Soft X-ray telescope (SXT) built on the principle of grazing incidence optics was launched on-board AstroSat on September 28, 2015, and made operational on October 26, 2015. The telescope optics consists of two conical sections of approximate paraboloid and hyperboloid mirror segments arranged in Wolter type-I design. It comprises a total of 320 mirror segments made from aluminum foil of 0.2 mm thickness coated with gold on the front (reflective) side by the replication process that was first used in the Suzaku Observatory. The mirrors focus X-rays in the energy range of 0.3–8.0 keV on to a charged coupled device-based focal plane camera assembly at a distance of 2 m. We present here the pre-launch, ground-based calibration, and evaluation of SXT optics carried out at the X-ray Optics Laboratory at the Tata Institute of Fundamental Research. The SXT optics assembly was calibrated and evaluated experimentally, using scans by a collimated optical beam from a laser source, as well as using a full-aperture optical beam from an inverse telescope. A collimated beam of a red laser source was used to ensure the accuracy of mounting of individual mirror segments and the full aperture beam white light-emitting diode (LED) source was used to estimate the point spread function (PSF) of all 320 mirrors together. The approximate PSF obtained during the ground calibration was around 136.6 arcsec. These results were in accordance with the results obtained during the in-orbit calibration post launch.
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The authors would like to thank Mr. Irfan Mirza, Mr. Devendra Pathare, Mr. Sandeep Vishvakarma, Mrs Nilima Kamble, Mr. Jayprakash Koyande, Mr. Balu Bagade, Dr Nisha Yadav, Dr K Mukerjee, the staff members of DAA, TIFR, for their contribution to the experimental work carried out in the Soft X-ray Laboratory, TIFR. We specially acknowledge the efforts of former staff members of DAA, TIFR, Mr. Umesh Tonpe and Mr. Ganesh Gawde for their valuable contribution in the initial experiments of calibration of stand-alone optics. We thank Prof. G S Lodha and Dr Sanjay Rai from Raja Ramanna Centre for Advanced Technology, Indore, for their support in the X-ray characterization of individual mirror segments. We thank Dr N J Westergaard of Danish Technical University, Denmark and Dr H Kunieda, Dr Y Tawara and Dr Y Ogasaka of the Nagoya University, Japan for guidance in the design of the mirror assembly and fabrication of mirror segments. We also thank Mr. Koteshwar Rao and his team from the Indian Space Research Organisation, for their valuable guidance in setting up the apparatus for full beam aperture scans.
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Navalkar, V., Singh, K.P., Shah, H. et al. Pre-flight evaluation of the soft X-ray telescope optics aboard AstroSat. J Astrophys Astron 42, 103 (2021). https://doi.org/10.1007/s12036-021-09754-y
- X-ray optics
- soft X-ray telescope
- Wolter type-I optics
- X-ray astronomy