Design and analysis of a five-mirror derotator with minimal instrumental polarization in astronomical telescopes.

It is well known that derotator is commonly used to compensate for image rotation in astronomical telescopes. The classical derotator, consisting of three mirrors, is named K-mirror. The main disadvantage of the K-mirror is that it has high instrumental polarization, causing a larger polarization measurement error in polarimetry. In the paper, we proposed a new design of the derotator based on the Mueller matrix transformation theory of mirrors in polarized light. The new designed derotator contains five mirrors and is called five-mirror derotator. It has a very small instrumental polarization, overcoming the shortcomings of the classical K-mirror. A novel method is applied to compensate for the beam shift induced by the thickness of the reflector. From the result of numerical polarization analysis, it is showed that the five-mirror derotator has a significant advantage for the telescopes with a small field of view, especially in polarimetry at longer wavelengths.