Jim’s 10 inch f/8 catadioptric Herschelian Schiefspiegler telescope

Even amateur planetary observers and photographers need big aperture telescopes because when it comes to resolution, size does matter. A larger scope will resolve smaller planetary details. Every amateur astronomer also functions on a budget. Apochromatic refractors larger than 8” are almost impossible to fabricate because the telescope industry depends on camera companies like Canon and Sigma to supply special dispersion glass blanks to grind and 8” is the upper limit for even their exotic telephoto lenses. f/5 also appears to be the fastest limit for good color correction and with an 8” objective would yield a 1 meter long optical tube. So from a cost and portability perspective, large telescopes have to be reflecting telescopes.

The traditional favourite for many decades has been the Celestron 14” Schmidt Cassegrain, compact enough to travel with and to be lifted and mounted by a single person. Along with a convenient eyepiece location there are few alternate designs that also offer serious aperture.

The only issue with the cassegrain design is the central obstruction of the secondary mirror. This causes energy to be diverted to the first diffraction ring and reduces image contrast which can result in real loss of resolution.

As a result, astronomers have been experimenting with designs that tilt the primary out of the line of sight of the secondary mirror so that there is no longer any obstruction. This unfortunately introduces coma and astigmatism into the image which can only be corrected with custom ground refractive elements making construction likely beyond the ability of most amateur astronomers.

Illustration-02-HerschelAndHisSisterAtTheTelescope-250b — William Herschel observing with the largest telescope of his time, a Newtonian reflector with a 40 ft focal length and a primary mirror diameter of 50 inches. Herschel was the first to tilt the primary so the focal point was exiting off to the side of the optical tube so that he could eliminate the secondary mirror. Mirrors of the time were constructed from speculum metal and polished to a high sheen. Still reflectivity was low, less than 70% and by eliminating the secondary mirror he could maximize the brightness of light going through his handheld eyepiece. His devoted sister Caroline stands below and was his assistant who later became one of the first women to be recognized for their scientific discoveries. The scope was commissioned by King George III in 1789 for the price of €4000 but the resultant coma and astigmatism made the scope useless except for low powered wide field viewing. Herschel quietly reverted to his original scope, a slower 18″ f/13 twenty foot reflector which allowed magnifications up to 160X.

Still, people persisted with the idea of tilted, unobscured reflectors. German film director Anton Kutter introduced his Schiefspiegler which means “oblique refractor” in German in the 1950s and continued to refine their design through to the 1970s. It has a tilted concave primary and a tilted convex secondary but does not correct for coma hence requiring a slow scope in the order of f/25.

Trischief — Kutter addressed the coma problem with the trischiefspiegler in the pages of *Sky & Telescope* in 1975 with the addition of a third concave mirror.

catschief — Kutter also introduced refractive elements to simplify the design and eliminate residual coma. Here he placed a tilted planoconvex lens about halfway between the secondary mirror and focal point. All these designs were very successful and yielded apochromatic unobscured large aperture scopes but are very difficult to make because they require custom grinding of lenses and mirrors to a specific prescription.

Fortunately in 2008, retired optician Ed Jones introduced the world to his personal tilted telescope design, the Catadioptic Herschelian Schiefspiegler or Chiefspiegler. The main advantage of his design is that it’s possible to buy off the shelf optics to make it. The primary mirror is typically an 8” Newtonian mirror f/10 or slower and the planoconvex and planoconcave lenses are 2” in diameter (50 mm) and of equal but opposite focal length. The lenses can be made from BK7, quartz, K5 etc but the lenses have to made from the same type of glass.

To better locate the eyepiece, the optical path is folded with a optical flat at some point before the corrective optics. Axial astigmatism can be corrected by changing the tilt of the lenses. The front lens is tilted counterclockwise and the rear lens clockwise and the production of axial lateral color can be minimized by slightly decentering the rear lens. Although almost any focal length can be used, 500 mm is better than 200 mm because the shorter focal length lenses require greater tilting and more separation and more axial color production. The color produced is less than the difference between achromatic and apochromatic refractors so may be perfectly tolerable. But you can eliminate it all together by using lenses of unequal radii and a slower Newtonian mirror. It’s just easier to buy a matching pair of PCX and PCV lenses premade, but you may need to regrind one of them to get superior color correction.

Since most commercially available lenses are rarely larger than 2″ in diameter, this limits the primary mirror to no larger than 10″ diameter to prevent vignetting.