Since about 1987, all astronomers observing at the main telescopes of
the La Silla observatory are requested to log the seeing angle
experienced during their observations.
Although this request is not always followed and the seeing values
are logged rather irregularly, this data base
provides significant information on the large differences
from telescopes that are in many respects quite
similar and located on the same site.
Unequivocally these differences can only be caused by the enclosure and
the atmospheric environment in the immediate surrounding of the
telescope
A plot of the data (fig. 
) outlines in
particular the
difference between the 3.6-m and the 2.2-m telescopes.
All values are corrected to get the seeing at zenith and at 1 
m
wavelength.
The data of the DIMM seeing monitor (see page )
starting mid 1991 quantify even more dramatically the degradation of
optical image quality at the other telescopes
determined by the type of enclosure and building.
Also since 1991 onward, the NTT establishes a new quality benchmark at the
observatory.
[IMAGE ]
Figure: Monthly average seeing at various telescopes at La Silla:
).
![[IMAGE ]](img206.gif){kind=link}