The Cassini–Huygens SSP refractometer : ref

The refractometer (REF) in the Huygens probe Surface Science Package (SSP) is a linear critical-angle refractometer ; it covers the refractive index range 1.2500–1.4500 with a discrimination of 0.0005, giving an accuracy of about 0.5% in the relative abundance of methane and ethane in the expected mixture on the possibly liquid surface of Titan. Light-emitting diodes at 635 nm provide illumination ; the output image shows a cut-off edge whose position is a linear function of refractive index. The image intensity profile is sensed by a 512-element photodiode array, and digitised ; it is then differentiated numerically, as this is found to enhance the discrimination, and may give information about any suspended matter. The main structure of the instrument is made of titanium, with a synthetic sapphire prism.