90-inch Vainu Bappu Telescope
The 90-inch Vainu Bappu Telescope (VBT) is operated as a National Facility for optical astronomy. It has two foci, an f/3.5 prime focus (with an image scale of 27 arcsec/mm) and an f/13 Cassegrain focus (with a scale of 6.7 arcsec/mm). At the prime focus, CCD cameras are used for imaging with various filters. A high resolution Echelle spectrometer is in operation where the light is fed through an optical fibre from the prime focus directly to the spectrograph slit. A spectral resolution of 70000 has been achieved with the spectrograph in the visible range. By narrowing the slit, it is possible to obtain a spectral resolution better than 100,000. At the Cassegrain focus, an OMR spectrograph is used for low-to medium resolution spectroscopy. A Boller and Chivens CCD spectrograph earlier used for spectroscopy has been converted to a spectro-polarimeter.
Backend Instruments
The VBT-HiRES is a high resolution fibre-fed Echelle Spectrograph at the prime focus.
| Location | Prime Focus |
| Plate Scale | 27"/mm |
| Optic-fibre core aperture | 100 microns, ~2.7" |
| CCD | UKATC 4K x 4K (4096x4096) pixels 12x12 micron pixels gain: 0.85e/ADU Read noise: 4.4e |
| Spectral Resolution (R) slit width: 100 microns | 27000 Limiting Mag (V): ~11 Typically 3 x 40min exposures for S/N ~60, clear sky |
| Spectral Resolution (R) slit width: 60 microns | 72000 Limiting Mag (V) ~9 Typically 2 x 40min exposures for S/N ~60, clear sky |
The VBT also has a Low Resolution Spectrograph (OMR) at the Cassegrain focus
| Location | Cassegrain Focus |
| Plate scale | 6.7"/mm |
| CCD | Tek 1K x 1K (1024 x 1024) pixels 24 x 24 micron pixels gain: 4.33 e/ADU Read noise: 13.5 e |
| Grating |
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| Slit Jaws | Min.width: 50 microns Max.width: 950 microns 2 pixel resolution => 300 microns (2" in sky) |
| Full Slit Height | 25 mm (adjustable by moving the decker) |
| Camera focal length | 150 mm |
| Collimator focal length | 1000 mm |
| Reduction factor | 6.7 |