The Einstein Tower: Establishment and Observation

The tower stands on the Telegraphenberg site of the Potsdam Astrophysical Observatory, south of its large refractor. Carl Zeiss in Jena began manufacturing the optical system and supporting structure in 1919. Construction started in summer 1920, and the main outer building was completed the following year.

Laboratories and working spaces were fitted out in 1922–1923, and the telescope was installed in 1924. Carsten Denker and colleagues’ technical paper dates the first observations to 6 December 1924. At the end of 1925, K. L. Wolf’s contemporary account described completed equipment and initial research. Preparations, building construction and scientific operation formed successive phases.

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Erwin Finlay-Freundlich sought to turn Albert Einstein’s theoretical predictions into measurable questions. He directed the scientific programme, architect Erich Mendelsohn designed the tower, and Einstein supported the project. Attention generated by the 1919 eclipse observations helped raise funds for a solar observing facility.

The Prussian state and the Albert Einstein Foundation established by German industry each provided half the construction funding. The foundation’s trustees included Einstein as chair, Freundlich and the observatory director, overseeing operating funds, additional instruments and building damage. Behind the tower’s striking appearance stood an institutional arrangement continually coordinating scientific needs and finance.

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The tower design avoided mounting heavy precision spectroscopic equipment on an ordinary telescope moving with celestial objects. Two plane mirrors at the top compensated for the Sun’s apparent motion and directed light down into the tower. An objective lens formed the image, and an inclined mirror turned the beam horizontally into the laboratory and spectrograph room.

Wolf’s 1925 account records a 60-centimetre objective and a separate wooden support tower carrying the experimental apparatus, sheltered from wind and weather by the outer building. The internal structure passed through floors without touching them and rested on dedicated piers, reducing the effect of footsteps and other vibrations. The optical path, supports and outer shell served different purposes.

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One central task was comparing the spectral-line positions of the same elements in terrestrial light sources and sunlight. Gravitational redshift predicts that light from the Sun’s stronger gravitational field shifts towards lower frequencies and longer wavelengths. Temperature and pressure also influence the lines, so measurements had to distinguish these effects.

The underground spectrograph room connected to both the telescope and physics laboratory, allowing celestial and laboratory light sources to be compared with the same equipment. Nearby facilities included a furnace producing high temperatures at low pressures, a darkroom and a photometry room. Electrical controls and reading telescopes reduced the need for staff to enter the temperature-stable room. The arrangement supported measurement of small shifts and intensity differences.

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Research extended beyond the building. Harald von Klüber, who had worked with Freundlich, recalled visiting researchers and experimental physics associated with astrophysics. Freundlich also organized Potsdam’s two eclipse expeditions to Sumatra, at Benkoelen in 1926 and Takengon in 1929.

The expeditions measured deflection of starlight near the Sun and collected coronal spectra and photometric data. Klüber noted that the 1929 deflection exceeded the theoretical prediction, a discrepancy Freundlich continued to investigate. Tower experiments compared solar and terrestrial spectral lines, while eclipse expeditions measured starlight deflection, testing different predicted phenomena.

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After the Nazis took power in 1933, Freundlich left Germany and the research institution was renamed and reorganized; observations continued. The later full-disk solar photographic archive recorded more than 3,500 images from 1943 to 1991. Pal and colleagues’ 2020 data-release paper states that the digitized record covers 1943–1990. Scanned images were calibrated for coordinates and brightness and made available for research.

Full-disk photographs record long-term changes in sunspots and other solar activity and can be compared with observations from other observatories. Through changes in instruments and personnel and digitization of its archives, the Einstein Tower continued beyond its early spectroscopy to support solar observations, instrument calibration and historical data preservation.

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