Abstract: In a telescope having primary and secondary reflectors, wherein the actual primary reflector surface deviates from an ideal primary reflector surface, such deviation is compensated for. At least one intermediate element forms an image of the primary surface onto the secondary surface, so each point on the secondary surface corresponds to a point on the primary surface. The secondary surface is formed with a deviation from an ideal secondary surface, with the "piston" distance of each point on the actual secondary surface equal to the piston distance of a corresponding point on the actual primary surface from the ideal primary surface. It is found that this results in electromagnetic (e.g., light) rays which strike a deviating area of the actual primary surface being brought to the same focus as if the actual primary surface did not have a deviation from an ideal primary surface.