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.

Results of the initial calibration of the Ultra-Violet Imaging Telescope (UVIT) were reported earlier by Tandon et al. (2017a). The results reported earlier were based on the ground calibration as well as the first observations in orbit. Some additional data from the ground calibration and data from more in-orbit observations have been used to improve the results. In particular, extensive new data from in-orbit observations have been used to obtain (a) new photometric calibration which includes (i) zero-points (ii) flat fields (iii) saturation, (b) sensitivity variations (c) spectral calibration for the near Ultra-Violet (NUV; 2000−3000Å) and far Ultra-Violet (FUV; 1300−1800Å) gratings, (d) point spread function and (e) astrometric calibration which includes distortion. Data acquired over the last three years show continued good performance of UVIT with no reduction in sensitivity in both the UV channels.

The presence of fine structures in the sunspot vector magnetic fields has been confirmed from Hinode as well as other earlier observations. We studied 43 sunspots based on the data sets taken from ASP/DLSP, Hinode (SOT/SP) and SVM (USO). In this Letter, (i) We introduce the concept of signed shear angle (SSA) for sunspots and establish its importance for non force-free fields. (ii) We find that the sign of global α (force-free parameter) is well correlated with the global SSA and the photospheric chirality of sunspots. (iii) Local α patches of opposite signs are present in the umbra of each sunspot. The amplitude of the spatial variation of local α in the umbra is typically of the order of the global α of the sunspot. (iv) We find that the local α is distributed as alternately positive and negative filaments in the penumbra. The amplitude of azimuthal variation of the local α in the penumbra is approximately an order of magnitude larger than that in the umbra. The contributions of the local positive and negative currents and α in the penumbra cancel each other giving almost no contribution for their global values for whole sunspot. (v) Arc-like structures (partial rings) with a sign opposite to that of the dominant sign of α of the umbral region are seen at the umbral-penumbral boundaries of some sunspots. (vi) Most of the sunspots studied, belong to the minimum epoch of the 23 rd solar cycle and do not follow the so-called hemispheric helicity rule.