100 cm Far Infrared Balloon Borne Telescope
The TIFR Infrared Astronomy Group has been carrying out far infrared (FIR) observations using the TIFR 100 cm balloon-borne telescope (T100). The telescope is f/8 Cassegrain system with a 100 cm primary aperture. The secondary mirror is 27 cm in diameter and is vibrated at a frequency of 10 Hz in order to effect sky chopping (for online background subtraction) with an amplitude of ~4.5 arcmin. The FIR telescope is being regularly flown from TIFR Balloon Facility at Hyderabad for studying Galactic star-forming regions at long wavelengths beyond 100 microns. The T100 is one of the largest payload to be flown by balloon world-wide and weighs about 1000 kg with ballast. This has been indigenously developed at TIFR.
Recently, as a part of the TIFR-Japan collaboration in balloon-borne FIR astronomy, a Japanese (Institute of Space & Astronautical Science and Nagoya University) Fabry-Perot Spectrometer (FPS) has been successfully interfaced with the T100 for a launch from TIFR Balloon Facility at Hyderabad. The FPS (R ~ 1700) is tuned to the astrophysically interesting fine structure line of [C II] at 157.74 microns. [C II] line is one of the most dominant cooling lines in Photo Dissociated Regions (PDRs), especially in low gas density PDRs. The spectrometer consists of a pair of Fabry-Perot interferometers, viz., the Scanning Fabry-Perot (SFP) and the Fixed Fabry-Perot (FFP). Each interferometer consists of etalon plates which are composed of nickel mesh, suitable for FIR observations. The SFP has an order of > 70 and has a finesse of ~26. The FFP has an order of only 3 and acts as an order sorting filter allowing only the desired wavelengths to pass through it and reach the detector. The detector for the FPS is a single stressed Ge:Ga photoconductor which is cooled to 1.8 K using liquid 4He at low vapour pressure.
The T100 along with the upgraded FPS tuned to astrophysically important line of [C II] at 158 microns, was last launched on March 18, 2018, from the TIFR Balloon Facility, Hyderabad, during the winter/summer 2018 flight campaign. The T100 performed as designed with a Japanese FPS at the focal plane. The performance of the FPS system is being investigated using the FIR data collected during the ~5.5 hours of float at 31.5 km altitude.