Plate scale variation detecting method for LINC-NIRVANA

LINC-NIRVANA is a near-infrared image-plane beam combiner with advanced layer-oriented multi-conjugated adaptive optics (MCAO) for the Large Binocular Telescope. The instrument will combine the lights from the two 8.4 m primary mirrors in “Fizeau” mode in which the wavefronts will interfere in the focal plane, not in the pupil plane, so it is a true imaging instrument. The field of view can be several arcminutes, only limited by the performance of the adaptive optics (AO) system to obtain near-zero aberration wavefronts over large sky angles. However, if the secondary mirrors of the MCAO system have mechanical displacement in a way that would make the plate scale of the two systems change and then may cause variant PSFs and a degradation of the image quality which may affect the fringe tracking performance. Thus it is very important to know whether the plate scale of LN changes or not. To try to compensate for this problem, a plate scale variation detecting method is proposed. The useful information is gotten from the un-overlapping wavefront which is measured by the high wave-front sensor (HWS), and then processed by an algorithm to extract the long-time exposure average value of wavefront tip and tilt which is related to and response to the plate scale change. The simulation study shows that the method is feasible, and the paper gives practical guidelines for the application of plate scale variation detecting for LINC-NIRVANA.