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We present the Mercator Telescope together with the P7-2000 photometer as its first-light instrument. Mercator is a 1.2-meter Ritchey-Chretien telescope, installed at the Roque De Los Muchachos Observatory on La Palma and fully operational since Spring 2001. The Geneva Observatory developed this telescope and a twin, known as the Euler Telescope, was already inaugurated at the La Silla Observatory in 1998. Mercator is an alt-azimuthal telescope designed for semi-automatic operation and high operational robustness. P7 is a high-precision 2-channel differential photometer, built by the Geneva Observatory and in permanent use for over 25 years on various telescopes. It allows quasi-simultaneous observations in the 7 filters of the Geneva photometric system with a variable sampling rate up to 100 samples per second. This vintage instrument was completely refurbished in 2000 to function in an automatic mode on the Mercator Telescope. Electronics were completely renewed and are now based on a digital signal processor (DSP), which controls the instrument and performs basic data reduction. The optical system was left unmodified, apart from the addition of a field camera that is also used for auto-guiding. We also added instrument temperature control and a mechanical derotator. Since the 7 filters are acquired simultaneously and the absolute calibration of the colors is strictly homogeneous, the Mercator-P7 combination is a unique tool to study stellar variability on many different timescales. The current scientific program focuses on multi-periodic phenomena in early-type stars with the goal to identify the frequency spectrum and to constrain stellar models by asteroseismology studies. More than 43000 observations have been performed since 2001 and a precision of few milli-magnitudes is routinely achieved. Our photometric measurements result in the continuous calculation of the atmospheric extinction coefficients and these data are available online for other observers as well. In this paper, we describe the telescope, the photometer and their software, followed by the presentation of some first results. Finally, we discuss an upcoming upgrade and the complete instrumentation plan for Mercator.
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