Calibration of the CCD camera of the 1m telescope at the Vainu Bappu Observatory, Kavalur, to theB V R system is reported here based on the observations of stars in the ‘dipper asterism’ in the open cluster M 67 (NGC 2682). Transformations involvingB andV have negligible colour terms, while those involvingR are slightly colour dependent. The possibility of using scaled-downR band fluxes to estimate the continuum flux at Hα is investigated by comparing the counts inR band with those through an interference filter centred at Hα. The scaling factor is found to remain constant over a wide range of colours. The sensitivity of the telescope-filter-CCD combination is estimated to be 2.0 per cent, 8.3 per cent and 9.7 per cent inB, V andR bands, respectively. The star F117 appears to be a small-amplitude (∼ 0.05 mag) variable.

The system gain of two CCD systems in regular use at the Vainu Bappu Observatory, Kavalur, is determined at a few gain settings. The procedure used for the determination of system gain and base-level noise is described in detail. The Photometrics CCD system at the 1-m reflector uses a Thomson-CSF TH 7882 CDA chip coated for increased ultraviolet sensitivity. The gain is programme-selected through the parameter ‘cgain’ varying between 0 and 4095 in steps of 1. The inverse system gain for this system varies almost linearly from 27.7 electrons DN^-1 at cgain = 0 to 1.5 electrons DN^-1 at cgain = 500. The readout noise is ≲ 11 electrons at cgain = 66. The Astromed CCD system at 2.3-m Vainu Bappu Telescope uses a GEC P8603 chip which is also coated for enhanced ultraviolet sensitivity. The amplifier gain is selected in discrete steps using switches in the controller. The inverse system gain is 4.15 electrons DN^-1 at the gain setting of 9.2, and the readout noise ∼ 8 electrons.