The selection of optical-system parameters plays a key role in star sensors, which can affect the sensor′s performance. When determining the parameters, sky background radiance and parameters of image sensors should be considered. Sky background radiance under different situations is simulated by Modtran. A more accurate formula to estimate the detectable stellar magnitude limit is proposed, considering the effects of energy focusing degree and sensor noise. With this formula, the factors affecting the detectable stellar magnitude, detection probability and accuracy are deeply analyzed. The feasibility of typical optical-system parameters is studied. The results offer important references in the determination of optical system of all-day used star sensors.