Combining Light From Telescopes

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We have seen that the resolution of a celestial image depends on the diameter of the primary mirror and the wavelength used to detect it. The diameter cannot be expanded beyond a point technically or financially. Hence an innovative technique has been developed to combine the light gathered by two or more telescopes placed apart and get the resolution of a bigger telescope, whose virtual diameter will be equal to the distance between the smaller telescopes. In other words merely adding a telescope would not increase the resolution of the final image; the baseline connecting the telescopes should be increased to get the benefit.

Combining light from telescopes faces a basic problem, as the signals from the celestial source arrive at different times at different telescopes, because of the rotation of the Earth. Advances in optical transmission have enabled the scientists to do something unheard of before: delaying the light wave! The signal of the telescope nearest to the source is delayed and made to reach when the farther telescope receives its signal. The signals would be corrected and combined to get a signal from a larger telescope. The technique is so sophisticated that a difference of just one nano second leads to an error of 30 cm!

In Mauna Kea, Hawaii, seven of the largest telescope are virtually combined to function as one big instrument (apparently 800 meter across) to get an ultra – high resolution of the near – infrared universe . This is 80 times more than what the 10 meter Keck telescope can give. Known as the optical Hawaiian Array for Nanradian Astronomy Array or interferometer (abbreviated as OHANA), It would provide the world’s highest resolution in infrared imagery viz. 0.5 mill arc seconds, which is about the size of an edge of this page as seen from 160 km! The light of the telescopes will be delayed by optical lines to compensate for the sidereal rotation of the stars, as if the telescope were physically tracking the stars.

Combination of light from several optical telescopes would need mirrors that are carried on carriages set up underground, and in radio telescopes the signals are electronically combined. An example of the latter is the Very Large Array Radio Telescope at Socorro in the USA , where its 28 dish antennas – each weighing 230 tons – will be combined’ to merge their inputs into a central special purpose computer to enhance their sensitivity to such an extent that it can pick up a cell phone signal on Jupiter!

The use of segmented mirrors will be the key technique in building the world’s biggest ground based optical telescope with a composite mirror of 42 m diameter at the European Southern Observatory. Christened the E-ELT, each of its 906 segments will be 1.45 m wide. This is expected to make it 100 times more sensitive than today’s largest optical telescope viz. the twin 10-m Keck telescopes in Hawaii.

Another telescope which will have segmented mirrors is the giant Magellan Telescope (planned by the USA and Australia to be ready in 2016), which will have six 8.4 m ‘off-axis’ mirrors surrounding a seventh, like petals in a flower to provide four and half times the collecting area of any current optical telescope and with a resolving power of 25.6 m diameter telescope or 10 times the power of the Hubble space telescope. An ‘off -axis’ mirrors focuses light at an angle away from its axis , unlike a symmetrical mirror that focuses light along its axis.

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