Ultraviolet Imaging Telescope (UVIT) is the long wavelength eye of the multi-wavelength satellite ASTROSAT, which carries telescopes giving a spectral coverage from hard X-rays to ultraviolet. The satellite was launched on September 28, 2015. Soon after the launch testing of the X-ray telescopes followed. The subsystems of UVIT, e.g. the detectors, were also checked with the doors closed to isolate the optics from degassing of the S/C. The doors of UVIT opened on November 30 and it was pointed to a relatively bright field at high declination and images were taken. This note describes the impressions from these images regarding performance of the payload.

 UVIT is configured as two Cassegrain-telescopes each of ~ 375 mm aperture. One of the telescope images in 130-180 nm (FUV) and the other images in 200-300 nm (NUV) and 320-550 nm (VIS). A suit of filters allow further selection of a narrower wavelength band for all the three detectors. The field is ~ 28’ in diameter and the most important specifications for imaging are: a) spatial resolution (FWHM) of the ultraviolet images <1.8”, b) sensitivity in 130-180 nm ~ AB mag 20 in 200 s of exposure.

 For the first light observations an open cluster of stars in our Galaxy, NGC 188, was chosen. This selection was based on the need to have the first pointing in a direction well away from the equatorial plane to keep the operations of pointing simple, and to have a source which has a variety of stars but not a very high density of stars. This open cluster (a group of stars which are gravitationally bound) is many billion years old and contains ~ 1500 stars. It is located at a distance of ~ 6000 light years in the constellation Cepheus. The cluster has several bright stars which are easily recognisable and are suitable for a first light exposure. Though this source is not a primary standard, it has been studied in details by the past missions so that a good idea of the performance of UVIT can be obtained from these images. Imaging of weak sources, with the proposed resolution of 1.8”, requires corrections for the small drift of the S/C (<0.5”/s) by images obtained with the visible detector. Therefore, in order to keep the analysis simple, it was important to have bright sources so that sensible images could be obtained in a few seconds in the ultraviolet and used for self-correction of the drift.

 The images were taken for several minutes with all the three detectors; the UV detectors worked in photon-counting mode to detect each individual photon, while the visible detectors worked in integration mode (like a typical CCD-camera). As the pointing has some drift, the images are taken in short (<~ 1s) exposures and are added together after correcting for the drift. This procedure is easily implemented for this field as bright stars (mag < 15) are available in the field. The final images are presented in the figure below.

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