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Kanzelh\"ohe Observatory (KSO) was founded during World War II by the"Deutsche Luftwaffe" (German Airforces) as one station of a network ofobservatories, which should provide information on solar activity in order tobetter assess the actual conditions of the Earth's ionosphere in terms of radiowave propagation. The solar observations began in 1943 with photographs of thephotosphere, drawings of sunspots, plage regions and faculae, as well as patrolobservations of the solar corona. At the beginning all data was sent toFreiburg (Germany). After WWII international cooperation was established andthe data was sent to Zurich, Paris, Moscow and Greenwich. Relative sunspotnumbers are derived since 1944. The agreement between relative sunspot numbersderived at KSO and the new International Sunspot Number (ISN) \citep{SIDC} lieswithin $\sim10\%$. However, revisiting the historical data, we also findperiods with larger deviations. The reasons for the deviations were twofold:(1) On the one hand a major instrumental change took place during which arelocation 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, theinstrumental improvements led to better image quality. Additionally we find along term trend towards better seeing conditions since the year 2000. We mayspeculate that this is related to climatic changes.

70 years of Sunspot Observations at Kanzelh\"ohe Observatory: systematic study of parameters affecting the derivation of the relative sunspot number