Imaging lens system for tracking celestial body

An imaging fixed-focus lens system for tracking a celestial body, which is a complicated-refractive-type Cooke lens structure composed only of refractive optical elements,

• wherein all lens elements are made entirely of glass;
• all lens elements are separate from one another, with no cemented lenses;
• all surfaces of the lens elements are spherical, and
• the imaging lens system comprises, sequentially in a direction from an object side to an image side of the imaging lens system:
• a first lens group composed of a plurality of lens elements, wherein a ratio of a focal length of the first lens group to an effective focal length of the imaging lens system is greater than 0.678 and less than 1.076;
• a second lens group composed of a plurality of lens elements each having a negative dioptric power value, wherein a ratio of a focal length of the second lens group to the effective focal length of the imaging lens system is greater than −0.461 and less than −0.377;
• and
• a third lens group composed of a plurality of lens elements, wherein a ratio of a focal length of the third lens group to the effective focal length of the imaging lens system is greater than 0.498 and less than 0.583,
• wherein the imaging lens system has an f-number greater than 1.4 and less than 1.6, and
• wherein the imaging lens system has a characteristic of an inverted telephoto configuration, namely with a ratio of a total length from a first lens surface of the imaging lens system in the direction from the object side to the image side of the imaging lens system to an image plane to a total effective focal length of the first to the third lens groups being greater than 1.82 and less than 2.61.