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Design and performance of a Prime focus camera for the Thai National Telescope

The Thai National Telescope (TNT) is the largest telescope in Southeast Asia with a primary mirror of 2.3-meter diameter located at altitude 2457 meters, Chiang Mai, Thailand. This telescope is equipped with two photometric cameras and a medium resolution spectrometer. The maximum instantaneous field of view (FOV) is circular of diameter equal to 15 arcminutes, provided by the camera called the “TNT Focal Reducer”. In this paper, we present the design and performance estimation of a prime focus camera for the TNT with the objective to reach a FOV equal to 1 degree. The TNT prime focus camera is specified to operate over the spectral domain 0.400-0.850 μm for spectral bands g’, r’ and i’ of the Sloan Digital Sky Survey (SDSS) photometric system. This camera is designed to reach a resolution better than 2 arcseconds, slightly above the seeing limit in median atmospheric conditions. The prime focus camera is planned to be installed at the secondary mirror (M2) and mounted on a hexapod manufactured by Physik Instrumente (PI) company. This hexapod will provide a positioning accuracy of ±2.5 μm of 3-axis translation. The orientation will be adjusted with an accuracy equal to ±1.03 arcseconds of rotation. The prime focus camera comprises five lenses made of S-FPL53, N-BAK2, SF1, BAF50 and SK3. This camera includes one aspherical concave surface of conic constant equal to -0.076 and the other surfaces are spherical. We have dedicated tolerance analysis to calculate the effects of manufacturing errors, alignment errors and stability on the operational performance. The results show that the operational angular resolutions over the full field of view should be better than 1.61 arcseconds. The results of the stray light analysis show that the ghost irradiance should be five orders of magnitude smaller than the image irradiance, assuming that the second lens of the camera and the detector window have an Anti-Reflective coating of reflectivity lower than 1%.