Medial telescopes were invented in a seminal paper in 1899 and the term was introduced by Ludwig Schupmann.

The goals of this development by the Aachen engineer were:

  1. the complete elimination of chromatic aberration for refracting telescopes
  2. the construction of lens objectives larger than 60 cm in diameter

The lenses are similar in composition to classic achromats, except that the ‘layer’ between the two lenses of different glass types is not thin, so that the two lenses form a single lens. In Schupmann lenses, the two lenses are often separated by half or even the entire focal length (see below).

Schupmann himself developed these ideas very meticulously and with detailed experimental studies from 1893 until his death in 1920. He made several smaller devices, including for the public Urania Observatory in Berlin and for the famous moon observer Philip Fauth, who tested them extensively and praised them highly.

Task: Start your research on this topic with the term ‘telescope’ (Greek: teleskop) and find out about imaging errors in lens and mirror telescopes!

Nevertheless, the Medialfernrohre did not catch on.

Reasons:

  • It is a complicated system, which is relatively difficult for amateurs to replicate. The effort involved does not justify the benefits.
  • It was not until 1953 that definitive proof was provided that the system was truly free of colour errors and capable of surpassing the 1 m lens of the Yerkes Observatory. At that point, it was soon deemed no longer worthwhile to analyse the bright objects observed through lenses using Earth-based research:
    • The moon and planets were explored through observation by satellites and space probes
    • Luminous night clouds soon ceased to be a subject of research or the field of meteorology
    • Instead, observational astrophysics turned its attention to distant galaxies, quasars and other objects for which reflectors are better suited

FYI: A conference paper I wrote is available on my Academia page.

Errors

Lenses:

  • chromatic aberration (due to optical dispersion, i.e. the wavelength-dependent refraction of light)
  • astigmatism (cylindrical vision): cylinder axis

Mirrors:

  • spherical aberration
  • coma
  • distortions: barrel-shaped or cushion-shaped

Study

  • Research the path of light in a refracting telescope (lens telescope) and a reflecting telescope (mirror telescope)!
  • Sketch the light paths in the Kepler and Galileo telescopes and the light paths in the Newton and Cassegrain telescopes!
  • What distinguishes these two large families of telescopes?
  • Watch the following film clip and then summarise how a brachymedial telescope works!
  1. Mr Rolf is talking about a ‘refractor’ here. Is he right? Is it really a refracting telescope? Or is it a reflecting telescope, or perhaps something else entirely?
  2. How does Mr Rolf manage to use all the light from the entrance lens in his telescope?
  3. What special features has he built into the tube to achieve an undisturbed beam path? (Can you imagine why he takes such care with the air in the tube?)
  4. Research what a Schmidt mirror is! (Beam path) … Do you think this is also a form of mediumship? – Justify your answer!

This first brachymedial with a diameter of 70 cm (i.e. larger than 60 cm, as required by Schupmann) was not completed until 1953. E. Rolf’s final report concludes that perfect achromatisation has been achieved here. It still stands today in the Brandenburg town of Rathenow, where it was built, and due to monument protection regulations, it cannot be removed.

Consider why, despite this excellent result of the 1950s study, no brachymedial telescopes are used in research today.