Optical polishing for development of highly specular sunshield for radiant coolers of meteorological satellites of ISRO

Abstract

Radiant cooler is used to cool the infrared and water vapour detectors of meteorological payload of several geosynchronous satellites of ISRO. A three stage cooler was developed at Thermal Systems Group, ISAC to achieve temperatures required for different missions. It consists of a patch (cold stage) on which detector is mounted, intermediate cooling stage radiator, vacuum housing to enclose optics of Meteorological payload and sunshields as the first stage. These sunshields are mounted on Vacuum Housing. The thermal control surfaces of sunshield facing patch is chosen to have low solar absorptance (αs) and low emissivity (εIR) to minimize the thermal load on patch. Also to minimize the reflected solar radiation from sun shield surfaces on the patch, specular reflecting surface in solar spectrum wave length range is needed. To achieve such a specularly reflecting metal mirror, a specialized process was developed by carrying out a sequence of operations in collaboration with NAL, IIA and LEOS. The precision machining on the aluminium sun shield panels was carried out to achieve good flatness and finish. The machined panels were plated with electroplated nickel and heat treated to relieve the stress. The panels after lapping were electroless nickel plated, polished to optical finish and coated with vacuum evaporated aluminium. The surface characteristics like surface finish (40 to 60 Ao), solar absorptance (0.11), Emittance (0.03) and solar specularity (98%) were measured. The surface so produced has met the requirement of IR detector cooling to 105 K in coolers onboard INSAT 2A/2B/2E, KALPANA-1(METSAT), INSAT-3A Satellites. To achieve IR detectors cooling requirement of 95 K for advanced Imager and Sounder Meteorological Payloads, the improved processes was developed for specularly reflecting sunshields. It comprises of Precision Machining (Flatness 10μm), Single Point Diamond Turning, Electroless Nickel Plating (100 μm thickness), Optical Polishing (finish: <20 Ao) and improved Aluminium coating (αs:0.078, εIR: 0.022, specularity >99%) The new process was implemented on INSAT 3D Imager, Sounder and Filter Wheel Cooler sunshields (ETM and Flight Models). The integrated cooler assembly with sunshields was subjected to the acceptance tests (Electrical Insulation test, Vibration test, Thermal cycling and thermal balance test under simulated space environment) and it passed the tests successfully. The satellite is expected to be launched in 2010. The report gives a comprehensive account of the development of metallic sunshields for earlier ISRO mission and the journey covered to reach the latest one. This report describes the methodology used in optical polishing and surface quality achieved. It discusses on orbit performance of KALPANA-1 cooler, which show a good match of predicted and observed temperatures. The results of ground tests conducted on the coolers of INSAT-3D are also presented along with on orbit predictions.