The historic Great Melbourne Telescope of 1869 originally utilised Huygenian (pronounced “high-gee-knee-yan”) eyepieces. These original eyepieces still exist and are preserved in Museum Victoria’s collection, but the precise specification of these eyepieces was not fully known. Recently a thorough and detailed measurement of each eyepiece was conducted. The overall dimensions of each eyepiece element and the curvature of the lenses were measured, and a Foucault knife edge test was performed to determine the focal lengths.
ASV volunteer Barry Adcock utilised his own home made equipment to perform the Foucault knife edge test.The test measured the back focal length of the eyepiece lens, using red, yellow and blue LED light sources. The figures obtained were used to calculate the refractive index at each colour and thus the dispersion of the glass. Both refractive index and dispersion are necessary in order to specify the glass used.
The Foucault test can be illustrated using the exaggerated diagram below. The light source is narrowed to simulate a point source similar to a star. The light rays pass through the lens being tested and are reflected off a flat mirror behind the lens. The reflected light passes back through the lens again and is observed at the knife edge at the place of the original LED light source. The spacing between the lens and the knife edge is adjusted (while the eye remains still and can still see the light source), until the reflected image of the light source looks to be shadowed, i.e., is nulled. At this point the system is in focus and the back focal length can be measured with a rule as the spacing between the lens and the knife edge. Calculations are then performed to determine the actual focal length of the lens.
The curvature of the eyepiece lenses was measured using a spherometer. The spherometer has a micrometer located centrally in the circular tool. Around the perimeter on the base of the tool there are three ball bearings held captive in a triangle formation in a precision machined groove.The diameter of the spherometer base may come in different sizes to measure different sized lenses. The spherometer is first calibrated on an optically flat piece of glass so that the micrometer central point and ball bearings are all in equal contact with the surface of the flat glass plate. The spherometer is then placed onto the lens to be tested and the micrometer adjusted to record the sagitta, which is the difference between the plane of the ball bearings relative to the position of the centrally located micrometer point.
Once the curvature and focal length of a lens are known, the refractive index of the glass can be calculated. This data is essential to design new replica eyepieces for use with the restored telescope and it also gives an insight to the availability and specification of optical glass of the 1860s.