Schupmann Medial Telescope

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The Schupmann Medial Telescope is a type of telescope developed by Ludwig Schupmann (1851–1920) . It is a catadioptric optical system in which elements of a telescope are combined with elements of a reflecting telescope to eliminate chromatic aberrations.

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Due to the different refraction of light , each lens produces a more or less severe color error that reduces the performance of a telescope. These errors can be compensated for with the telescope by combining different glasses (see achromat and apochromat ). In Schupmann's time, however, the so-called flint glasses had a yellow tinge and were relatively unstable. The manufacture of large, defect-free glass lenses is extremely difficult; with increasing thickness, the glasses increasingly absorb light. In addition, refractors with large focal lengths require a lot of space - the focal length corresponds to the length of the tube . Reflectors ( mirror telescopes ), on the other hand, have no color defects and can be made shorter because the beam path is "folded". At the time, however, mirror diameters of more than 60 cm were problematic, as the mirrors sag under their own weight, massive mounts were required, the mirror surfaces tarnished quickly and therefore had to be re-mirrored several times a year.

At the beginning of the 20th century it was not clear to which system - lens or mirror - the future in the professional field belonged.

Construction principle

Schupmann devised a type of telescope that would combine the advantages of both systems. To do this, he arranged corrective lenses and mirrors behind the objective lens, which eliminated all color errors. He published his thoughts on the subject in 1899 in his work “The Medial Telescopes - A New Construction for Large Astronomical Instruments”, in which he described two technical solutions to the problem - the medial refractor (medial means “in the middle” between one Refractor and a reflector) and the brachymedial refractor (a telescope with a very short design). In the same year he applied for a patent for a medial refractor in the USA .

The objective lens of the medial refractor consists of crown glass with a low refractive index and is ground slightly biconvex . Behind the objective lens is a field lens connected to a totally reflective prism . The light rays of different colors are guided through the beam path in the prism in such a way that they are reunited at the subsequent correction optics. It also deflects the light beam at right angles, which leads to a comfortable viewing position. The field lens also partially corrects the color deviations. The light then hits a Mangin mirror , consisting of a convex - concave pair of lenses, the rearmost surface of which is mirrored. From the Mangin mirror, the light reaches the eye of the beholder via an eyepiece .

When the light passes through the optical arrangements, the different wavelength ranges of the light are combined again to form white light. The disadvantage is that the construction causes light losses of 0.2 to 0.3 size classes .

Application history

In 1901, a larger medial refractor, manufactured by Reinfelder & Hertel in Munich , was tested under professional conditions at the Urania observatory in Berlin and compared with the 30 cm refractor of the observatory. The very good image quality was praised, but light loss due to the many surfaces (the telescope had ten refractive and two reflective surfaces) and the narrow field of view were criticized. In addition, there were significant difficulties in attaching a micrometer to the device.

Schupmann tried out improvements for ten years before he dared to manufacture another large instrument. In the meantime, the Americans George Ellery Hale and George Willis Ritchey at the Mount Wilson Observatory succeeded in building a reflector with a diameter of 1.5 m, the image quality of which exceeded all existing refractors. This made it clear that in the future the world's major observatories would be equipped with reflectors, and Schupmann's system became less important.

One of Schupmann's friends was Philipp Fauth , the most important visual observer of the moon and advocate of world ice theory . In 1911, at Fauth's request, Schupmann had a medial refractor with a 38.5 cm opening made. The instrument showed excellent image quality and was used by Fauth to produce lunar maps until 1941. It was destroyed towards the end of the Second World War .

In 1917 a slightly smaller model with a 32.5 cm opening was made, which was extensively tested by the employees of the Königsstuhl observatory and was also found to be excellent. The device is now at the Pfaffenwald University Observatory .

Anton Kutter , the inventor of the Schiefspiegler , built three Schupmann medials with an opening of 12.2 to 27 cm.

Schupmann's idea was to create large telescopes with an opening of more than 1 m for the professional sector. However, this could not be realized. Despite the good performance, the Medial telescopes could not ultimately prevail. This may also be due to the fact that ten months after Schupmann's death the “Zeitschrift für Instrumentenkunde” reported on the telescopes. The article, which could no longer be read by Schupmann, was incomprehensible and contained numerous errors. In the USA , however, there is a "fan base" of amateur astronomers who build Schupmann's own medial telescopes.

The Rolf refractor in Rathenow (also Rathenower refractor ) is a Schupmann medial telescope with a 70 cm aperture and a focal length of 20.6 m. It was privately built by the engineer and amateur astronomer Edwin Rolf from 1949 to 1953. The listed, renovated telescope is now on the grounds of the Rathenow Optics Park .

It was not until 50 years later that a larger telescope was built with the optical configuration of a Schupmann medial. It is the Swedish 1-m " Swedish Solar Telescope ", which was built in 2002 on the Canary Island of La Palma .

See also

Rathenower refractor

literature

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