diff --git a/pyobs/modules/telescope/basetelescope.py b/pyobs/modules/telescope/basetelescope.py
index 36834325..125a91d7 100644
--- a/pyobs/modules/telescope/basetelescope.py
+++ b/pyobs/modules/telescope/basetelescope.py
@@ -310,12 +310,10 @@ async def _update_celestial_headers(self) -> None:
             "SUNDIST": (None if sun_dist is None else float(sun_dist.degree), "Solar Distance from Target"),
         }
 
-    def _get_derotator_offset_from_header(self, hdr, obstime):
-        lat, lon, height = hdr["LATITUDE"][0], hdr["LONGITUD"][0], hdr["HEIGHT"][0]
-        location = EarthLocation(lat=lat * u.deg, lon=lon * u.deg, height=height * u.m)
-        target = SkyCoord(ra=hdr["TEL-RA"][0] * u.deg, dec=hdr["TEL-DEC"][0] * u.deg, frame="gcrs")
-        parallactic = Observer(location=location).parallactic_angle(time=obstime, target=target).deg
-        return float(parallactic - hdr["TEL-ALT"][0] - hdr["TEL-ROT"][0])
+    def _calculate_derotator_position(self, ra: float, dec: float, alt: float, obstime: Time) -> float:
+        target = SkyCoord(ra=ra * u.deg, dec=dec * u.deg, frame="gcrs")
+        parallactic = self.observer.parallactic_angle(time=obstime, target=target).deg
+        return float(parallactic - alt)
 
 
 __all__ = ["BaseTelescope"]