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Register a new userCalibration and performance of the photon-counting detectors for the Ultraviolet Imaging Telescopes (UVIT) of the Astrosat observatory
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J.Postma
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We describe calibration data, and discuss performance of the photon-counting flight detectors for the Ultraviolet Imaging Telescopes on the Astrosat observatory. The paper describes dark current, flat field and light-spot images for FUV, NUV, and Visible band detectors at more than one wavelength setting for each. We also report on nominal gain and low-gain operations, full- and sub-window read rates, and non-photon-counting modes of operation, all expected to be used in flight. We derive corrections to the event centroids from the CMOS readout arrays, for different centroid algorithms. We derive spatial resolution values for each detector and plots of point-source signal saturation for different flux levels. We also discuss ways to correct for saturation in extended object images.
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Ultraviolet Imaging Telescope (UVIT) is one of the payloads onboard AstroSat, Indias first multi-wavelength Astronomy mission. UVIT is primarily designed to make high resolution images in wide field, in three wavelength channels simultaneously: FUV (130 - 180 nm), NUV (200 - 300 nm) and VIS (320 - 550 nm). The intensified imagers used in UVIT suffer from distortions, and a correction is necessary for these to achieve good astrometry. In this article we describe the methodology and calculations used to estimate the distortions in ground calibrations.
We present the in-orbit performance and the first results from the ultra-violet Imaging telescope (UVIT) on ASTROSAT. UVIT consists of two identical 38cm coaligned telescopes, one for the FUV channel (130-180nm) and the other for the NUV (200-300nm) and VIS (320-550nm) channels, with a field of view of 28 $arcmin$. The FUV and the NUV detectors are operated in the high gain photon counting mode whereas the VIS detector is operated in the low gain integration mode. The FUV and NUV channels have filters and gratings, whereas the VIS channel has filters. The ASTROSAT was launched on 28th September 2015. The performance verification of UVIT was carried out after the opening of the UVIT doors on 30th November 2015, till the end of March 2016 within the allotted time of 50 days for calibration. All the on-board systems were found to be working satisfactorily. During the PV phase, the UVIT observed several calibration sources to characterise the instrument and a few objects to demonstrate the capability of the UVIT. The resolution of the UVIT was found to be about 1.4 - 1.7 $arcsec$ in the FUV and NUV. The sensitivity in various filters were calibrated using standard stars (white dwarfs), to estimate the zero-point magnitudes as well as the flux conversion factor. The gratings were also calibrated to estimate their resolution as well as effective area. The sensitivity of the filters were found to be reduced up to 15% with respect to the ground calibrations. The sensitivity variation is monitored on a monthly basis. UVIT is all set to roll out science results with its imaging capability with good resolution and large field of view, capability to sample the UV spectral region using different filters and capability to perform variability studies in the UV.
Ultra Violet Imaging Telescope on ASTROSAT Satellite mission is a suite of Far Ultra Violet (FUV 130 to 180 nm), Near Ultra Violet (NUV 200 to 300 nm) and Visible band (VIS 320 to 550nm) imagers. ASTROSAT is the first multi wavelength mission of INDIA. UVIT will image the selected regions of the sky simultaneously in three channels and observe young stars, galaxies, bright UV Sources. FOV in each of the 3 channels is about 28 arc-minute. Targeted angular resolution in the resulting UV images is better than 1.8 arc-second (better than 2.0 arc-second for the visible channel). Two identical co-aligned telescopes (T1, T2) of Ritchey-Chretien configuration (Primary mirror of 375 mm diameter) collect celestial radiation and feed to the detector system via a selectable filter on a filter wheel mechanism; gratings are available in filter wheels of FUV and NUV channels for slit-less low resolution spectroscopy. The detector system for each of the 3 channels is generically identical. One of the telescopes images in the FUV channel, while the other images in NUV and VIS channels. Images from VIS channel are also used for measuring drift for reconstruction of images on ground through shift and add algorithm, and to reconstruct absolute aspect of the images. Adequate baffling has been provided for reducing scattered background from the Sun, earth albedo and other bright objects. One time open-able mechanical cover on each telescope also works as a Sun-shield after deployment. We are presenting here the overall (mechanical, optical and electrical) design of the payload.
We present a novel method to measure precisely the relative spectral response of the fluorescence telescopes of the Pierre Auger Observatory. We used a portable light source based on a xenon flasher and a monochromator to measure the relative spectral efficiencies of eight telescopes in steps of 5 nm from 280 nm to 440 nm. Each point in a scan had approximately 2 nm FWHM out of the monochromator. Different sets of telescopes in the observatory have different optical components, and the eight telescopes measured represent two each of the four combinations of components represented in the observatory. We made an end-to-end measurement of the response from different combinations of optical components, and the monochromator setup allowed for more precise and complete measurements than our previous multi-wavelength calibrations. We find an overall uncertainty in the calibration of the spectral response of most of the telescopes of 1.5% for all wavelengths; the six oldest telescopes have larger overall uncertainties of about 2.2%. We also report changes in physics measureables due to the change in calibration, which are generally small.
Results of the initial calibration of the Ultra-Violet Imaging Telescope (UVIT) were reported earlier by Tandon et al. (2017). 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 - 3000 Angstroms) and far Ultra-Violet (FUV; 1300 - 1800 Angstroms) gratings, (d) point spread function and (e) astrometric calibration which included distortion. Data acquired over the last three years show continued good performance of UVIT with no reduction in sensitivity in both the UV channels.
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