Collection:
Louwman Collection of Historic Telescopes
Accession #:
LC 30
Sources:
Louwman Catalogue, p. 46.Louwman, P.J.K., and Zuidervaart, H.J., "A Certain Instrument for Seeing Far: Four Centuries of Styling the Telescope Illustrated by a Selection of Treasures from the Louwman Collection of Historic Telescopes". Wasscnaar, 2013. p.60.
Public Notes:
"Objective lens, polished by Christiaan Huygens, signed: ‘CHR. HVGENIVS Ao 1656 – PED II’ [= 11 feet or 335 cm]. The actual focal length has been determined in the optical laboratory of Rolf Willach as 365 cm. Biconvex lens Ø 6.2 cm. Radius of curvature: on the front side (with inscription) 400 cm; on the back 408 cm. Maximum thickness: 0.34 cm. Colour: grey-green. Many small bubbles in the lens. A paper diaphragm with an opening of 3.4 cm belonging to this glass has been preserved as well" (Louwman, and Zuidervaart, 2013).
Objective Notes:
This lens sits in a pasteboard well with a paper diaphragm on the reverse. The outer diam of the cell is 75.9 mm. There is writing on the black underside of diaphragm. Measured focal length of 3450 mm for outer edge (70% zone) at 5460 mm. After removing the stop blocking the central portion of the lens, and placing the lens in its original holder, which reduces the aperture from 62.3 mm to 30.9 mm, allowing only the central rays to pass through, the focal length increases significantly to 3635 mm. Ronchigrams show significant spherical aberration outside the zone of the original aperture stop, as expected. Objective is nearly equally biconvex, with the starside registering just over 4100 mm radius of curvature, the eyeside is typically a little shallower at 4150-4200 mm, when using a 40mm spherometer bell. A smaller spherometer bell (12.5 mm) showed a surprising variation from +-spot to spot, not just from center to edge. The 25 mm bell showed starside c 3500 mm (+/- 150), eyeside shallower but ranging from 3400 to 4000. The 12.5 mm bell reveals confusing data, all concave readings.
Dioptrice is made possible by the generous
support of the National Science Foundation, the National
Endowment for the Humanities, the Program in the History and
Philosophy of Science at the University of Notre Dame, and the Adler Planetarium & Astronomy Museum.
