Over the last 5 years, UVIT has completed observations of more than 500 proposals with $\sim$800 unique pointings. In addition, regular planned monitoring observations have been made and from their analysis various key parameters related to in orbit performance of UVIT have been quantified. The sensitivities of the UV channels have remained steady indicating no effect of potential molecular contamination confirming the adequacy of all the protocols implemented for avoiding contamination. The quality of the PSF through the years confirms adequacy of thermal control measures. The early calibrations obtained during the Performance Verification (PV) phase have been further revised for more subtle effects. These include flat fields and detector distortions with greater precision. The operations of UVIT have also evolved through inorbit experience, e.g. tweaking of operational sequencing, protocol for recovery from bright object detection (BOD) shutdowns, parameters for BOD thresholds, etc. Finally, some effects of charged particle hits on electronics led to optimised strategy for regular resetting. The Near-UV channel was lost in one of suchoperations. All the above in-orbit experiences are presented here.

Curvit is an open-source Python package that facilitates the creation of light curves from the data collected by the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat, India’s first multi-wavelength astronomical satellite. The input to Curvit is the calibrated events list generated by the UVIT-Payload Operation Center (UVIT-POC) and made available to the principal investigators through the Indian Space Science Data Center. The features of Curvit include: (i) automatically detecting sources and generating light curves for all the detected sources and (ii) custom generation of light curve for any particular source of interest. We present here the capabilities of Curvit and demonstrate its usability on the UVIT observations ofthe intermediate polar FO Aqr as an example. Curvit is publicly available on GitHub at https://github.com/prajwel/curvit.

Performance of the Level-2 pipeline, which translates the UVIT data created by the ISRO’s ground segment processing systems (Level-1) into astronomer ready scientific data products, is described. This pipeline has evolved significantly from experiences during the in orbit mission. With time, the detectormodules of UVIT developed certain defects which led to occasional corruption of imaging and timing data. This article will describe the improvements and mitigation plans incorporated in the pipeline and report its efficacy and quantify the performance.

Ultra Violet Imaging Telescope (UVIT) is one of the 5 instruments on AstroSat satellite, which was launched on September 28, 2015. UVIT was designed to make images with a resolution of <1:8$''$, simultaneously in two ultraviolet channels: Far Ultraviolet (130–180 nm) and Near Ultraviolet (200–300 nm). Images are also made in visible region (320–550 nm) for tracking drifts in pointing. The shortest wavelengths to be observed with UVIT can be heavily absorbed by mono-molecular deposits/contamination on the optical surfaces.Keeping contamination under control in UVIT was a major challenge and it required a variety of actions: (i) strict control of the payload materials and process, (ii) mechanical configuration, (iii) baking of all the parts to release all the adsorbed molecules etc., (iv) assembly in ultra cleanrooms, (v) pre-inspection and auditing of all the areas, in which UVIT was placed, for any potential for contamination, (vi) continuous purging, with ultrapure nitrogen gas, till a few days before the launch, etc. In order to minimise any possible cross contaminationsfrom the other payloads/satellite, the doors of UVIT were opened 2 months after the launch. The high performance in the orbit and high stability of the sensitivity over 4 years in the orbit shows that the contamination was negligible. This paper presents the processes and protocols followed during the integration and testingphase to minimise the contamination in order to prevent any performance degradation.

We describe a versatile pipeline for processing the data collected by the Ultra-Violet Imaging Telescope (UVIT) on board Indian multi-wavelength astronomical satellite ASTROSAT. The UVIT instrument carries out simultaneous astronomical imaging through selected filters/gratings in far-ultra-violet (FUV), near-ultra-violet and visible (VIS) bands of the targeted circular sky field ($\sim$0.5$^{\circ}$ dia). This pipeline converts the data (Level 1) emanating from UVIT in their raw primitive format supplemented by inputs from the spacecraft sub-systems into UV sky images (and slitless grating spectra) and associated products readily usable by astronomers (Level 2). The primary products include maps of Intensity (rate of photon arrival), error on Intensity and effective Exposure. The pipeline is open source, extensively user configurable with many selectable parameters and its execution is fully automated. The key ingredients of the pipeline include extraction of drift in the pointing of the spacecraft, and disturbances in pointing due to internal movements; application of various corrections to measured position in the detector for each photon – e.g., differential pointing with respect to a reference frame for shift and add operation, systematic effects and artefacts in the optics of the telescopes and detectors, exposure tracking on the sky, alignment of sky products from multiepisodeexposures to generate a consolidated set and astrometry. Detailed logs of operations and intermediate products for every processing stage are accessible via user-selectable options. While large number of selectable parameters are available for the user, a well characterized ‘standard default’ set is used for executing this pipeline at the Payload Operation Centre (POC) for UVIT and selected products are archivedand disseminated by the Indian Space Research Organization (ISRO) through its ISSDC portal.