| Galilean Telescope Homepage | Site Map |
In the following tables, Galileo's five surviving lenses are identified by the Lens ID codes explained on our page about Building a Galilean Telescope. The Thickness and Full Diameter refer to the whole lens. All the lenses are mounted with stops, and the Clear Aperture is the size of the central diameter actually used in the telescope. Front Radius and Rear Radius refer to the radii of curvature of the front and rear optical surfaces, respectively. A positive radius on the front indicates a convex (outward bulging) surface. On the rear surface a positive radius indicates a concave (inward bulging) surface. The Focal Length is a measured parameter. The Refractive Index of the glass is inferred by computation based on the measured radii, thickness and focal length. The refractive index is dimensionless; all other parameters are given in millimeters.
| Lens ID | Full Diameter | Clear Aperture | Thickness | Front Radius | Rear Radius | Focal Length | Refractive Index |
|---|---|---|---|---|---|---|---|
| GAL 1A | 51 | 26 | 2.5 | 995.5 | -3,465 | 1,327 | 1.580 |
| GAL 1B | 26 | 11 | 3.0 | infinite | 48.5 | -95.2 | 1.509 |
| GAL 2A | 37 | 16 | 2.0 | 535 | -50,500 | 956 | 1.550 |
| GAL 2B | 22 | 16 | 1.8 | -51.5 | 51.5 | -48.8 | 1.527 |
| GAL 3 | 58 | 38 | 4.0 | 941.6 | -14,363 | 1,689 | 1.523 |
| Lens ID | Full Diameter | Clear Aperture | Thickness | Front Radius | Rear Radius | Focal Length | Refractive Index |
|---|---|---|---|---|---|---|---|
| GAL 1A | 51 | 26 | 2.5 | 950 | -2,700 | 1,330 | 1.528 |
| GAL 1B | 26 | 11 | 3.0 | infinite | -- | -94.0 | 1.516 |
| GAL 2A | 37 | 16 | 2.0 | -- | infinite | 980 | 1.546 |
| GAL 2B | 22 | 16 | 1.8 | -- | -- | -47.5 | 1.539 |
| GAL 3 | 58 | 38 | 4.0 | 940 | -12,000 | 1,710 | 1.510 |
In the cases indicated by "--", Greco, Molesini and Quercioli did not attempt to repeat Ronchi's measurements. They state they found the refractive indices to range from 1.510 to 1.546, but do not identify which numbers go with which lenses. We have independently calculated the refractive index of each lens based on values given in the table. Ronchi thought that the glass of GAL 1A had an anomalously high index. Greco, Molesini and Quercioli apparently believe this was due to an erroneous measurement, by Ronchi, of the radius of its first surface (995 vs. 950 mm). Please note that the eyepiece designated here as GAL 2B is known to be a later replacement for the missing original eyepiece, and is, therefore, not connected with Galileo.
The only dispersion measurements we know of are those made by Ronchi on the three objective lenses.
| Wavelength [Å] | GAL 1A | GAL 2A | GAL 3 |
|---|---|---|---|
| 4500 | 1,304 | 938 | 1,663 |
| 5000 | 1,317 | 950 | 1,678 |
| 5500 | 1,327 | 956 | 1,689 |
| 6000 | 1,333 | 962 | 1,697 |
| 6500 | 1,337 | 965 | 1,702 |
| 7000 | 1,339 | 969 | 1,705 |
The focal length of a singlet lens is completely determined by its thickness, the radii of its front and back surfaces, and the refractive index of the glass. Hence, given the measured thickness, radii and focal length the refractive index of the glass can be deduced. A graph of the refractive indices of Galileo's glasses, and how they vary with wavelength (their "dispersion"), as inferred from Ronchi's focal length measurements together with his measured thicknesses and radii, is given on our Building a Galilean Telescope page. Somewhat different indices, particularly for GAL 1A, are inferred if the newer radius measurements are used.
| Galilean Telescope Homepage | Site Map |
Last modified: July 14, 2006