Astronomy

Atmospheric Distortion In Telescope Viewing

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We have seen why ground – based telescopes cannot produce optical images with angular separations as small as their theoretical limit because of the atmosphere. Telescopes in space to not have the problem , as they orbit above the Earth’s atmosphere . But the size and cost of a space telescope are limiting factors.

On the ground, though a larger telescopes gatherers more optical energy, its diameter limits the angular resolutions of the image. The resolution of a given instrument is operational to the diameter of its objects, and inversely operational to the wavelength of the light being observed. So simply increasing the size of the telescope will not give clearer images.

We can make the best ground telescope but we would be at the mercy of the atmosphere, which will distort the images. Astronomers use the term ‘seeing’ to describe the effect of distortion in the path of starlight through different layers of the atmosphere.

The turbulence is caused by heat flow and winds in atmosphere, causing fluctuations in the density of air. Conventions in and around the building and winds at various heights – all cause turbulence. The ’cells ‘of varying size that refracts light differently cross the light path rapidly, distorting the shape of the waterfront. This led to the formation of what are called ‘eddies’ or fluctuations in the way in which a light path is affected by the air.

The turbulence is minimal just after sunrise but steeply goes up until afternoon. It is mainly because of solar heating of the ground . Besides the telescopes and its enclosure have been found capable of degrading the image. For instance, the mirrors in the telescope get heated up and distort the image. Remedial measures include elimination of the heat produced by electric equipment on the telescopes and elsewhere , and maintaining a uniform temperature in and around the primary mirror. Seeing improves, for example, when the mirrors is warmer than ambient dome air .

A major breakthrough in observational astronomy was achieved when the degraded images resulting from short exposures was identified as ‘speckles’ and a technique called single aperture speckles Interferometer developed to counter it . The basic interference phenomenon is called speckle. The term ‘speckle’ refers to the grainy structure seen, when a uneven surface of an object is illuminated by a coherent source. A speckle interferometer is a camera that records magnified short – exposure images. Correcting prisms can be used to counter the dispersion caused by the atmosphere. Speckle spectroscopy is also possible. By using filters or concave grating, instead of the correcting process, one can get several spectral channels with narrow bandwidth, down to 0.03 Nanometers.

interferometers

Image Credit:wanda.fiu.edu

A rigid but light camera has been developed by a group of scientists led by Prof. S.K Sash, of the Indian institute of Astrophysics (IIAP), for the 2.34- m telescope at the Vainu Bappu Observatory, Kavalur. The instrument has been made, using computer – aided design and analysis with precision machine tools.

The instrument basically consists of a mirror with 0.35 – mm aperture, a microscope assembly, a filter and detector, besides a guiding system. The instrument can operate at the prime focus as well as the Cassegrain focus of the Vainu Bappu Telescope. The new interferometer has made it possible to observe many interesting celestial objects and map their high-resolution feature. The images are reconstructed on the basis of complicated formula and algorithms.

Single aperture interferometers have been found quite useful in enhancing the clarity of stellar images in the terms of their diameter, the separation of unresolved close binary stars , imaging emission line of the active gal active nuclei (AGN), spatial distribution of matter surrounding objects and gravitationally lensed quasars.

Interferometer techniques can bring out the high –resolution information on some of the fundamentals processes on the Sun that occur on shorter than arc second scales. In particular, magnetic fields on the Sun and solar features of the size of about 100 km or smaller can be observed. The process is limited if solar granulation evolves too rapidly to be detected. A speckle reconstruction of solar feature was done during the partial eclipse of the Sun as viewed from Bangalore on 24 October, 1995.

As an experiment, an interferometer developed by IIAP was used to record the images of Jupiter during the collision of the fragments of comet Shoemaker-Levy 9 in 1994. The Japal –Rangapur Observatory, Hyderabad, recorded it with the aid of the interferometer.

Speckle interferometer is capable of detecting faint objects at about 16th magnitude. However, the angular resolution of the image will be limited by the diameter of the telescope. Hence, another technique called, long baseline interferometer is used to improve the resolution in the visible wavelength.

Speckle cameras have advantage over conventional imaging polar meters in monitoring objects subject to short variations in atmospheric transmission. Polarized light carries information about its origin in the source, its magnetic fields and chemical interactions, etc. Polarimetry in astronomy is useful in realizing many objectives such as finding out the size and shape of the dust grains and analysis of binary and other stars.

Initially, photographic plates were used to detect the objects. CCDs have been increasingly used, especially to obtain snap shots of very short phenomena. Photon counting detector systems have emerged and are in use. A key advantage of such a technique is that the signal can be short exposures.

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