3.6m Devasthal Optical Telescope

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A modern 3.6 meter new technology optical telescope as a major national initiative in Astrophysics


Scientific Objectives

India has already developed a world class observing facilities at radio wavelengths (GMRT) and currently developing space based observing facilities at X-ray and ultra-violet wavelengths ( ASTROSAT and TAUVEX ). However, we still lack of a world class observing facilities for ground - based optical astronomy. It felt that the time for the Indian astronomical community has come to set up a ground based modern optical observing facility of at least 3 - 4 m class in near future and plan for a larger telescope.


In near future, the installation of a 3.6 m optical telescope will be advantageous for many scientific programmes, some of the science drivers are as follows:



Current Status


The proposed 3.6 m telescope will be optimized for the spectroscopic observations. ARIES made serious efforts to involve international community for technical and financial support in this project. The success came as Russian Academy of Sciences and Belgium astronomers decided to participate in this project. National Institutes like IIA, TIFR are also participating in this project. It is also proposed that (I) A high resolution (~ 50,000 ) stellar spectro-polarimeter (II) Faint object spectrograph camera and (iii) Near IR camera are to be developed as the first generation instruments.


A contract for design, manufacture, integration, testing, supply and installation of the telescope has been entered between ARIES and AMOS, Belgium on 29th March 2007. The optical subsystem specifications are as follows :


  1. Size of primary mirror : 3.6m ( Clear aperture)
  2. Material of the primary mirror : SCHOTT Zerodur
  3. Material of the secondary mirror : Sitall- Co - 115m
  4. Primary focal ratio : f/2
  5. Configuration: Ritchey-Chretien (RC) with Cassegrain focus
  6. Effective focal ratio: f/9
  7. Operational waveband: 350 to 5000 nm
  8. Science field of view: 10 arc min without corrector and 30 arc min unvignetted field for axial port and 10 arc min for side ports
  9. Optical image quality: 50% energy in < 0.3 arc sec diameter, 80% energy in < 0.45 arc sec diameter and 90% energy in < 0.6 arc sec diameter ( for 10 arc min field without corrector)
  10. Location of Cassegrain focus: 1. 50 cm (approx.) behind the instrument flange


Mechanical Specifications


  1. Mounting: Alt-Azimuth
  2. Zenith blind spot: Less than two degree conical diameter
  3. Instrument envelope: Cylindrical space of minimum 2.5 meter below the focal plane for axial port and approximately 3.0 meter diameter around optical axis
  4. Pointing accuracy: < 2 arc sec RMS for any point in the sky with elevation > 10 degree, < 0.1 arc sec RMS for 10 arc min off set
  5. Tracking accuracy: < 0.1 arc sec RMS for < one minute in open loop, < 0.1 arc sec RMS for < one hour in closed loop (with auto guider) and < 0.5 arc sec peak for < 15 min in open loop
  6. Max slow speed: 20 per sec
  7. Retargeting time: < 30 sec for 20 move
  8. The rotation freedom: In Azimuth ± 270 0 and in Altitude 100 to 900
  9. Acquisition and guiding unit will be provided
  10. Five axis motion of secondary mirror will be provided


Control Systems


  1. Suitable modern computer based control system for operating and controlling the telescope under all possible observing situations
  2. Complete computer control of the telescope with network operational capability

To achieve the above specifications, AMOS plans to implement active optics on this telescope. The active optics concept is

  1. Primary mirror will have its out-of-plane shape actively driven by a set of force-controlled axial actuators. The lateral supports will be passive.
  2. Secondary mirror will have passive axial and lateral supports but will be positioned in 5 axes (focus, centering and tilts) by an hexapod to compensate misalignment with primary.
  3. Both active sub systems will receive their set points online from wave front sensor.


The total project time is around 48 months.