A means for examining the sun in monochromatic light in which the separation of the mirrors of either a catadioptric, or pure two-mirror, telescope is altered to produce a virtual object for a converging lens, termed a "forcing lens," this lens being so located between the virtual object and a narrow-band, Fabry-Perot, etalon-type, interference filter, that the convergent beam of light from the telescope passes through the interference filter at a high focal ratio, relative to the telescope's conventional focal ratio, before it traverses the forcing lens, which in turn forms the optical system's only real image, at a different, and much lower, focal ratio. Image contrast is markedly enhanced, relative to the contrast obtained by the same telescope at its conventional focus. By means of a graduated series of entrance pupils, which may be generated by a graduated series of broad-band pre-filters, a step-wise-zoom, constant-contrast, solar monochromator may be constructed. Formulas are provided for parfocalizing a series of forcing lenses. By means of such a series, real-image height may be adjusted, allowing the optimization of an eyepiece/forcing-lens combination for viewing the solar disk and surrounding prominences simultaneously, out to a considerable distance from the solar limb.

A primary mirror 22 defines an active concave surface 24, a first planar 40 and a first concave 42 alignment surfaces, each facing an active surface 30 defined by a secondary mirror 28. The secondary mirror also defines a second planar 46 a second concave 48 alignment surfaces, each opposite the active surface 30. Alignment beams 64, 66, 68, 70 are reflected from each of the four alignment surfaces, which all face the same direction, to adjust the mirrors. The first planar alignment surface is used to adjust tip and tilt of the primary mirror; the first concave alignment surface is used to position a retro reflector 50 relative to the primary mirror; the second planar alignment surface is used to adjust tip and tilt of the second mirror, and the second concave alignment surface is used to adjust the second mirror in the x, y and z direction relative to the first mirror. The apparatus and method are described for two and three mirror telescopes.

A method of manufacturing an optical system comprises assembling an optical element on a mounting frame thereof. The mounting frame is disposed on a rotary table having an axis of rotation, and the mounting frame is adjusted such that a predefined axis of symmetry thereof is parallel to the axis of rotation. The optical element is placed on the mounting frame, and an interferometric measurement of a surface of the optical element is performed. The interferometric measurement is analyzed to arrange the optical element relative to the mounting frame such that a predefined axis of the optical element is parallel to the axis of rotation.

A reference-height calculating unit calculates an original height of a distortion detecting mirror when a distortion detecting mirror and a reference mirror are set in parallel to each other. A height measuring unit measures mirror heights when the distortion detecting mirror is tilted in directions of an X axis and a Y axis. A contour-line calculating unit calculates a pair of contour lines based on the mirror heights. An intersection calculating unit calculates an intersection of the contour lines. A storing unit stores coordinates to which height of predetermined coordinates are moved by distortion in a correction table. A distortion correcting unit corrects the distortion based on the correction table.

An image pickup optical system has an optical element of a transparent, optical material including an entrance surface, at least one reflective surface, and an exit surface. The optical element has an optical power, the image pickup optical system has an image pickup element including a plurality of pixels. On the image pickup element an image is formed by light from an object through the optical element. The optical material is a material having an index change amount .DELTA.n from an absolute dry condition at the temperature of 50.degree. C. to saturation under a circumstance of the temperature being 50.degree. C. and the humidity being 90%, the index change amount .DELTA.n satisfying the following condition: .DELTA.n.ltoreq.F.delta.r.sub.0.sup.2 /(2f.sup.2 L) where F is the F-number of the image pickup optical system, f is the focal length of the image pickup optical system, .delta. is the pitch of the pixels of the image pickup element, L is the optical path length of the optical element along the reference axis, and r.sub.0 is half of the length of the optical element along a direction perpendicular to a plane including the reference axis.