| The VMC260L’s catadioptric optical system is a sophisticated variation of the Maksutov-Cassegrain design that eliminates much of the weight and cost of conventional Maksutov optics. Originally developed by Russian optical designer Yuri A. Klevtsov, it provides a large aperture for deep space visual observing and imaging at a reasonable cost. The Vixen optical tube starts with a Maksutov’s two high-precision Pyrex spherical mirrors that help eliminate spherical aberration and yield a sharp image with no hint of false color (for example, chromatic aberration is up to 170x smaller than that of an achromatic refractor). But it doesn’t use the thick, expensive, and heavy full aperture meniscus corrector lens found at the front of a conventional Maksutov’s optical system. Instead, the VMC260LD uses a two-element corrector lens placed in the light path in front of the secondary mirror. One element is a quasi-afocal meniscus corrector lens, while the second is a negative Mangin lens. The combination serves the same optical purpose as a Maksutov’s full aperture meniscus corrector lens, but in a much smaller, lighter, and less expensive form. Like a Maksutov, the Vixen version offers excellent performance within the solar system, for high contrast visual observing and high-magnification eyepiece projection imaging, as well as high resolution/high power deep space observing of binary stars, globular star clusters, compact planetary nebulas, etc. Its unique design permits a faster focal ratio than a conventional Maksutov however (f/11.5 versus a Maksutov’s f/15). This makes the Vixen better suited than a Maksutov for observing a wide variety of faint deep space objects – diffuse and compact nebulas, galaxies, open star clusters, etc. This gives it an advantage in all-around observing over the more expensive pure-Maksutov design. Optical quality for deep sky viewing is excellent. With a quality long focal length wide field eyepiece the edge of the field is flat and very sharply focused. The VMC260L uses an internal moving primary mirror focusing system. Image shift is very low when compared to that of a similar-aperture Schmidt-Cassegrain telescope, however, typically on the order of only 7 arc seconds, even at very high power. Images snap cleanly into focus, unlike many other catadioptric scope types. The non-standard thread at the scope’s Cassegrain focus has an adapter ring that accepts an optional 2" star diagonal and 2" photo accessories. Two thumbscrews on the adapter ring hold the diagonal or accessories in place. The open tube (no full-aperture corrector lens) design eliminates the dewing on a conventional catadioptric scope’s full-aperture corrector lens that is common with Schmidt- or Maksutov-Cassegrain systems. No cumbersome dew shield or power-hungry dew heater system is needed to keep the VMC260L free of image-degrading dew all night long. The optical tube’s standard equipment carrying handle, reasonably compact size (only 26.4" long), and relatively light weight (only 24.3 lbs.) make it very easy to transport. It comes with a full length, but non-standard width, dovetail mounting rail that measures 78mm (3" wide) x 590mm (22.25" long) x 25mm (1" tall). A sliding balance bar fits onto the dovetail rail. This balance bar has a number of holes (see the feature image below for their spacing) that can be used to bolt it to a conventional short dovetail plate on an equatorial mount. The sliding balance bar allows the optical tube to be moved fore and aft to balance the weight of photo accessories. While the 24.3 lb. weight of the optical tube is theoretically light enough to be used on an optional medium payload German equatorial mount such as the Vixen Sphinx or GP-D2 Great Polaris, adding photo accessories would probably push the total scope weight beyond the payloads for which these mounts are rated. A more suitable mount for the VMC260L would be the Celestron CGE or Losmandy G11. The Vixen VMC260L comes standard with a wide-field 7x50mm finderscope in a quick-release dovetail bracket, a 2" eyepiece adapter, a dual T-thread flip-mirror 1.25" star diagonal with dual 1.25" eyepiece adapters, a convenient carry handle with a 1/4"-20 thread piggyback mounting bolt, and a dovetail mounting rail with slide balance bar. With 69% more light-gathering capacity than an 8" scope, and a large and flat field that is aberration-free, the VMC260L is a superb imaging platform well-suited for use with the new larger CCD cameras and 35mm film cameras. Here is the conclusion from an on-line review the VMC260L received: “I believe the imaging use of the scope is superb. Having owned C11, M10, C14 and numerous other instruments I can heartily say that this scope is a strong performer for imaging. I am not much of a visual user but visually the images are extremely pleasing. If your primary goal is imaging, this OTA is a very good optical step above the SCT/RCT and approaches the more expensive astrographs in quality."
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Terestrial Photography
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