high power view is a plus, such as resolving tightly packed globular clusters and double stars. The most notable early amateur astronomical type was the Questar 3-1/2 Maksutov Cassegrain introduced in 1954, a small-run, expensive model still available on the consumer market. The mid-1970s saw the introduction of mass-produced models by some of the major commercial manufacturers. More recently, low-cost Russian and, lately, Chinese mass-production has pushed the prices down even further. Many manufacturers currently produce Maksutov–Cassegrains, such as Explore Scientific, Intes, Intes-Micro, LOMO, Orion Optics, Telescope Engineering Company (TEC), Vixen, the Meade Instruments's ETX line, and the Synta Taiwan produced Celestron, Sky-Watcher and Orion Telescopes lines. The spot Maksutov–Cassegrain design has been used extensively in military, industrial, and aerospace applications. Since all of the optical elements can be permanently fixed in alignment and the tube assembly can be environmentally sealed, the design is extremely rugged. That makes it ideal for tracking, remote viewing, and radar calibration / boresighting, where instruments are subjected to severe environments and high g-forces. Rutten Maksutov–Cassegrains The Rutten Maksutov–Cassegrain (also called a Rumak or Sigler Maksutov) has a separate secondary mirror mounted on the inner surface of the meniscus corrector, sometimes similar to the corrector/mirror holder configurations found in commercial Schmidt–Cassegrains. This provides an extra degree of freedom in correcting aberration by changing the curvature of the corrector and the secondary independently. Specifically it allows the designer to aspherize the secondary to provide a much wider flat field than traditional spot Maksutovs, with less off-axis coma. Mounting the secondary on the corrector also limits diffraction spikes. This version is named after the work of Dutch optical designer Harrie Rutten. Sub-aperture corrector Maksutov–Cassegrains Maksutov noted in his designs that instead of using a full-aperture corrector, a small sub-aperture corrector could be placed in the converging light cone of the primary mirror and achieve the same effect. In the 1980s Dave Shafer and Ralph W. Field came out with sub-aperture Cassegrain designs based on this idea. The design reduces the mass and "cool-down time" of a full-aperture corrector. It has the drawbacks of an open, unsealed tube and requires a spider assembly to hold the secondary mirror and corrector, which inevitably affects image quality through diffraction artifacts. Also since the light passes through the corrector twice, the number of surfaces involved is increased, making it difficult to achieve good aberration correction. Sub-aperture corrector Maksutovs are currently manufactured