We consider a semi-infinite open-ended cylindrical waveguide with uniform dielectric filling placed into collinear infinite vacuum waveguide with larger radius. Electromagnetic field produced by a point charge or Gaussian bunch moving along structure's axis from the dielectric waveguide into the vacuum one is investigated. We utilize the modified residue-calculus technique and obtain rigorous analytical solution of the problem by determining coefficients of mode excitation in each subarea of the structure. The main attention is paid to analysis of penetration of Cherenkov radiation into vacuum regions of the outer waveguide. Numerical simulations in CST Particle Studio are also performed (for long enough bunch exciting the first Cherenkov mode only) and an excellent agreement between analytical and simulated results is shown. The discussed structure can be used for generation of Terahertz radiation by modulated bunches (bunch trains) by means of high-order Cherenkov modes. In this case, due to high frequencies numerical simulations become extremely difficult while the developed analytical technique still remains the efficient approach for calculation of the radiation characteristics.