Schmidt and Schmidt-Cassegrain optical design systems are well known methods of achieving wide field of view optical imaging which are particularly desirable because they use primarily reflective elements. In a Schmidt system, a reflective spherical mirror focuses energy from a scene of interest onto a spherical focal surface. To correct for spherical aberration introduced by the spherical mirror, an aspheric refractive corrector plate (“Schmidt plate”) is placed at the center of curvature of the spherical mirror. Because of its simplicity, a Schmidt system is relatively easily aligned.
In a Schmidt-Cassegrain system, a second curved reflective element is placed between the primary spherical mirror and the focal surface of a simple Schmidt system, in order to produce an optical system which has a flat focal plane and is more compact, and which retains the basic correction capacity and other advantages of the Schmidt system.
A basic Schmidt-Cassegrain telescope is illustrated in FIG. 1. While there are many variations, (both mirrors spherical, both mirrors aspherical, or one of each) they can be divided into two principal design forms: compact and non-compact. In the compact form, the corrector plate is located at or near the focus of the primary mirror. In the non-compact, and as shown in FIG. 1, the corrector plate 42 remains at or near the center of curvature (twice the focal length) of the primary mirror 40.
One very well-constructed design example would be the concentric (or monocentric) Schmidt-Cassegrain, where all the mirror surfaces and the focal surface are concentric to a single point: the center of curvature of the primary. Optically, non-compact designs often yield better aberration correction and a flatter field than a compact design, but at the expense of longer tube length.
Schmidt-Cassegrain telescopes have several limitations. First, higher order (oblique) spherical aberration will limit the field of view which can be utilized without image degradation, even if monochromatic light is used. Second, at narrower fields of view, the image quality is limited by spherochromatism (spherical aberration caused by variation of wavelength in the incident light), which can be reduced by addition of a second plate of different material (an “achromatic corrector plate”) at the center of curvature of the spherical mirror.
In a number of applications requiring broader spectral coverage, the difficulty in correcting chromatic aberration, and the limitations in materials tend to reduce the effectiveness of the Schmidt plate correction concept. As a result, systems requiring broad band coverage and wide field of views have tended to use purely reflective designs (which eliminate the problem of chromatic aberration). Some of these systems use the Schmidt principle, with the aspheric deformation applied to a mirror, rather than to refractive plate. These systems generally involve off-axis apertures and/or off-axis fields of view, resulting in relatively larger system sizes. In all cases however, the aspheric surface has been rotationally symmetric, whether placed on a flat or curved folding mirror. For a detailed description of these prior design concepts, see I. R. Abel and M. R. Hatch, “The Pursuit of Symmetry in Wide-Angle Reflective Optical Design,” Proc. 1980 International Lens Design Conference, SPIE Vol. 237, p. 271 (1980).
The foregoing discussion of the prior art derives primarily from U.S. Pat. No. 4,576,452 which proposes to utilize in a reflective Schmidt or Schmidt-Cassegrain system configuration a corrector mirror in the form of a tilted nonrotationally symmetric aspherized mirror placed between the optical system and the target of interest, generally at or near the center of curvature of the primary focusing mirror. According to the inventors, the corrector mirror encodes both the correction for traditional aberrations associated with the Schmidt designs, as well as the wavefront error introduced by tilting the fold mirror.
One way to enlarge the field of view is to use a camera with a larger sensor, but these are very expensive. Another way is to use a telescope with a shorter focal length, such as a refractor, but these are generally much smaller telescopes. Further, high quality refractor telescopes suitable for imaging can be quite expensive, and their smaller apertures restrict the resolution and limiting magnitude of the system. Finally, the focal ratio of a small refractor is typically f/5 to f/7, but still long enough to require an equatorial mount and guide camera.
Even with a larger sensor in the camera, there are many potential issues remaining. For instance, many telescopes do not produce sharp stars very far off axis. Most SCTs experience coma, and all SCTs have field curvature. Thus, while the field of view might be reasonably large with a big sensor, the star images will not be ideal.
The amount of sky one can capture in an image is partially dependent on the focal length of the telescope used and the size of the sensor in the camera. A big field of view is a definite advantage because most deep-sky objects are quite large.
One solution is to collect image data at the focal point of the primary mirror (prime focus). A typical Schmidt-Cassegrain telescope uses a primary mirror which has a focal ratio of around f/2 and a secondary mirror which provides a 5× magnification, yielding an overall focal ratio of f/10. Because of the focal length, the amount of exposure time required to collect enough light for a useful image necessitates polar alignment and the guiding of the SCT to follow the target across the sky as the earth rotates. Removal of the secondary mirror converts the telescope to a focal ratio of approximately f/2. Because the exposure time required for this focal length is much shorter, the telescope does not need to be polar aligned, nor is guiding necessary. The magnification lost in the secondary mirror is not useful for viewing unresolved objects such as stars.
U.S. Pat. No. 6,587,265 suggests collecting data at the prime focus of a telescope. The system used in this prior art reference is large and complex, being designed primarily for very large telescopes. Because of the large size and weight, the system includes an adjustment mechanism to correct the relative position of the apparatus as the telescope moves. This adjustment mechanism, which comprises a series of six jacks called a Stewart platform, adds to the complexity of the system.
Moreover, the prior art includes no means for correcting optical aberrations that may occur as a result of the primary mirror. Removal of the secondary mirror, without providing a corrective lens, produces very poor images at the prime focus. The primary mirror alone generally suffers from spherical aberration and field curvature which is normally minimized by the secondary mirror.