2 The Bordeaux and Valinhos CCD drift scan cameras

 In drift scan mode, the rows of the CCD detector are parallel to the diurnal motion of stars, and electric charges are moved from each column of the detector to the next one at a rate matching precisely the rate of transit of the star image calculated for the declination of the centre of the field. Charges accumulated during the integration period, which is the time spent by the stars in front of the CCD detector, are collected by quickly emptying the last column of pixels during each charge transfer. Sky strips which are narrow in declination (13' in Valinhos and 28' in Bordeaux), and wide in right ascension, can be numerically reconstructed, up to several hours.

  

Table 1: Main features of the Valinhos and Bordeaux meridian circles and CCD detectors

Basic features of the Valinhos and Bordeaux instruments and cameras are given in Table 1. Both cameras work in MPP (multi-pinned phase) mode and are cooled down to $-40^\circ$C by a two-stage thermoelectric Peltier module and water circulation. The overall dark noise of the CCD detector is about 65 e$^{-}\!/$s. Charge transfer at the sidereal rate is controlled by a rubidium clock. The signal is converted by a 16-bit AD1377 converter. The observed star field is continuously displayed in real time. In order to reduce saturation effects for bright stars, an antiblooming circuit, producing electrical pulses on one of the 4 phases of the CCD detector during the integration period, is used. Software for the control of the instruments was developed at Bordeaux Observatory and is the same for both meridian circles.

The CCD detector covers a wide range of wavelengths (Fig. 1). On both instruments, filters are used to limit the spectral bandpass to values close to the focal minimum of the lens, for which changes in chromatic refraction are not too large. Filters GG495 + BG38 were selected, giving a spectral bandwidth of 520-680 nm and a mean wavelength of 605 nm. With these filters, chromatic corrections, with respect to stars of spectral type K3, vary from -0.04'' tan(z) for B0 stars to 0.04'' tan(z) for M 3 stars, where z is the zenith distance.

  

Figure 1: Spectral response of the CCD detector, as given by the manufacturer