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9 January 1984 A 75 cm Balloon-Borne Telescope For Far-Infrared Astronomical Observations
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A balloon-borne 75 cm aperture Cassegrain telescope has been developed for far-infrared astronomical observations. This telescope uses a three axis stabilised platform which is designed to orient the optic axis with an accuracy of R',0.5 min of arc. A star sensor is used for the guidance; the axis of the star-tracker can be offset by angles upto ±4 deg. along two axes for pointing the telescope towards dark fields. The light weight mirrors (primary %40 Kgm) of the telescope are made in epoxy resin backed by aluminium blanks through a copying process. The secondary mirror is chopped at 20 Hz and the signals from the infrared detector are analysed by a phase sensitive detector (PSD). The gain of the PSD can be changed during the flight to provide an overall dynamic range of 1.5 x 105. The zero level of the PSD output and the relative phases of the chopper and PSD can be adjusted during the flight for optimum performance. The telescope was flown at an altitude of 32 km from Hyderabad on December 10, 1980 for photometric observations in 80um to 120um wave-length band. An aperture of 4 min of arc was used and a sensitivity of 100 Jy was expected. The flight performance of the telescope can be summarised by the following: (i) The r.m.s. pointing accuracy was r%)0.7 min of arc, (ii) stars upto mv%3.9 were used for guidance, (iii) the offset signal due to background infrared emission was less than 70000 Jy and (iv) the sensitivity deteriorated during the orientation mode to a value about 2000 Jy.
© (1984) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
S N Tandon, R R Daniel, S K Ghosh, K V. K. Iyengar, T N Rengarajan, and R P Verma "A 75 cm Balloon-Borne Telescope For Far-Infrared Astronomical Observations", Proc. SPIE 0445, Instrumentation in Astronomy V, (9 January 1984);


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