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 already in 1612 he 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 their eclipses (by Jupiter's shadow) 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). As a result the objective needs to be stopped down to 30mm in order to get a reasonable image, making the scope f/50 (or f/49.67 to be more exact). It was for this poor quality that I wanted the objective lens in my observatory.

In order to make the quality of the objective lens clear, I did a single-pass Ronchi-test using a Gerd Neumann jr. 10 lines per mm (254 lines per inch) photographic Ronchi eyepiece, a ZWO ASI290MM camera with 35mm CCTV-camera lens, and Procyon (a star of magnitude 0.34 in the constellation Canis Minor) as a target.

The test was done both at full aperture (upper Ronchigram in adjacent image) and while being stopped down to 30mm (lower Ronchigram). As expected the Ronchigrams show the deliberate poor figure of the lens. At full aperture the central zone of the lens clearly deviates from the outer part, an indication of it's poor performance when not stopped down. But even when stopped down the lens is not perfect, showing curved Ronchi-lines. Being of bad figure and polish the objective lens needs to be stopped down to 30mm aperture (see figure 3), a method commonly applied in the early days of astronomy, in order to get reasonable images.

As projecting a full solar disc was impossible with the small diameter eyepieces Roger provided I later added two concave eyepieces for magnifications of 10x and 7.5x.

The OTA I created myself using a motorised Moonlite CS model focuser, an 80x1.5mm aluminium tube (see figure 8), 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. Soon I found out that focus with this type of telescope is not very critical (focus is achieved along several centimetres focus-range), so I removed the focus-motor and built in onto the Lunt LS80THA. Having a 60mm aperture I named the telescope GTT60 (Galilean Type Telescope of 60mm aperture).

Initially mounted in the Lunt rings (see figure 9), 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 4 and figure 7) 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.

When the ADM side-by-side plate was replaced in August 2020 by a Baader 8" one, I modified the rings so that they now are mounted directly on that plate.

In order to overcome the vignetting issues with the original eyepieces I ordered new lenses for the GTT60 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 12 and figure 13).

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.