The Ultra-Violet Imaging Telescope (UVIT) is one of the payloads in AS-TROSAT, the first Indian Space Observatory. The UVIT instrument has two 375 mm telescopes: one for the far-ultraviolet (FUV) channel (1300-1800Å), and the other for the near-ultraviolet (NUV) channel (2000-3000Å) and the visible (VIS) channel (3200-5500Å). UVIT is primarily designed for simultaneous imaging in the two ultraviolet channels with spatial resolution better than 1.8 , along with provision for slit-less spectroscopy in the NUV and FUV channels.The results of in-orbit calibrations of UVIT are presented in this paper.

The performance of the ultraviolet telescope (UVIT) on-board ASTROSAT is reported. The performance in orbit is also compared with estimates made from the calibrations done on the ground. The sensitivity is found to be within ~15% of the estimates, and the spatial resolution in the NUV is found to exceed significantly the design value of 1.8" and it is marginally better in the FUV. Images obtained from UVIT are presented to illustrate the details revealed by the high spatial resolution. The potential of multi-band observations in the ultraviolet with high spatial resolution is illustrated by some results.

We present early results from the Ultra-Violet Imaging Telescope (UVIT) onboard the ASTROSAT observatory. We report the discovery of a hot companion associated with one of the blue straggler stars (BSSs) in the old open cluster, NGC188. Using fluxes measured in four filters in UVIT's Far-UV (FUV) channel, and two filters in the near-UV (NUV) channel, we have constructed the spectral energy distribution (SED) of the star WOCS-5885, after combining with flux measurements from GALEX, UIT, UVOT, SPITZER, WISE and several ground-based facilities. The resulting SED spans a wavelength range of 0.15 µm to 7.8 µm. This object is found to be one of the brightest FUV sources in the cluster. An analysis of the SED reveals the presence of two components. The cooler component is found to have a temperature of 6 000±150 K, confirming that it is a BSS. Assuming it to be a main-sequence star, we estimate its mass to be ∼ 1.1 -1.2M . The hotter component, with an estimated temperature of 17 000±500 K, has a radius of ∼ 0.6R and L ∼ 30L . Bigger and more luminous than a white dwarf, yet cooler than a sub-dwarf, we speculate that it is a post-AGB/HB star that has recently transferred its mass to the BSS, which is known to be a rapid rotator. This binary system, which is the first BSS with a post-AGB/HB companion identified in an open cluster, is an ideal laboratory to study the process of BSS formation via mass transfer.

We present the UV photometry of the globular cluster NGC 1851 using images acquired with the Ultra-violet Imaging Telescope (UVIT) onboard the AS-TROSAT satellite. PSF-fitting photometric data derived from images in two far-UV (FUV) filters and one near-UV (NUV) filter are used to construct colormagnitude diagrams (CMD), in combination with HST and ground-based optical photometry. In the FUV, we detect only the bluest part of the cluster horizontal branch (HB); in the NUV, we detect the full extent of the HB, including the red HB, blue HB and a small number of RR Lyrae stars. UV variability was detected in 18 RR Lyrae stars, and 3 new variables were also detected in the central region. The UV/optical CMDs are then compared with isochrones of different age and metallicity (generated using Padova and BaSTI models) and synthetic HB (using helium enhanced Y 2 models). We are able to identify two populations among the HB stars, which are found to have either an age range of 10-12 Gyr, or a range in Y ini of 0.23 -0.28, for a metallicity of [Fe/H] =−1.2 to −1.3. These estimations from the UV CMDs are consistent with those from optical studies.The almost complete sample of the HB stars tend to show a marginal difference in spatial/azimuthal distribution among the blue and red HB stars. This study thus show cases the capability of UVIT, with its excellent resolution and large field of view, to study the hot stellar population in Galactic globular clusters.

We report the discovery of a hot white dwarf (WD) companion to a blue straggler star (BSS) in the globular cluster (GC) NGC 5466, based on observations from the Ultra-Violet Imaging Telescope (UVIT) on board AstroSat. The Spectral Energy Distribution (SED) of the Far-UV detected BSS NH 84 was constructed by combining the flux measurements from 4 filters of UVIT, with GALEX, GAIA and other ground-based observations. The SED of NH 84 reveals the presence of a hot companion to the BSS. The temperature and radius of the BSS (T eff = 8000 +1000 −250 K, R/R = 1.44 ± 0.05) derived from Gemini spectra and SED fitting using Kurucz atmospheric models are consistent with each other. The temperature and radius of the hotter companion of NH 84 (T eff = 32, 000 ± 2000 K, R/R = 0.021 ± 0.007) derived by fitting Koester WD models to the SED suggest that it is likely to be a hot WD. The radial velocity derived from the spectra along with the proper motion from GAIA DR2 confirms NH 84 to be a kinematic member of the cluster. This is the second detection of a BSS-WD candidate in a GC, and the first in the outskirts of a low density GC. The location of this BSS in NGC 5466 along with its dynamical age supports the mass-transfer pathway for BSS formation in low density environments.