Most telescopes used for astrophotography are equatorially mounted. However, the more popular computer controlled telescopes are alt-az mounted and are usually deemed unsuitable for astrophotography. The reason is that alt-az mounted telescopes have one more degree of freedom that is not corrected because it does not affect visual observations of celestial objects. Unfortunately, by leaving it uncorrected, alt-az telescopes cannot be used for long exposure astrophotography. In order to overcome this flaw in alt-az mounted telescopes, we have participate in the design and assembly of a derotator for the Turkish national 4-m ground telescope based in Erzurum.