An array of small crosses or dots exposed on to a photographic film or plate before processing, or overlain on the photocathode of an electronic detector, to provide a standard basis for measurement of positions of astronomical objects.
科学参考收录了60776条科技类词条，基本涵盖了常见科技类参考文献及英语词汇的翻译，是科学学习和研究的有利工具。