Astronomy

Control Of Telescopes At Hanle

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Indian astronomers and engineers of IIA could have built most of two-metre stellar optical telescope for Hanle indigenously. After all, they had made the 2.3-m telescope for Kavalur. However, it would have taken a long time to make Hanle telescope a totally indigenous effort. The overwhelming consideration was to quickly develop the site and start a facility, where, additions could be built. Hence it was decided to import a telescope, but it was not just bought off the shelf. Indian astronomers who are aware of the state-of-the-art trends in the field called for the best possible deals and participated in the design review and ensured their inclusion.

The telescope has a diameter of 2.01 m. it has an instrument called faint object spectrograph for optical imaging and spectroscopy, developed by the Copenhagen Observatroy, Denmark. A charge coupled device (CCD) array (with 2048 by 4096 pixels) detects images and transfers them to a control computer. Another instrument works in the near-infrared portion of the spectrum (1 t0 25 micrometres). The instrument was fabricated in the United States. IIA developed its control system and user interface.

A remote control center was set up at Hosakote, near Bangalore to operate the dedicated satellite link. The Karnataka government gave 240 ha of land for the centre.

The satellite link can transmit data at one mega bits per second in either direction. The telescope and its dome movement can be remotely controlled. There is provision for weather and cloud monitoring as well as video conferencing with the personnel at Hanle.

Control of the telescope is automatic, though at any time, engineers on duty can take over. At the control room in Hosakote, there are three computers dedicated to the control of the dome, the instruments and the telescope, respectively. The control is quite sophisticated; for example, one can point the telescope blindly and still get an accuracy of 2.5 arcseconds and then fine-tune it by 0.1 degree. Once the coordinates of the cosmic objects are fed into the system, the telescope can track them on its own. A small team of engineers at the control center in Hosakote and Hanle ensures trouble-free operation. The engineers and the staff on either side can have a video conference at any time. Preventive maintenance is done on full-Moon days, as observers prefer to work on dark nights.

The telescope has four side ports and one axial port at the bottom. Light from the sky is directed by default to the port, which can record an optical image or spectrum of the source. There is a provision to direct the light to the side ports, for example, for imaging the infrared radiation. A wavefront sensor is fitted to the telescope to evaluate the optical performance of the telescope. The deformations of the primary mirror can be corrected within a tenth of a wavelength.

Computer control adjusts the focus of the secondary mirror inside the telescope to compensate for its tilt or tip and keep the optical alignment with the primary mirror fixed at all orientations, besides taking care of temperature variations that expand or contract the truss, thus changing the distance between the primary and secondary. There is provision to test the quality of the sky by using an online web/camera to see the telescope enclosure and part of the day-time sky, and another one to see the night sky.

The pointing accuracy of the telescope is less than 0.1 arcs econd, which is equivalent of targeting the figure of a lion’s head (2 mm) on a rupee coin from a distance of two km. the resolution of the image is 0.9 arc second. (As arc second is 1/3600th of a degree.)

Continuous monitoring of any desired field is possible by looking on to the celestial object. Long spectroscopic exposure is also done. The auto-guider system for this has been developed collaboratively by the Copenhagen University Observatory, Niels Bohr Institute, Denmark, and IIA. It was installed in 2005.

A high-resolution spectrometer will be built to augment the efficiency of the Hanle telescope. A detailed concept study was made to develop a spectro-polarimeter, which would measure not only the spectrum of stars but also their atmosphere, velocity, as well as the polarity of their light for determining the direction and strength of their magnetic fields.

As solar power is available in plenty in this part of the Himalayan region, the entire power needs of the observatory—some 30 KW—are derived from solar panels set up close by. An automated weather station provides online information on ambient temperature, relative humidity, pressure, wind-speed and direction as well as precipitation. A large plant capable of providing a reflective coating to 2-m mirrors has been set up to provide on-the-spot service.

Hanle has become quite popular with astronomers at home and abroad. In fact, online viewing of the night sky at Hanle is possible from any where in the world: A continuous all-sky camera, developed by Michigan Technological University, USA, has been installed for this purpose. It is part of an international network of all-sky cameras operating at night and available on the Internet.

The duration of full nights of working with the telescope depends on the season: 15 hours in winter, and 8 hours in peak summer (when daylight hours are longer). Maintenance work will also last the whole night and may extend to the day time if need be. One many take corrective action during the day time and test its effect at night.

The telescope time is oversubscribed by two and half times. Once in four months, a committee that allots time decides on the list of observers. It is said that one should be happy, if one gets even two consecutive nights for observation. Despite the tight schedule, priority would be given to observe any unexpected celestial phenomenon. The astronomers call it a target of opportunity, which can be triggered by a simple e-mail nowadays from any part of the world. Such an occasion would interrupt the programmed observation schedule.

The Hanle user community, including students, is growing. So are the fields of investigation, which cover a wide range of topics from objects in the solar system to distant cosmic phenomena. The data collected would be deemed public property after a lock-in period of one or two years when the initial usage is complete. The Virtual Observatory India team at Inter-University Centre for Astronomy and Astrophysics, in collaboration with M/s Persistent Systems, Pune, has developed the software to access the data archive. It is also possible to access the Internet from Hanle.

The Himalayan Chandra Telescope marks the first step towards developing a large national optical and infrared telescope. The infrastructure and the remote operating facility are spurring further development in ground-based gamma array astronomy as well as geophysical and atmospheric sciences.

Image-credit: http://aryabhatfoundation.blogspot.in/

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