The Eschelle Spectrometer


How good is the Eschelle spectrometer? Astronomers quantify the efficiency in terms of a number derived by dividing the wavelength by the increments of a wavelength. The number, 60,000, for example, indicates that the wave can be split into 60,000 lines in the spectrum. The more the lines, the more would be the resolution and the space between the lines (orders as they are called) decreases. One can go up to a spectral resolution better than 100,000 lines, by narrowing the slit in the spectrometer. But there is a trade-off. If the number goes up, the available light is spread out too thin and the spectrograph would be faint. With all the sophisticated optics, only 15 per cent of the received light could be used in spectroscopy. Only for very bright stars, the resolution can go up to 100,000. The VBT optics has been optimized at 70,000 (lines) in the visible range

The high-resolution Eschelle spectrometer has been in use since 2005 at VBT. It has been useful to detect stellar elemental abundances. In some stars, certain elements (such as hydrogen) are found depleted. And the distribution of elements is different from the expected pattern. It is more like the interstellar medium. Stellar variability and astroseismology demand high-precision radial velocity measurements. The results obtained compare well with those taken by advanced foreign telescopes.

Another mode of observation, introduced in 1990, is called the Cassegrain focus, where the light collected is reflected back, as designed in this type of telescope. The scale of the image is 6.7 arcseconds per millimeter. It can, with an opto-mechanical research spectrograph, provide high spatial resolution (for e.g. it can distinguish two closely located objects, which would otherwise appear as one blurred object). However, the spectral resolution would be medium or low. Besides an imaging camera and a spectrograph, a polarimeter that helps determine the precise position of stars, and a near-infrared photometer are available at the Cassegrain focus.

The observatory has a Zeiss telescope. It has a primary mirror with a diameter of 102 cm, which can give an image at the scale of 15.6 arcseconds per millimeter. The telescope is guided manually. A one-metre mirror has been ground and polished to replace the primary mirror of the telescope. Another telescope (0.75 m) has a near-infrared photometer, besides an imaging camera.

Working of an Echelle Spectrometer.

Working of an Echelle Spectrometer.

A plant for making liquid nitrogen is in operation in Kavalur, as it is necessary to cool certain components of the system such as CCDs. In addition, an on-site aluminium coating plant does the sophisticated aluminium coating of the mirror to refurbish it once in two years. Aluminium vapour is sprayed on the mirror, which is removed from the telescope for this purpose and attached again—quite a delicate operation.

The indigenous production of the telescope has resulted in savings as there is no operational cost by way of royalty payable for an imported system. As Shri A.K. Saxena of the IIA, who guided the polishing of the mirrors in Kavalur points out, the indigenous capability would be an asset for designing larger telescopes and in installing state-of-the-art devices.

The demand for telescope time at Kavalur is growing, both from within the country and abroad. As the number of clear sky nights is limited to some six months in a year, a 1.3-m telescope is being installed to meet the requirements.

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