The Thirty Meter Telescope (TMT) adopts a recently developed technology known as Stressed Mirror Polishing for the polishing of its 492 mirror segments. In this process, first the meniscus type spherical shape glass blanks are converted in to a desired aspheric shape by the application of forces around the edges using warping arms followed by spherical polishing in the stressed condition. After that, the blank edges will be cut in to its final hexagonal shape. These warping as well as the hex cutting process generate significant stress within the glass which in later stage, will cause the propagation of micro cracks and results in blank breakage. So prior and after the hex cutting process, it is essential to ensure that the glass blanks are free from stress accumulation. Hence the glass blanks need to be stress relieved before the hex cutting process. To achieve this stress relaxation, the glass blanks need to be kept over a platform or a support system which will provide a zero gravity condition for a time period of at least 48 hours. As a part of this, we designed, developed and analyzed a whiffletree based support system which will equally distribute the entire mirror blank mass into three points which are equally separated by 120⁰ from each other and thus balance itself as if it is in a floating condition. This support system which additionally gives optimized support for the glass blank which in turn minimizes the surface deformation due to its self weight sagging. This paper also discusses the positional sensitivity, reaction force sensitivity and alignment sensitivity analyses which are essential to obtain the tolerance values in the fabrication point of view.
Annapurni Subramaniam, Shyam Tandon, John Hutchings, Swarna Ghosh, Koshy George, V. Girish, P. U. Kamath, S. Kathiravan, Amit Kumar, J. Paul Lancelot, P. K. Mahesh, Rekhesh Mohan, Jayant Murthy, S. Nagabhushana, Ashok Pati, Joe Postma, N. Kameswara Rao, Kasiviswanathan Sankarasubramanian, P. Sreekumar, S. Sriram, Chelliah Stalin, Firoza Sutaria, Yuvraj Harsha Sreedhar, Indrajit Barve, Chayan Mondal, Snehalate Sahu
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.
Amit Kumar, S. Ghosh, P. Kamath, Joe Postma, S. Kathiravan, P. Mahesh, S. Nagbhushana, K. Navalgund, N. Rajkumar, M. Rao, K. Sarma, S. Sriram, C. Stalin, S. Tandon
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.
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