A traditional Gregorian telescope features an intermediate focus, which makes the system longer than an equivalent fnumber
Cassegrain design. One could shorten the Gregorian system by inserting a flat mirror before the secondary
mirror. We explore the potential of this compact configuration for sky survey imaging with relaxed requirements for
angular resolution. A 0.5 m f/1.4 telescope with 4 deg full field is presented. The modified design consists of two
elliptical mirrors and a folding flat in between. A plano-convex field flattener is used near the focal plane. The telescope
optical performance is analyzed and possible improvements are discussed based on aberration balancing. A special
emphasis is given to stay light analysis and baffle designs are considered.
A new class of gradient refractive index (GRIN) lens is introduced and analyzed. The interior iso-indicial contours mimic the external shape of the lens, which leads to an invariant geometry of the GRIN structure. The lens model employs a conventional surface representation using a coincoid of revolution with a higher-order aspheric term. This model has a unique feature, namely, it allows analytical paraxial ray tracing. The height and the angle of an arbitrary incident ray can be found inside the lens in a closed-form expression, which is used to calculate the main optical characteristics of the lens, including the optical power and third-order monochromatic aberration coefficients. Moreover, due to strong coupling of the external surface shape to the GRIN structure, the proposed GRIN lens is well suited for studying accommodation mechanism in the eye. To show the power of the model, several examples are given emphasizing the usefulness of the analytical solution. The presented geometry-invariant GRIN lens can be used for modeling and reconstructing the crystalline lens of the human eye and other types of eyes featuring a GRIN lens.
Aspherical surfaces are difficult to manufacture, therefore we consider a possibility of using only spherical surfaces.
Two optical designs for 800-mm F/4.5 seeing limited telescope are considered: Ritchey-Chrétien and a two-mirror
Catadioptric system. The Ritchey-Chrétien system has a hyperboloidal primary mirror, a planoid aspheric secondary
mirror, and a two-lens field corrector, whereas the all-spherical system contains a Mangin mirror and a meniscus
corrector as the secondary group and a three-lens corrector. Our analysis of the image quality achieved in the two designs
shows that the all-spherical catadioptric system is an acceptable alternative to the Ritchey-Chrétien design.