RV precision. Since VBT Echelle is a general purpose spectrograph and used in a broad range of science observations, the grating needs to be moved to observe the wavelength of interest on a case-to-case basis. Due to contingent movement, it is difficult to maintain a well-defined zero-position of the grating. The problem is further aggravated by the backlash error in the grating-drive, non-uniformity of the gears and imperfect locking and holding. This would also result in inaccurate zero-positioning when the grating motor is turned off at the end of the observations on a day-to-day basis. With the instrumental restrictions, the grating has a positional uncertainty of $\pm  $ 25 arc-sec ($ \approx \pm $ 0.01 degree).

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We have used Th-Ar observations to estimate the repeatability of the calibration lines, which in turn affects the calibration consistency. The grating was commanded to move back and forth between two angular positions to determine the position repeatability. Between the moves, a Th-Ar spectrum was taken at each reference position and a total of 8 frames/position were obtained in about 1 hours duration. The first Th-Ar spectra for each position was cross-correlated with its subsequent frames to estimate the repeatability of the grating 