In this paper we report on a velocity map imaging spectrometer with the implementation of an on-axis geometry where the electrons travel parallel to the ionizing beam towards the detector. Owing to a usually low photon flux from table-top soft x-ray sources, the instrument is optimized for high detector efficiency and high target density. The collinear design offers the possibility to freely choose the state and orientation of the polarization of light fields in streaking experiments. First results of an experiment using THz pulses for streaking using a Xe gas target are presented and compared to the simulated performance of the spectrometer.