GTT60: Galilean Type Telescope of 60mm aperture
One the subjects encountered during my research on nautical instruments that fascinated me most was Galileo Galilei's early seventeenth century proposal for longitude finding at sea. Galilei had discovered the first four moons of Jupiter (Io, Europa, Ganymede, and Callisto) in 1610 (for pictures see the astrophotography page) and soon realised that they formed a heavenly clock that could potentially be used as a reference clock in the determination of longitude by comparing tabulated times of the moments its moons touched the Jovian limb with times from onboard observations.
In order to test Galilei's method a Galilean Type telescope was added in September 2018. For this I acquired a 60mm objective lens (focal length 1490mm) and a 30x concave ocular (focal length -50mm) from Dr. Roger C. Ceragioli, an optical engineer and professional telescope maker at the Richard F. Caris Mirror Lab, University of Arizona, Tucson (USA). In December he provided additional concave oculars for 20x and 40x magnification. He had made these four lenses for his own research and deliberately made the objective lens of bad figure and polish to mimic the quality that could be achieved in Galilei's time (results of this can be found at the ATS Forum).
Later I added two concave eyepieces for magnifications of 10x and 7.5x in order to be able to project a full solar disc. Being of bad figure and polish the objective lens needs to be stopped down to 30mm aperture (see figure 2), a method commonly applied in the early days of astronomy, in order to get reasonable images.
The OTA I created myself using a motorised Moonlite CS model focuser, an 80x1.5mm aluminium tube (see figure 7), some additional home-made aluminium and bronze parts, and a 2" star diagonal. The scope saw first light at daytime on 28 September 2018 and at night on 3 October that year. Having a 60mm aperture I named the telescope GTT60 (Galilean Type Telescope of 60mm aperture).
Initially mounted in the Lunt rings (see figure 8), the GTT60 was given its final position between the C11 and Esprit in the last week of November 2018 (see figure 1). In order to be able to align it with the other telescopes I created a set of rings (see figure 3 and figure 6) that can be adjusted in right ascension and declination. These rings are mounted on cross-bars that are fastened to the ADM Losmandy style side-by-side plate.
So far the eyepieces I had were not capable of projecting a sufficiently large and unvignetted image of the Sun.
In order to overcome this I ordered new lenses for my Galilean Type Telescope in March 2021. They are 25.4mm in diameter, plano-concave, and have focal lengths of -100mm and -75mm. As they would not fit a standard eyepiece I constructed a whole new eyepiece for them out of stock aluminium. A first test using the Moon as object revealed they both allow full projection of it (and thus of the Sun).
On 25 March I had first light with the -75mm lens at the Sun and indeed it gave a crisp view of it, though of course being slightly affected by chromatic aberration due to the GTT60 single plano-convex objective.
The projected sun has a diameter of 80mm at a projection distance of 340-350mm from the eyepiece (see figure 11 and figure 12).
The data I collect in this way is used by Leif Svalgaard to build a calibration point for early modern sunspot series in order to get a better understanding of the long-term magnetic cycle of the Sun. Leif Svalgaard can always use more data, so for those interested in joining the project, more information can be found on the AAVSO website.
On 10 June 2021, the GTT60 was also used to observe the partial solar eclipse in the Netherlands. Images of the eclipse can be found in the Sun section.
If you have any questions and/or remarks please let me know.
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SkyWatcher Esprit 150ED Celestron C11 XLT EdgeHD Lunt LS80THA GTT60 SkyWatcher Explorer 300PDS 10Micron GM3000HPS Bath-interferometer Foucault-test