The ESO 3.6 meter Telescope


The ESO 3.6m Telescope was commissioned in 1977, and completely upgraded in 1999. In 2004 a new secondary mirror cell was manufactured, improving considerably the image quality of the telescope. In 2007 the control system of the lateral pads was upgraded. The image quality of the telescope is better than 0.2 arcsec at Zenith. The telescope has a horseshoe/fork mounting, and an interchangeable top unit allowing the secondary mirror to be changed from a F/8 to a F/35 Cassegrain focus. The pointing error is about 5 arcsec RMS. The pointing is limited to 70 deg. zenithal distance, and 5.5h HA, but a small region under the pole is reachable. Operation and maintenance of the telescope is the responsibility of ESO.
Since April 2008 HARPS, using the F/8 Cassegrain focus, is the only instrument available at the 3.6m.

The telescope is at a geographical location of:

70° 43' 54.1" W  -29° 15' 39.5" S (WGS84)

2400 metres above sea level

This page provides information about the main characteristics of the telescope:

Technical Characteristics

Mounting Equatorial, Horseshoe
M1 Diameter 3.566 m
Cassegrain Hole diameter 0.698 m
M1 clear area 8.8564 m2
M2 diameter 1.200 m
Focal Ratio f / 8.09
Scale at Focal Plane 7.12 arcsec/mm

Image Quality

The Point Spread Function (PSF) measured at the telescope depends on several factors, including the site characteristics (atmospheric conditions), telescope optics and tracking.

The best PSF that one may expect with the 3.6m telescope is about 0.5 arcsec (measured with EFOSC2) and more generally, in good seeing conditions is in the range of 0.7 - 0.8 arcsec. La Silla seeing statistics are available from the La Silla Astroclimatology page and from the La Silla MeteoMonitor and its archive.

The image quality at Zenith is better than 0.2arcsec, the largest residual aberration being the astigmatism.

Secondary mirror

The secondary mirror can move in x-y and z, z being the vertical axis of the focus. The movement along x-y is used to minimize aberrations as a function of the telescope position. Currently an optimum position is encountered that minimizes he coma, and the secondary mirror is fixed to this position. This setting is revised and optimized as part of the regular maintenance.


One should always focus the telescope every hour or so even when observing the same field. In general any shift in Z of the telescope by 15-20 degrees on the sky or a change in the Serrurier temperature of more than 1 degree Celsius should be accompanied by a focus procedure. The accuracy of the focus procedure is affected by mereological conditions. For example, with a seeing above 1.5arcsec it is not worth to push for the utmost accuracy on the focus.

Zenith Distance


Mirror Stresses

It has also been noticed that pointing the telescope to extreme positions (> 4 hr zenith distance) results in bad image quality even after the telescope is brought back close to the zenith. This persistence of distortion is believed to be due to unrelieved stresses on the telescope primary. It can usually be corrected by executing a standard sequence of telescope pointings, known to the telescope operators, which requires 10-15 minutes. The observer may request the telescope operator to carry out this procedure if desired.


Field rotation is available only for EFOSC2. The rotator is set to a default angle (270 degrees) for all the other instruments. The adapter hosts the guiding functions as well as the EFOSC2 calibration lamps and calibration screen.


Pointing Accuracy

The RMS pointing error is about 5 arcsec rms. It get slightly worse when pointing to the North.

Pointing Limits

accessible sky areaThe sky area accessible to the 3.6m+HARPS telescope/instrument configuration is characterized by the conditions:

  1. limit in hour angle HA: -5 h 30 m < HA < 5 h 30 m
  2. limit in zenith distance ZD: ZD < 70°
  3. limit in declination DEC: -120° < DEC < +29.5°

In condition (3) declinations less than -90° refer to an accessible sky region "below" the southern pole, i.e. 12 h away in hour angle with respect to condition (1).

The telescope has an equatorial mount and so has no problem observing sources at zenith, while observations within 3 degrees of the pole must be avoided.

The procedure checks if the target is outside the limit at the start of the OB and if so it aborts the observation. However, if the target starts within the allowed region but subsequently goes beyond the limit, the telescope will stop tracking at the limit without warning.

If the telescope reaches the altitude limit (20°) it may take up to 30 minutes to recover. Observers should keep this in mind while following a source towards the western horizon.


  • The dome hatch #2 has to be brought down when observing within 5° of the zenith - otherwise it could result in significant vignetting
  • The dome hatch #1 has to be raised when observing at elevation below 24° to avoid vignetting.


Tracking (without guide star) currently results in a systematic shift of about 0.1arcsec/minute (position dependent). For very fast exposures with EFOSC2, guiding might not be indispensible.


Guiding is done with different cameras for the different instruments. The high resolution spectrographs can use the EFOSC2 guide camera to guide "off axis". TIMMI2 uses its own guide camera. Apart the case of TIMMI2 all guide cameras are installed within the adapter. The autoguider keeps the telescope guiding accurate to about 0.1 arcsec in both RA and DEC, for an essentially unlimited time.

Autoguiding can be done on the object (HARPS/CES/EGGS/TIMMI2) or on a suitable guide star in the field of view (2' x 2') of the autoguider camera. The high resolution fiber spectrographs can use the slit viewer camera to guide on the science object or the TEC camera (alike EFOSC2) to guide on a nearby object.

Detector Technical CCD IMG/TEC
Dynamical Range 0 - 65536 ADU
Magnitude Range 0 - 16 mag
Guiding Accuracy < 0.1 arcsec (rms)

Hatches opening

The hatches of the 3.6m allow four different openings:

Hatches opened Available sky area (altitude) Available sky area (Zenithal distance)
4 90 - 58 degrees 0 - 32 degrees
4+3 90 - 42 degrees 0 - 48 degrees
4+3+2 86 - 24 degrees 4 - 66 degrees
4+3+2+1 68 - 7 degrees 22 - 83 degrees

Typically hatches # 4,3,2 are opened. Care has to be taken for observation at Zenith, as Hatch # 2 covers 4 degrees at Zenith when it is opened. In case of need the Hatch # 2 can be closed from the control room, but reopening it requires the telescope operators to drive up to the telescope, leading to an overhead of about 10 minutes. For observations below 24 degrees of altitude the hatch # 1 is vignetting and should be opened implying, also in this case, an overhead of about 10 minutes.

Weather Limits

The telescope dome shall be closed when any of the following weather conditions occur :

  • Wind speed > 20 m/sec (on the 3.6m monitor)
  • Humidity > 80% (on the 3.6m monitor)
  • Temperature within 3° of dew point (on the 3.6m monitor)
  • Dew on the dome (the Telescope and Instrument Operator will check the dome in person when there is reason to believe that condensation may occur)
  • Sky overcast with dark clouds threatening rain

The telescope shall not be pointed into the wind when the wind speed is more than 14 m/sec (3.6m monitor)

Note: Weather conditions at the 3.6m telescope may be significantly different from those near the NTT and the La Silla Meteo Monitor. In particular the wind speed is a few m/sec higher and the humidity is generally lower.

The Telescope Operator is the ultimate responsible for the telescope and will make the decision to close the dome as necessary. Visiting Astronomers should accept the decision of the Telescope Operator. At any rate, in case of a disagreement the dome should first be closed and subsequently the Visiting Astronomer may take up the issue with the support astronomer and finally the La Silla co-ordinator - though it is highly unlikely that the decision will be altered.

The dome may be re-opened if weather conditions improve and stay below the operating limits for at least 30 minutes. This waiting period is particularly important in case of humidity. It is likely that after a snow storm the telescope will have to stay closed despite clear skyes, due to presence of ice on the dome.