Telescope

Image Quality

Image quality depends on several factors including the site characteristics (atmospheric conditions), telescope optics and tracking.

The best seeing that one may expect with the 3.6m telescope is about 0.5 arcsec (measured with EFOSC2) and more generally good seeing is in the range of 0.7 - 0.8 arcsec. The typical seeing measured at the telescope is about 1.0 arcsec. La Silla seeing statistics are available from the La Silla Astroclimatology page and from the La Silla MeteoMonitor and its archive.

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. It is planned, for Period 78, to built a lookup table which will allow to set the secondary mirror position as a function of telescope position in order to minimize aberrations. This could slightly improve the image quality.

Focus: 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: The PSF size and distortion worsen when one observes at large hour angles (> 4 hr) or far to the north. So it is best to observe sources as close to the zenith as possible. It may be noted that this telescope has an equatorial mount and hence observing close to the zenith is not a problem, while observations within 3 degrees of the pole must be avoided.

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.

Adapter/Rotator

Pointing

Pointing Accuracy

Pointing Limits

The sky area accessible to the 3.6m+HARPS telescope/instrument configuration is characterized by the conditions: limit in hour angle HA: -5 h 30 m < HA < 5 h 30 m limit in zenith distance ZD: ZD < 70° 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.

Note :

• 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

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

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. For very high accuracy radial velocity work guiding on the science object itself is mandatory.

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 > 90% (on the 3.6m monitor)
• Temperature within 2° 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.