天文学以科学为导向、观测为基础、发现为驱动。大天区面积高精度多波段成像测光巡天已成为发现和探索新天体、新现象，深入研究各类天体性质及其形成和演化的最重要的天文学研究手段。至今为止，所有国内外的大视场巡天计划均只能在同一时间对观测天区进行单一波段成像，无法获取天体的实时颜色信息。因此，基于这些巡天“拍摄”的天体运动、变化的影像仅仅是“黑白影片”或“伪彩色影片”。开展变源、暂现源研究的瓶颈在于获取高精度的实时颜色信息以对其各类候选体进行快速分类与证认，而现有的巡天均无法有效解决该难题。本项目拟研制建设的云南大学“多通道测光巡天望远镜”（Multi-channel Photometric Survey Telescope，简称Mephisto），通过创新性的光学设计，将在国际上首次实现较大通光口径、较大视场、多通道高精度测光巡天。
Large-scale astronomical surveys play a key in modern astrophysical studies, leading to new discoveries, and revolutionizing our understandings of the origin and evolution of the universe. The fast technological advances are changing the ways we view the sky, from snapshot taking in surveys such as SDSS and XSTPS-GAC, to movie shooting by observing the same sky thousands of times in surveys, such as ZTF, Skymapper, WFST and LSST. The developments open up the possibility to probe the universe in multi-dimensions, in particular in time domain. Build upon the experiences of those surveys and to make a step forward, we plan to build a wide-field multi-channel survey telescope -- the Multi-channel Photometric Survey Telescope (Mephisto), the first of this type in the world.
Mephisto是一台较大通光口径（1.6米）、较大视场（3.14平方度）的多通道测光巡天望远镜，其基本参数见表一。Mephisto创新性地采用了加改正镜的Ritchey-Chretien（RC）系统与立方分色棱镜，使望远镜焦面在主镜之下，且焦面与主镜之间有较大的距离，从而能在后端光路安置分色系统及多台相机，同时保证了整个视场上的优质像质（图一）。Mephisto将配备三台CCD相机，总像素高达10亿，能同时在三个波段（ugi或vrz）对同一天区进行高质量成像测光观测，获取高精度的实时颜色信息，“录制”宇宙天体运动和变化的“彩色记录片”。Mephisto不仅显著提高了巡天的效率和颜色定标精度，更重要的是通过提供高精度的实时颜色信息，有效地解决了变源与暂现源研究面临的瓶颈。高精度的颜色信息对获取天体的一系列重要物理参数（如温度、金属丰度、距离、年龄等）也具有极为重要的意义。Mephisto研制期四年（2018-2021年），预计2022年正式投入观测。
Mephisto has a 1.6 m primary and is equipped with three CCD cameras of a field-of-view 3.14 deg2 (Fig. 1), capable of simultaneously imaging the same patch of sky in three bands (ugi or vrz). The three cameras boast a total of 1.0 Giga pixels. Mephisto will yield real-time colours of astronomical objects with unprecedented accuracies, and deliver for the first time a coloured documentary of our evolving universe. The development of Mephisto is expected to take four years (2018-2021) , and it is expected to be operational in 2022.
Table 1 Main specifications
图一 Mephisto光路图 Figure 1 Mephisto optical path
Mephisto巡天计划包括两部分：1）对北半球可观测天区开展多通道测光巡天（W巡天）；2）采用不同采样频率对多个不同面积的指定天区开展多通道测光巡天（D、H和M巡天）。
W巡天：在巡天启动的第一年（2022年），用ugi、vrz两组滤光片对北半球可观测天区（δ>-15°，约26,000平方度）进行拍摄。曝光时间2×20秒，极限星等r≈22.37等。每个观测夜Mephisto将对同一天区进行两轮拍摄，时间间隔15-60分钟。全年预计完成两组滤光片各五组拍摄，叠加后极限星等达到r≈23.27等。经定标处理的真彩色图像将向全世界发布，供研究者使用。在随后的九年里（2023-2031年），Mephisto还将用30%的时间观测W天区，平均每年完成三轮拍摄。至2031年底，完成两组滤光片总共18轮拍摄，最终极限星等r≈23.98等，颜色定标精度0.2-0.5%。
D、H与M巡天：自巡天第二年（2023年）起，Mephisto将用70%的观测时间对多个1800平方度D天区、180平方度H天区及3-18平方度M天区进行ugi或vrz拍摄，采样频度分别为天、小时和分钟量级，开展时域天文学观测研究。D、H和M巡天将获取天体在不同时间跨度、不同采样间隔、不同采样次数、不同深度的高精度多波段亮度与实时颜色变化曲线。相比W巡天，D、H与M巡天采样频度更高、极限星等更深，为研究变源及暂现源提供极为丰富的高质量数据。
The Mephisto surveys will have two components: the Mephisto-W covering the whole northern sky of ～26,000 deg2 and the Mephisto-D, Mephisto-H and Mephisto-M covering sky areas of respectively thousands, hundreds and tens of square degrees with a corresponding cadence of days, hours and minutes, targeting specifically fast variables and transients.
Mephisto-W. The Mephisto-W survey will target the whole northern sky of ~26,000 deg2 at δ>-15°. The first year of the survey (2022) will be dedicated to Mephisto-W. The full survey area will be imaged five times over the year, in both ugi and vrz filter combinations, using pairs of 20-second exposures twice per night, delivering a typical 5σ depth of r~22.37 mag. For the following nine years (2023 -- 2031), 30% of the observing time will be allocated to Mephisto-W. After 10 years of operation, Mephisto will uniformly observe the whole survey area 18 times (for both the ugi and vrz filter combinations), leading to a coadded depth of r~23.98 mag and colour accuracies of 0.2-0.5%.
Mephisto-D, H and M. Starting from the second year (2023), 70% of the observing time will be allocated to Mephisto-D, H and M surveys. Mephisto-D will cover several regions of interest, each of an area of ~1800 deg2. Mephisto-H and Mephisto-M will target regions with higher sampling frequencies, with each D and M region covering an area of 180 deg2, and 18 deg2, respectively. The D, H and M areas will be imaged in ugi or vrz filter combinations with the time intervals of days, hours and minutes, respectively. Comparing to Mephisto-W, Mephisto-D, H and M will have better time samplings and deeper limiting magnitudes, providing high quality data for time-domain science.
Mephisto的核心科学目标围绕其特有的实时高精度颜色测量及大视场、高效率等优势展开，涉及时域天文学、星系宇宙学、银河系考古学及近场宇宙学、恒星物理、太阳系等天文学基础前沿研究领域多个关键核心科学问题。具体包括：
Mephisto独一无二的精确实时颜色变化信息将对开展各类变源及暂现源的搜寻、快速分类与证认及后续研究提供极大的帮助，如引力波事件电磁对应体、极端贫金属超新星的搜寻。Mephisto的大视场能快速扫描引力波事件的可能天区，提供视场中变源的实时颜色变化信息，从而快速有效地证认出其可能的对应体。Mephisto提供的实时颜色变化曲线能有效地限制超新星前身星的金属丰度，以快速有效地搜寻到极端贫金属超新星。此外，Mephisto采样频率从分钟到天量级的D、H与M巡天将有助于系统搜寻、研究宇宙中各类有重要科学意义的爆发事件，如黑洞潮汐瓦解事件、伽玛暴余辉以及其他未知的快速变化的宇宙暂现源事件。
Mephisto获取的高精度实时颜色信息能够快速有效地证认Ia型超新星并进行分类。预计巡天将获得超过三万颗Ia型超新星精确的光度及实时颜色变化信息，从而帮助揭开Ia型超新星前身星及其爆发机制的谜团，并精确校准Ia型超新星绝对星等与颜色、颜色变化曲线形状、光变幅度和光变曲线形状之间的关系。Mephisto还将探测到超过2000万颗活动星系核与类星体，提供的精确光变、颜色与自行等信息对深入研究该类天体的性质具有重要的意义。Mephisto将极大扩展强引力透镜系统样本，获取时间延迟信息。Mephisto获得的大样本Ia型超新星、活动星系核以及强引力透镜系统，有助于深入研究宇宙中星系的形成与演化历史，限制宇宙学模型、暗物质与暗能量的性质等。
Mephisto测光系统包括了对恒星大气基本参数即为敏感的v滤光片，结合其高效的巡天能力及高精度的颜色定标，Mephisto将极大地提升对极端贫金属星的搜寻能力。预计Mephisto可探测超过两万颗极端贫金属星，这将极大地促进深入理解宇宙第一代恒星的形成、演化与核合成过程，及银河系早期形成历史。Mephisto将获取银河系超过1%恒星（≥20亿）的精确参数（位置、距离、自行、化学丰度、年龄等），为精确描绘银河系结构子结构、星族、尘埃分布及其化学动力学性质，揭示银河系的集成与演化历史提供高质量的数据。
Mephisto获取的精确的多波段与实时颜色变化曲线，在深入研究各类脉动变星的内部结构与脉动机制，精确校准有重要宇宙学应用价值的脉动变星（如天琴座RR型变星、长周期变星以及造父变星等）的周期光度关系等方面具有独特的优势。Mephisto还将通过精确的颜色信息对不同类型的白矮星进行搜寻、证认、参数测定与研究。通过不同的方法，Mephisto可以测量大量恒星精确的年龄。Mephisto还可以有效地寻找疏散星团成员星，并进一步研究恒星的形成与演化规律。
Mephisto将对整个黄道面进行采样频率为天量级的观测，完成对整个太阳系各类小型天体的全景式普查。初步估计，Mephisto将能探测到约500万颗主带小行星，四万颗海（王星）外天体，五万颗特洛伊型小行星等。Mephisto特有的实时高精度颜色测量有助于对小行星进行快速准确的分类，揭示太阳系的起源与演化。此外，Mephisto还将对近地天体、空间碎片进行搜寻和检测。据估计，Mephisto可以探测、监视、编目和证认90%以上有潜在威胁、直径大于140米的近地天体。
Mephisto巡天项目的实施还将为我国培养一批实测和理论天体物理学家，扩大天文学的社会影响力。它的实施将为我国跻身大视场巡天科学世界先进行列、增强我国的全球竞争力做出重要贡献。
The unique capability of recording high-precision real-time colours, thus allowing quick and robust classification of fast transients, makes Mephisto particularly powerful for time domain astrophysics. Celestial objects and forefront problems that Mephisto will target and tackle include, for example,
Astrophysical transients and the underlying physics. The real-time multi-channel and high cadence observations of Mephisto surveys will enable efficient searches and classifications of different transients, including electromagnetic counterparts of gravitational waves, supernova explosions of extremely metal poor stars, tidal disruption events, gamma-ray bursts, and physically still unknown transients.
Galaxies and cosmology. Mephisto will be able to produce large samples of Type Ia supernovae, QSOs and AGNs, as well as strong lensing systems. The large sample sizes, the high image qualities, the precise real-time colours (and variations), multi-band magnitudes (and variations), and positions (and variations) will provide enormous information that allows us to probe the nature of those objects. This, in turn, will reveal to us how the galaxies form and evolve. Cosmologically, the deep understanding of these objects can greatly improve the constraints on properties of dark matter and dark energy because of the tight systematic controls of different cosmic tracers, such as Type Ia supernovae.
Extreme metal-poor stars and Galactic archeology. Mephisto will carry out a systematic search for the most metal-poor stars in the Galaxy using sensitive filter sets the same as that of Skymapper. Mephisto will also provide us accurate stellar parameters for billions of Galactic stars, such as the effective temperature, metallicities, distances and ages, etc. These will offer an unique opportunity to reveal the true multi-dimensional structure and the formation and evolution history of the Milky Way.
Stars and variables. Based on the accurate real-time colour variations and multi-band magnitudes, Mephisto will make fundamental contributions to our understanding of variability in stars of different kinds, including various types of variable stars and white dwarfs. This can not only improve our knowledge about the stellar formation and evolution, but also refine the accuracy of distance ladders from variable stars.
Planetary systems including the Solar System. Mephisto will carry out a comprehensive survey of the small bodies of the Solar System, and help to understand the formation and evolution of the Solar System. Mephisto will also be able to detect and characterise the Near-Earth Objects.
Mephisto data products will be publicly available to the domestic and international communities. The successful development and operation of Mephisto will help enhance the global scientific and technique competitiveness of the Chinese astronomical community. It will also benefit significantly the growth of next generation of Chinese young scientists.
