An apparatus for providing an energy source for portable computer systems. Both of the first and second image forming cartridges has at least one handle. At least one of the valves is a check valve including a housing having a bore, a ball reciprocably guided in the bore by guide ribs, and a resilient retaining element capturing the ball in the bore and spring-loading the ball toward a valve seat. A recording power Pow_tilting of the tilting part of the tilting disk could be estimated by adding a given recording power Piwrtpwr to the power compensation Pow_comp. A second mask may be formed by removing a portion of the first mask.