A non-relayed reflective triplet and a double-pass imaging spectrometer including the reflective triplet configured as its objective. In one example the reflective triplet includes a primary mirror that receives and reflects electromagnetic radiation from a viewed scene and defines an optical axis of the optical system, a secondary mirror that receives and reflects the electromagnetic radiation reflected from the primary mirror, and a tertiary mirror that receives the electromagnetic radiation reflected from the secondary mirror and focuses the electromagnetic radiation onto an image plane to form an image of the viewed scene. The primary, secondary, and tertiary mirrors together are configured to form a virtual exit pupil for the optical system, the image plane being located between the tertiary mirror and the virtual exit pupil. The reflective triplet is on-axis in aperture and off-axis in field of view.

A three-aspherical-mirror anastigmat telescope comprises means for moving the third mirror linearly along the optical axis of the telescope so as to make the focal length of the telescope change to a plurality of focal lengths between at least a minimum focal length and a maximum focal length, a plurality of aspherical optical components respectively associated with the plurality of focal lengths, the third mirror having a new conicity determined from an initial conicity, the new conicity being determined so that the telescope has, in the absence of the aspherical components and for the minimum and maximum focal lengths, aberrations that are compensable by the aspherical components, the position and the form of the surface of each aspherical component being determined so as to correct the compensable aberrations of the telescope for the associated focal length and to optimize image quality in the first focal plane of the telescope according to a preset criterion.

Reflective optical telescopes and method of observing a scene. One example of a reflective optical telescope includes a primary mirror positioned along a linear axis extending between an entrance aperture and an image plane, configured to receive optical radiation via the entrance aperture, a secondary mirror positioned along the linear axis and configured to receive the optical radiation from the primary mirror and to reflect the optical radiation in a direction of the primary mirror, wherein the optical radiation received by the primary mirror is unobscured by the secondary mirror, a tertiary mirror positioned along the linear axis and configured to receive the optical radiation from the secondary mirror and reflect the optical radiation in a direction of the primary mirror, and a quaternary mirror positioned along the linear axis, configured to receive the optical radiation from the tertiary mirror and reflect the optical radiation to the image plane.

Reflective optical telescopes and method of observing a scene. One example of a reflective optical telescope includes a primary mirror positioned along a linear axis extending between an entrance aperture and an image plane, configured to receive optical radiation via the entrance aperture, a secondary mirror positioned along the linear axis and configured to receive the optical radiation from the primary mirror and to reflect the optical radiation in a direction of the primary mirror, wherein the optical radiation received by the primary mirror is unobscured by the secondary mirror, a tertiary mirror positioned along the linear axis and configured to receive the optical radiation from the secondary mirror and reflect the optical radiation in a direction of the primary mirror, and a quaternary mirror positioned along the linear axis, configured to receive the optical radiation from the tertiary mirror and reflect the optical radiation to the image plane.