# Precision Auto-Focusing Apparatus Based on Long-Working-Distance, Wide-Spectrum Microscope
## 1 Introduction  
The significant features of the new generation of remote spatial sensing technology are wide field of view and high resolution. To realize these features, remote sensing cameras in remote sensing satellites with large apertures and high resolutions require the use of a large, high resolution Charge Coupled Device(CCD) array device [1; 2; 3]. Due to technological limitations, a monolithic CCD cannot meet the large-size and large field-of-view requirements for the image sensor. In order to obtain a larger image sensor without high additional costs, the method commonly used is multi-CCD stitching assemblage, in which several CCDs with different wavelength filters are assembled in a particular arrangement to form a large CCD array [4; 5]. However, a significant challenge for this approach is that precise auxiliary assembly equipment is needed for precise stitching assemblage of the CCD array.  
The equipment needs to realize the observation of stitching state and effect evaluation by high magnification microscopic imaging of CCD pixel. In addition, the system should have the function of judging the defocusing state of the stitching focal plane, which can realize the precise focusing of the splicing surface. The use of an ordinary microscope is limited by the existence of a spectro-prism and mounting mechanism, which requires the microscope instrument to have an ultra-long working distance to ensure that CCD can be observed through the spectro-prism and mounting mechanism. In order to achieve precise stitching assemblage of a wide-spectrum CCD, the specialized instrument used for precise aiming and positioning must be apochromatic to ensure that each splicing CCD is on the same plane. The accuracy of CCD stitching assemblage is generally of the order of microns. Therefore, as well as having a compact structure, the instruments used for precise stitching assemblage must meet the requirements of long working distance, apochromatism, and high magnification to ensure accuracy and convenience. Considering the characteristics of ultra-long working distance, wide spectrum and high imaging quality, the micro-imaging instrument is not only needed for stitching assemblage equipment, but also for other fields such as microscopic observation in extreme environment, microcircuit welding, etc.  
Mirror-based reflective imaging systems have some unique advantages over refraction imaging systems. Reflective imaging systems have no chromatic aberration--neither axial nor transverse; in theory, reflective microscopic imaging can be applied to any spectral range [6]. Moreover, reflective systems have a lower cost and produce lower material transmission. Reflective imaging technology is widely used in specialized microscopic imaging fields such as X-ray imaging [7; 8; 9], extreme-ultraviolet/ultraviolet (EUV/UV) lithography [10], UV-band micro/nanofabrication [11] and near-infrared imaging [12; 13].  
In this article, we report the use of a mirror-based micro-imaging device to realize micro-sized observation of samples with working distances of several hundreds of millimeters. Based on this device, the automatic focusing system can realize observation and evaluation of the stitching state, and also accurately quantify the defocusing of the image via an automatic focusing image processing method. The automatic focusing system is designed for use in the precise stitching assemblage of a multi-window CCD camera.