  Kanzelh\"ohe Observatory (KSO) was founded during World War II by the
"Deutsche Luftwaffe" (German Airforces) as one station of a network of
observatories, which should provide information on solar activity in order to
better assess the actual conditions of the Earth's ionosphere in terms of radio
wave propagation. The solar observations began in 1943 with photographs of the
photosphere, drawings of sunspots, plage regions and faculae, as well as patrol
observations of the solar corona. At the beginning all data was sent to
Freiburg (Germany). After WWII international cooperation was established and
the data was sent to Zurich, Paris, Moscow and Greenwich. Relative sunspot
numbers are derived since 1944. The agreement between relative sunspot numbers
derived at KSO and the new International Sunspot Number (ISN) \citep{SIDC} lies
within $\sim10\%$. However, revisiting the historical data, we also find
periods with larger deviations. The reasons for the deviations were twofold:
(1) On the one hand a major instrumental change took place during which a
relocation and modification of the instrument happened. (2) On the other hand,
a period of frequent replacement of personnel caused significant deviations,
i.e. stressing the importance of experienced observers. On long-term, the
instrumental improvements led to better image quality. Additionally we find a
long term trend towards better seeing conditions since the year 2000. We may
speculate that this is related to climatic changes.
