gathering capacity. Large telescopes have large aperture – the diameter of a mirror to gathers more light and provide better angular resolution. by doubling the mirror aperture, one can collect four times as much light. A 100-mm telescope can resolve stars located at about 1 arc second apart. An arc second is 1/3600th of a degree. The maximum power a telescope can have is equal to about 50 times its aperture in inches. The human eye can resolve only about 60 arc seconds.
Small telescope collect less light but have less chance of “saturation”( over – exposure to bright sources). Moreover smaller telescopes would be good for long exposures. Several attachments could strengthen them. For example photometry could strengthen them. For example photometry can record minute fluctuations in the brightness of objects. Image intensification device are nowadays available. They can help in photographing stars as faint as those in the 20th magnitude and enable even a small telescope to look at galaxies beyond our own.
Astronomy has undergone a revolution over the past decades, basically because of the introduction of large mirrors, which could not even have been thought of earlier. For instance, the Keck twin telescopes in Mauna Kea, in Hawaii, have a mirror of 10 meter across, as against 5 meter at Mt Palomar (California). The world’s largest optical telescope mirror has been constructed at the Canary Islands. Built by Spain, it has a mirror of 10.4 m diameter. In contrast, the Hubble space telescope has an aperture of only 2.4 meter. Nowadays, large mirrors in telescopes are also segmented in order to overcome the problem of sagging under their own weight.