As per reports in September 2020, a Large Synoptic Survey Telescope (LSST) is being set up in Chile, South America, whose sensitivity would be a 100 million times greater than that of the human eye. Once in action, this telescope, an 8.4 metre wide reflector that focuses images of a wide swathe of the sky on to a 189 segment, 3,200 megapixel detector, would be able to image a golf ball from 24 kilometres away.
The Vera C Rubin Observatory is under construction in Chile, from where the LSST will survey the entire Southern Hemisphere sky every few days, till the next 10 years. About 15 terabytes of data would be collected every night, such that the LSST would have recorded more detail of the cosmos than all the previous astronomical surveys within just a month.
The LSST is set to be in action in 2022, but the sensor array for the camera has been assembled and was tested. The sensors are so sensitive that they need to be protected from all kinds of stray radiation, or ‘noise’. To this end, the array is cooled to 100°C below freezing.
The optical telescope needs lenses of the finest quality and of the largest diameter possible to get the best magnification and detail. The inherent flaws in glass lenses become serious when the lenses become large. Therefore, reflecting telescopes with very large apertures have been brought in, which made faint images visible with high-grain photographic film and long exposures.
However, there are limits, both to the detail and the sensitivity, when we use visible light. Detail is limited by the wavelength of visible light, given the dimensions of lenses or mirrors that are practical. And for sensitivity, much of the visible light from distant objects gets scattered before it reaches the Earth. The most distant objects are thus too dim to be seen.
The most distant objects can be spotted by the radio waves they emit, rather than light. As radio waves have wavelengths thousands of times greater than light, they are scattered much less, and radio telescopes are used to observe the farthest objects. The detection of radio images is by an array of radio antennas spread over a large area. While the larger wavelength of radio waves help in detecting faint signals, the same large wavelength reduces the detail that can be detected. This is compensated by the large area over which signals are collected, and this has the effect of a very large lens or mirror in the visible range.
As for the limit to detail that visible light presents, a way around was by using ultraviolet light, or even X rays. Now, the atmosphere is opaque to UV or X-rays. These telescopes hence had to be stationed outside the atmosphere, and this became practical with satellites placed in orbit around the Earth. The famous Chandra X ray telescope has been in orbit around the Earth since 1999, providing detailed images of high-energy processes, like supernovae and the surroundings of black holes.
Telescopes rapidly improved in quality, with the limitations of lenses finding an answer in using mirrors in their place, and the largest telescopes we have are now the ‘reflectors’.
There has been a progress in the development of cameras too. Cameras must use lenses. While the lenses were improved, for larger diameter, or aperture, to allow more light, the quality of the film on which the image was captured was also refined.
The array of electronic, light sensitive specks of silicon oxide on a sliver of silicon is now being used in place of photo film. In the photo film, the bright spots of an image cause chemical change, and the image is recorded on the film. However, with the electronic sensor, the bright spots of the image cause build-up of charge, in the specks on the sliver of silicon. These charges are then transferred to a collector, which creates a digital record of how much light fell on each of the specks in the silicon screen. The record can then be used to display the image for the user of the camera to frame the picture, or to save the record, when the picture is clicked.
While the resolution, or the detail, in the photo film depended on the fineness of the chemical specks deposited on the film, in the digital camera, the equivalent is the number of light sensors that act to capture the image. If the device has 1,000 rows and 1,000 columns of sensors, there would be a million sensors in all, and we would speak of a megapixel camera. The resolution of professional digital cameras goes as high as 50 megapixels.
Courtesy: The Statesman, Sept 16, 2020