The field rotation effect can be described as observing the gyration of an object with the pupil of the Eastern Anatolian Observatory (DAG) telescope around the optical axis under the influence of the latitude of the observatory while the telescope is following that astronomical object. This is possible as a result of the alt-azimuthal mount of the telescope. (the orientation of the astronomical observed object, the parallactic angle, is defined as “q”) <p> </p>Since the CCD has a low signal on noise ratio, it necessitates long integration time that can vary from a few minutes to hours. It is essential to correct and compensate the rotation of the optical field caused by the earth’s rotation during the monitoring of the astronomical object. <p> </p>A (field) derotator is a class of devices that is used to correct the optical field rotation. In a telescope of a Ritchey- Chretien, Nasmyth configuration, the device must be integrated between the scientific instruments and the M3 mirror. The anastigmatic and the anachromatic features of this type of derotator is the main reason that it is chosen. These characteristics are provided by the K-Mirror design. <p> </p>The aim of this study is to evaluate the possibility to integrate the derotator in the central hole of the telescope fork and to evaluate the mechanical/optical features of the model.
This paper summarize our work on the design of a field derotator for the adaptive optics instruments Nasmyth platform of DAG (Dogu Anadolu Gozlemevi), a new 4 m telescope for astronomical observations near the city of Erzurum, Turkey. While the telescope follows an astronomical object, its pupil sees a rotation of the object around the optical axis which depends on the telescope geographic coordinate and the object sky coordinate. This effect is called the field rotation. This rotation needs to be compensated during the astronomical object data acquisition. In this report we demonstrate the feasibility of placing the derotator (a K-mirror design) in the telescope fork central hole and propose a preliminary design, considering flexures.