Ground based radars and telescopes are the only means to provide information about the hazardous uncatalogued space debris objects. But due to their limited sensitivity and observational constraints they leave an observational gap of mm- and cm-sized objects in higher altitudes. However, information about smaller objects in all regions is required for the validation of debris models. Passive space based telescopes are the most cost effective means to provide the required data. Results from simulation runs with the PROOF tool show the impact of sensor orbit and viewing geometry on the kind and number of objects crossing the FOV cone. The influence of the design and operation parameters of the telescope on the detected objects is discussed. Observational aspects for three possible sensor orbits (SSO, GTO and GEO) are analysed. It is shown that space based telescopes employed in a low-cost mission can fill the observational gap to a high degree.