Eschelle Spectrometer: A Unique Device

tower star light

Normally, it would have taken as many as seven mirrors to focus light from a telescope to a spectrometer. At each reflection, 10 per cent of the light would have been lost. Astronomers had to await the emergence of good quality optical fibres to efficiently transmit the light. An optical fibre avoids multiple reflections and the light can be directly taken to the spectrometer. Moreover, the fibre nullifies the uneven light input and due to its internal multi-reflections provides a uniform light distribution. More than 90 per cent of the starlight can thus be captured. There is yet another advantage. The diameter of the 45 m long fibre chosen for the VBT is 100 micro metres, which corresponds to the resolution of the primary focus viz. 2.7 arcsec. The diameter is ideal, as it would cut off skylight and let in most of the star light.

Bringing the light out of the optical fibre is just the beginning of the exercise. The light emerges as a diversified beam that needs to be made parallel before it hits the spectrograph. A collimator (which lines up the optical components like mirrors and eyepieces and regulates the light wave) in a tub-like structure does this work with six lenses. The collimated beam is then sent through a prism (called cross dispersion) and it illuminates the eshelle grating, designed to disperse the light into a spectrum. In a unique arrangement, the dispersed beam from the grating retraces the same path to the prism, which cross-disperses it a second time, when the collimator acts like a camera and focuses the beam on to a CCD. Quite a complicated procedure, but it works.

Though the role of an eshelle spectrometer was known, the unique arrangements at Kavalur are the result of an imaginative design of IIA devised by Prof N. Kameswara Rao and his colleagues. It is significant to note that the system was not just bought off the shelf from the West. The grating, biggest of its kind, was built by the University College, London as specified by the IIA astronomers. The CCD was also built exclusively for the VBT. The IIA built the collimator and the structure. The spectrometer is kept in a vibration-free location in a fully air-conditioned place. All operations are done through remote control.

In order to confirm the spectrometer’s high precision, molecular iodine lines are used for calibration as standard candles. Iodine cells are chosen as they provide a large number of sharp absorption lines in the same spectral region that is used for measuring the spectrum of light from the stars. Light from the star goes through the iodine cell before it reaches the Spectrograph. The final spectrum recorded after the light is dispersed by the Spectrograph would register the small and fine changes in the spectra of the star and iodine simultaneously. By deferentially measuring one spectrum against the other, one can detect the effect of the instrument as well as the stellar displacement, if any.

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