For future divertor studies on TCV measurements of electron temperature and density and their spatial gradients along open flux surfaces will be of major interest. A Thomson scattering system is proposed for this purpose, which takes advantage of the good access via horizontal ports on three vertical levels. The modular design of the existing Thomson scattering diagnostic on TCV offers the possibility to modify it for simultaneous measurements in the main plasma and in the divertor region using the same laser. Arranging for a second passage of the laser beam at a radial position shifted towards the inside, measurements can be made along the open flux surfaces between the X point and the outer divertor strike point. The optical system is designed to share the camera lens with the existing system. After installation of additional fiber bundles and another set of filter spectrometers, the system will be capable of measuring temperature and density at ten selected locations in the divertor zone with a resolution in the vertical direction of 15 mm. Using numerical simulations, a set of interference filters has been selected to cover the range from 100 to 5 eV. (C) 1999 American Institute of Physics. [S0034-6748(99)53601-0].