Optical Profilometry as a Non-Destructive Technique to Quantify Engraving on Medieval Brass Astrolabes
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Optical Profilometry as a Non-Destructive Technique to Quantify Engraving on Medieval Brass Astrolabes
Brian D. Newbury and Michael R. Notis Materials Science and Engineering, Lehigh University, 5 East Packer Avenue, Bethlehem, PA 18015, U.S.A. ABSTRACT Optical profilometry has been performed on an astrolabe dated 1556 AD and attributed to the famous maker Gualterus Arsenius of Louvain, Belgium. In this non-destructive technique, a beam of white light is impinged on the sample and combined with the chromatic aberration technique to accurately measure surface features. With machine parameters set to a vertical resolution of 0.1 µm, lateral resolution to 6 µm and a 3 mm depth of field, various engraving features on the astrolabe were studied. Both hand-scribed and stamped markings are present on the astrolabe components. Inspection of the hand-scribed date on the instrument’s mater allows one to quantify the shape and dimensions of the tool used, as well as the direction and inclination of the engraving tool during inscription. The results of this analysis give information about how the astrolabe was constructed in one of the Renaissance’s greatest scientific instrument workshops. The results are compared to similar engravings on an undated and unsigned Islamic astrolabe tympan. INTRODUCTION The astrolabe is an analog computer which utilizes a stereographic projection map of the stars and Sun’s position to perform many astronomical and navigational calculations and to determine the time of day. Originally developed by the Greeks, the astrolabe was rediscovered by Islamic astronomers studying Greek texts in the 8th century [1]. The instrument thrived in Islamic lands and was introduced into Europe from Moorish Spain via the Pyrenees in the 11th century [2]. Often considered the first modern scientific instrument, the astrolabe remained in use relatively unchanged for over 600 years in Europe and over 1000 years in Islamic lands. Even though the astrolabe represented the pinnacle of pre-telescopic astronomical instruments, it was not uncommon for astrolabe manufacturers to leave their work unsigned. Without a signature to assign an instrument to a manufacturer, modern scholars have used stylistic analysis to attribute an instrument to a school or individual manufacturer based on design and engraving characteristics [3, 4]. However, these techniques are largely qualitative and subject to the judgment of the researcher. In this study, the emerging non-destructive technique of optical profilometry is employed to examine the engravings present on a 1556 A.D. Gualterus Arsenius astrolabe and an unsigned and undated Islamic astrolabe tympan1. It is the aim of this research to quantify the engraving characteristics of the astrolabe components such as graver dimensions and shape, allowing for quantitative comparisons between signed and unsigned astrolabes in future research.
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Tympan – a tablet that fits below the rete (see later footnote) which contains a stereographic projection of the celestial sphere’s coordinat
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