A modern Ritchey-Chretien Cassegrain 1.3-m diameter Devasthal Fast Optical Telescope (DFOT) has been installed at Devasthal, Nainital by the DFM Engineering Inc. USA and run by Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous research institute under the Department of Science and Technology (DST), Government of India. It was inaugurated by Dr. T. Ramasami, Secretary, Department of Science & Technology, Govt. of India on December 19, 2010.
The focal length to diameter ratio (focal-ratio) of the overall telescope optics is four making it a very fast system with a total field view of the sky up to 66 arcmin in diameter. The mirrors are made of Corning’s Ultra Low Expansion (ULE) glass/ceramic material. The mirrors are polished to optical wavelength accuracies and coated with Aluminum to obtain high reflectivity at visible wavelengths. The telescope mount is of fork-equatorial type which requires only one axis of rotation while tracking celestial sources. The focus can be adjusted using a five-axis (tip, tilt, and 3-axis translation) controller on the secondary mirror. The telescope uses friction drives to control motions in right ascension and declination axes without any backlash. The telescope can be pointed to a celestial object with an accuracy of 10 arcsec rms. The mechanical system provides a tracking accuracy at nearly 0.5 arcsec rms over 10-min without any external guider. There is also an onsite weather monitoring system to keep a watch on the outside weather. The telescope is housed in an open roll-of-roof type structure, designed and constructed by the institute itself, to help the telescope to cool faster in the ambient. The telescope is equipped with a motorized filter changer, design and developed at the institute where we can keep 8 filters at any moment among broad-band UBVRI, SDSS ugriz and narrowband H-alpha, O[III], S[II] interference filters. The telescope is equipped with low noise and fast modern Charge-Coupled Devices (CCD) detectors and high efficiency transmission filters. Two CCD cameras are currently available with the telescope for obtaining images of the celestial sky. The cameras are (1) 2048×2048 pixels, 13.5 micron pixel size conventional back-illuminated, deep thermoelectrically cooled (-80 deg C) CCD, (2) 512×512 pixels, 16 micron pixel size electron multiplying frame transfer back-illuminated, deep thermoelectrically cooled (-90 deg C) CCD. Both the cameras have high quantum efficiency E2V chip, assembled by ANDOR with low read noise electronics. The observations at DFOT are carried out through telescope control centre at Devasthal.
Although the aperture of DFOT is small in the context of present international scenario, the darkness and sub-arcsec seeing at the Devasthal site makes it an excellent facility for carrying out valuable astronomical research in the field of faint objects. As this optical observing facility lie between Australia in the East and Canary islands in the West, it suited ideally to observe optical transients and follow-up study of many sources being discovered by the space based Indian telescope ASTROSAT and GMRT Radio telescope. In fact, we defocus the telescope, we can obtain high-precision photometry for very bright stars and can achieve a stability of milli-mag detection. The main objective of installing DFOT at Devasthal was to meet the observational requirements for the institute’s main scientific programs such as monitoring of transients (Gamma Ray Bursts; GRB, Supernovae explosions, extrasolar planets), variability of stars in the Milky-way, star clusters, active nucleus in external galaxies, etc which were otherwise solely depend on the 40 year old 104-cm Sampurnanand telestcope at Manora Peak, Nainital. We found this telescope is ideally suited to detect and analysed extra solar planets which need very high-precision photometry at the few milli-mag level. The variability in Active galactic nuclei has also been detected with few mmag accuracy. This telescope is also well suited to do wide-field photometry like it is needed to do in case of open star clusters, star forming regions, wide field galaxies and supernova explosion. This telescope is totally dedicated to the scientific programs carrying out at ARIES, however, science programs by non-ARIES members including foreign scientists are welcome in the collaboration with ARIES scientists.
Two CCD cameras are currently available with the telescope for obtaining images of the celestial sky.
1. 2048×2048 pixels CCD Camera: Andor’s DZ436 camera is a back-illuminated, deep thermoelectrically cooled having high resolution and large dynamic range. It has negligible dark current, low read-out noise and high quantum efficiency of up to 95% which is ideal for the faint imaging astronomy. Although it covers a large field of view of 18×18 square arcmin area of the sky but through windowing, one can just observe small area hence fast imaging is also possible with this instrument.
2. 512×512 pixels CCD Camera: Andor’s iXon EM+ DU-897 camera is an electron multiplying frame transfer back-illuminated and deep thermo-electrically cooled CCD imager. The CCD is cooled using thermo-electric cooler. It has a quantum efficiency of more than 90% and containing a 512×512 frame transfer CCD sensor from E2V technologies which enables charge to be multiplies on the sensor before it is read out. It covers a field of view of 5.5×5.5 square arcmin area of the sky. This detector is most suitable for ultra-fast imaging where few milli-second exposure can be given.