ASM instruments

In this page you will find a brief information about all the instruments used in the ESO sites to monitor weather conditions. 

Astronomical Site Monitor

The Astronomical Site Monitor (ASM) is a critical tool for our operation both in the decision concerning the weather condition and in the selection of the OBs to be observed. The ASM is made of various subsystem.

Differential Image Motion Monitor (DIMM)

The Differential Image Motion Monitor (DIMM) is located on a 7.5 m high platform away from any disturbing infrastructure. This new DIMM is less sensitive to turbulence taking place at the surface layer than the previous system. Therefore its seeing estimate at zenith and in the V band is significantly more reliable, and once airmass is taken into account, is generally in closer agreement with measurements on the UTs. The DIMM seeing is recorded in the fits header files under the keywords TEL AMBI FWHM START/END. 

Note that the seeing measured with the DIMM is computed at 500 nm and at zenith. However, the seeing constraint in the OB corresponds to the data observed image quality at specified reference wavelength and airmass.

We also remind that the TEL.IA.FWHMxxx values are only optimal for images seen through an ADC, which is only the case at UT1-cassegrain focus or at low airmass. More information on the TEL.IA.FWHMLIN & TEL.IA.FWHMLINOBS keywords is available at (password required). These values are also available in a display that the TIOs are running on the TCS of each UT. 

Multi Aperture Scintillation Sensor (MASS)

The expected improvement on the science image quality brought by the new adaptive optics facility will be strongly dependent on the vertical distribution of the turbulence. The a priori knowledge of this atmospheric parameter is thus a requirement to schedule AOF observations most efficiently. 

Two dedicated instruments were added for fulfilling this task. The first one is called Multi Aperture Scintillation Sensor (some additional details and reference at The MASS is coupled to the DIMM and provide information about turbulence profile in the atmosphere.

The MASS installed in Paranal as part of the new ASM is working automatically and is running when the new DIMM is running. By combining the MASS and DIMM measurements, new estimates for the isoplanetic angle (Theta0) and the coherence time (Tau0) are now provided in real time (and without relying on predictions of the wind at 200 mbar as for the old ASM).

An example of this display is available here Tau0 display example

The MASS also provides a seeing estimate in a free atmosphere (above 1km) that is available in the ASM database.


The Slope Detection and Ranging (SLODAR, provides turbulence profiles for the ground layer of the atmosphere uo tp ~1500 m. The use of the the SLODAR data is critical for the HAWKI+GRAAL and MUSE+GALACSI Wide Field Mode.

LHATPRO radiometer

The RPG-LHATPRO radiometer measures the amount of Precipitable Water Vapor (PWV) and other parameters of the sky (such as the effective sky temperature).

In particular two parameters are displayed at all time in the control room: Water Vapour At Zenith and IR Temperature at Zenith. This is illustrated in this image

Presentation on Operational aspects of the LHATPRO radiometers.


VAISALA is the meteo monitor system from Paranal that provides all the information on external conditions (wind, humidity, dew point, particule count, pressure).

More information about the hardware can be found here.