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. At the end of the PV phase, the instrument calibration is almost complete and the remaining calibrations will be completed by September 2016. 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.
ASTROSAT is India’s first astronomy satellite that will carry an array of instruments capable of simultaneous observations in a broad range of wavelengths: from the visible, near ultraviolet (NUV), far-UV (FUV), soft X-rays to hard X-rays. There will be five principal scientific payloads aboard the satellite: (i) a Soft X-ray Telescope (SXT), (ii) three Large Area Xenon Proportional Counters (LAXPCs), (iii) a Cadmium-Zinc-Telluride Imager (CZTI), (iv) two Ultra-Violet Imaging Telescopes (UVITs) one for visible and near-UV channels and another for far-UV, and (v) three Scanning Sky Monitors (SSMs). It will also carry a charged particle monitor (CPM). Almost all the instruments have qualified and their flight models are currently in different stages of integration into the satellite structure in ISRO Satellite Centre. ASTROSAT is due to be launched by India’s Polar Satellite Launch Vehicle (PSLV) in the first half of 2015 in a circular 600 km orbit with inclination of ~6 degrees, from Sriharikota launching station on the east coast of India. A brief description of the design, construction, capabilities and scientific objectives of all the main scientific payloads is presented here. A few examples of the simulated observations with ASTROSAT and plans to utilize the satellite nationally and internationally are also presented.
Ultra Violet Imaging Telescope on ASTROSAT Satellite mission is a suite of Far Ultra Violet (FUV; 130 - 180 nm), Near Ultra Violet (NUV; 200 - 300 nm) and Visible band (VIS; 320-550nm) imagers. ASTROSAT is a first multi wavelength mission of INDIA. UVIT will image the selected regions of the sky simultaneously in three channels & observe young stars, galaxies, bright UV Sources. FOV in each of the 3 channels is ~ 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 the celestial radiation and feed to the detector system via a selectable filter on a filter wheel mechanism; gratings are available in the 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 telescope images in the FUV channel, and other images in NUV and VIS channels. One time open-able mechanical cover on each telescope also works as Sun-shield after deployment. We will present the optical tests and calibrations done on the two telescopes. Results on vibrations test and thermo-vacuum tests on the engineering model will also be presented.
The Ultra Violet Imaging Telescope on ASTROSAT Satellite mission is a suite of Far Ultra Violet (FUV: 130 - 180
nm), Near Ultra Violet (NUV: 200 - 300 nm) and Visible band (VIS: 320-550nm) imagers. ASTROSAT is the multi-wavelength
mission of ISRO. UVIT will image the sky simultaneously in three channels with a field of view diameter of
~ 28 arcminutes and an angular resolution < 1.8". Two identical co-aligned telescopes (T1, T2) of Ritchey-Chretien
configuration (Primary mirror of ~375 mm diameter) collect the celestial radiation and feed the detector systems via a
selectable filter on a filter wheel mechanism; gratings are available in the filter wheels of FUV and NUV channels for
slitless low-resolution spectroscopy. The photon-counting 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 via a
beamsplitter. Images from the VIS channel are principally used for measuring drift, used in construction of images on the
ground by shift and add, and to reconstruct absolute aspect of the images. Adequate baffling has been provided for
reducing the scattered background from the Sun, earth albedo and other bright objects. The one-time opening mechanical
cover on each telescope also works as a Sun-shield after deployment.
We will present the overall (mechanical, optical and electrical) design of the payload.
While the new, very large optical/IR telescopes setal most of the astronomical limelight, the relevance of smaller telescopes till remain undiminished as far as scientific output is concerned. Mature technology an reduced competition for observing time allow these telescopes and their backed instruments to be fine tuned for specific scientific programs. In order to maximize returns, it is essential to reduce time overheads in pointing and acquisition and ensure that the telescopes spend most of the time observing the targets. A sensitive and accurate auto- guidance system is another necessity. We have developed an acquisition and guidance system, which is flexible enough to be used with a wide variety of instruments at the different existing and planned telescopes in India. An implementation of this system, as part of an imaging polarimeter, has been working successfully for quite some time now. When used with a 1.2 m, f/13 telescope, this system is capable of acquiring the target within a few arcseconds and tracking, with subarcsecond accuracy, on stars as faint as V equals 15, available anywhere in a scan area of 0.05 square degrees, within about 15 arcminutes of the target.
The complexity of the structure of many astronomical objects (viz. galaxies, nebulae, comets etc.) is such that details of polarization frequently requires seeing limited resolution. A two channel imaging polarimeter has been described here which can provide seeing limited spatial resolution and highly accurate values of linear polarization by minimizing the harmful effects of atmospheric variations. This can be achieved by using a Wollaston prism analyzer to split a telescope image into two polarized components and simultaneously comparing them to eliminate effect due to atmospheric scintillation. The introduction of a grid at the focal plane, prevents the overlapping of o and e images on the CCD plane, where the image is refocussed. In order to keep the orientation of the two polarized components fixed in the instrument frame, the polarization vector of the image is modulated by mechanically rotating a half wave plate and readings are taken for several positions of the half wave plate to derive the polarization vector. We plan to use the above instrument at the cassegrain plane of 2.3 m V B telescope, India (f/13 beam) to carry out imaging polarimetry of extended astronomical objects. With 1000 secs. exposure time for each frame, on the GEC/EEF P8603A CCD chip, one can measure polarization values with an uncertainty of 0.3%, for the Seyfert N1068 (mv equals 17.5 per square arc sec) when observed in V band. This uncertainty is mainly dominated by photon count statistics. This instrument is presently being fabricated at Inter University Center for Astronomy and Astrophysics, Pune, India.