The ESO New Technology Telescope


The 3.6m New Technology Telescope (NTT) saw first light in March 1989. CCD frames were obtained on which stellar images had FWHM diameters of 0.33, indicating the excellent imaging quality of this telescope.

The NTT is an alt-az telescope. It resides in a rotating building; the control room was moved to a new building located close to the bottom of the NTT ramp in 2003, so now the observers don't rotate with the telescope any longer.

The NTT at night
The NTT at night, with the dome of the ESO 3.6m in the background.


The telescope chamber is ventilated by a system of flaps which optimize the air flow across the NTT minimizing the dome and mirror seeing. All motors in the telescope environment and hydraulic system are water cooled to prevent heat input to the building. For the same reason, all the electronics boxes located on the telescope are insulated and cooled. The concrete platforms and parking lots around the telescope are painted in a bright white in order to minimize the accumulation of heat during day time.

The primary mirror is actively controlled to preserve its figure at all telescope positions. The secondary mirror position is also actively controlled in three directions. The optimized airflow, the thermal controls,  and the active optics give the excellent image quality of the NTT. Note that the NTT has active instead of adaptive optics: it corrects the defects and deformation of the telescope and mirror, but dos not correct the turbulence; it ensures that the optics is always in perfect shape. Together with the thermal control, it allows the NTT to reach the ambient seeing, but it does not improve it. The main mirror is frequently cleaned using carbonic snow (weekly) and water (every six months), maintaining its reflectivity to an optimal value of ~88-90%.

The NTT has a rigid construction due to the reduced mass of the primary mirror and telescope structure compared with classical telescopes. This result is a very accurate pointing of 1.5'' r.m.s. over most of the sky. Degradation occurs close to the zenith and at zenith angle larger than 60 degrees. Tracking accuracy at 0.1'' matches the overall image quality of 80% energy in 0.15'' diameter. For short exposures (less than 10m on not moving targets) the tracking is so good that it is better not to use the guiding at all.

The telescope has an extensive sky coverage, with very reduced poiting limits; however some caution needs to be exercised at low elevations. It is not recommendable to execute image analysis at elevation lower than 40 degrees and it is absolutely forbidden to do any optics adjustment at elevation below 20 degrees, not even M2 focus adjustment. A software interlock will activate when the telescope reaches 10 degrees or 89 degrees elevation, while the hardware interlock will activate at elevations 8.9 and 90 degrees respectively. Users must be aware that it takes considerable time to recover the telescope after an hardware interlock has been triggered.

There are two Nasmyth focus platforms to mount two sets of instruments simultaneously. To switch from one to the other, a 45-degree third flat mirror is rotated by 180 degres. Change-over therefore takes little time. The instrumentation presently consists of EMMI for optical imaging in blue and red, low/medium/high resolution spectroscopy, SUSI2 for high-resolution imaging, and the new ESO infrared instrument SOFI.

Two guide cameras, one for each Nasmyth focus, are available: they have a pixel size of 0.2" and the orientation of the images is N on the top and E on the left. A slit viewer camera is available on Nasmyth B; it has a pixel size of 0.24" and its orientation is N on the left and E on the bottom. It is used to place targets in the EMMI slit for medium dispersion spectroscopy and it is reliable for objects down to 19 magnitude in R.

The "Big-Bang"

On the period July 1996 - July 1997, a major upgrade was accomplished (known as the NTT Big-Bang). The control system was replaced to be compliant with the control system to be employed on the UTs of the VLT. The new control system is, to all intents and purposes, identical to the VLT control system.

The NTT electronics (i.e.. encoders, drive motors and velocity controllers) were replaced.  Also, all the computers were replaced. The old TV cameras for the autoguiders, slit viewer, image quality and image analysis were replaced by VLT standard technical CCDs.

The VLT control system is based on a distributed architecture: Workstations provide the user level interaction and co-ordination, but all actions are taken by the Local Control Units (LCUs). The telescope axis, rotators, adapters, mirrors, building and hydraulics are controlled by 11 LCUs, the instruments and scientific CCDs are controlled by 7 LCUs and the technical CCDs are controlled by other 7 LCUs.

The control system time keeping relies on a dedicated GPS receiver (located on the roof of the NTT) for UT time. The receiver electronics has a backup clock should no satellites be available.

The control software, is also compliant with the VLT standards. This relies heavily on Rtap, a commercial real time system (HP). Rtap provides the workstation database and messaging systems for the control system.

More details about the Big-Bang can be found in the NTT Upgrade Plan (PS document)

Photos of the NTT

Click on the thumbnail to load the full-size image.

View of the NTT in front of the smaller telescopes, from the 3.6m

NTT in its enclosure

Closer view of the NTT

Active Optics Support of the NTT main mirror

Achievements of the NTT

Highlights of the scientific results obtained with the NTT are described in this other page.

Technical Characteristics

New Technology Telescope
Location 70° 44' 01.5" W  -29° 15' 32.1" S (WGS84)
Altitude 2375 m.
 Optical Design Richey-Chretien
Main Mirror Schott Zerodur
Diameter 3.58m, thin meniscus (24cm), 
Active Optics 75 actuators, 3 fixed points, 24 lateral actuators
Focal Ratio f/2.2
Secondary Mirror Schott Zerodur
Diameter 0.875m
Central Obstruction 1.16m
Tertiary Mirror Schott Zerodur
Major x minor axis 0.840 x 0.600m
Nasmyth foci
Field of view 30arcmin, 336.6mm
Focal Ratio f/11
Plate scale 5.35905 arcsec/mm
Image Quality 80% energy in 0.15" in all telescope position


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