An Automatic Observation Management System of the GWAC Network I: System Architecture and Workflow
Xuhui Han, Yujie Xiao, PinPin Zhang, Damien Turpin, Liping Xin, Chao Wu, Hongbo Cai, Wenlong Dong, Lei Huang, Zhe Kang, Nicolas Leroy, Huali Li, Zhenwei Li, Xiaomeng Lu, Yulei Qiu, Jing Wang, Xianggao Wang, Yang Xu, Yuangui Yang, Yong Zhao, Ruosong Zhang, Weikang Zheng, Yatong Zheng, Jianyan Wei
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AN AUTOMATIC OBSERVATION MANAGEMENT SYSTEM OF THE GWAC NETWORK I: SYSTEMARCHITECTURE AND WORKFLOW
Xuhui Han, Yujie Xiao, PinPin Zhang, Damien Turpin, Liping Xin, Chao Wu, Hongbo Cai, Wenlong Dong, Lei Huang, Zhe Kang, Nicolas Leroy, Huali Li, Zhenwei Li, Xiaomeng Lu, Yulei Qiu, Jing Wang,
5, 1
Xianggao Wang, Yang Xu, Yuangui Yang, Yong Zhao, Ruosong Zhang, Weikang Zheng, Yatong Zheng, and Jianyan Wei Key Laboratory of Space Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101,China. Universit´e Paris-Saclay and Universit´e de Paris, CNRS, CEA, AIM, F-91191 Gif-sur-Yvette, France. Changchun Observatory, National Astronomical Observatories, Chinese Academy of Sciences, Changchun 130117, China. LAL, Univ Paris-Sud, CNRS/IN2P3, Orsay, France. Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004,China. School of Physics and Electronic Information, Huaibei Normal University, Huaibei 235000, China. Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China. Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA.
Submitted to PASPABSTRACTThe Ground Wide Angle Camera Network (GWAC-N) is an observation network composed of multi-aperture andmulti-field of view robotic optical telescopes. The main instruments are the Ground Wide Angle Cameras Array(GWAC-A). Besides, several robotic optical telescopes with narrower field of views (than the GWAC-A) provide fastfollow-up multi-band capabilities to the GWAC-N. The primary scientific goal of the GWAC-N is to search for theoptical counterparts of Gamma Ray Bursts that will be detected by the Space Variable Object Monitor (SVOM)satellite. The GWAC-N performs many other observing tasks including the follow-ups of Target of Opportunities(ToO) and both the detection and the monitoring of variable/periodic objects as well as optical transients. To handleall of those scientific cases, we designed 10 observation modes and 175 observation strategies, especially, a jointobservation strategy with multiple telescopes of the GWAC-N for the follow-up of gravitational wave (GW) events.To perform these observations, we thus develop an Automatic Observation Management (AOM) system in chargeof the object management, the dynamic scheduling of the observation plan and its automatic broadcasting to thenetwork management and finally the image management. The AOM system combines the individual telescopes into anetwork and smoothly organizes all the associated operations. The system completely meets the requirements of theGWAC-N on all its science objectives. With its good portability, the AOM is scientifically and technically qualifiedfor other general purposed telescope networks. As the GWAC-N extends and evolves, the AOM will greatly enhancethe discovery potential for the GWAC-N. In the first paper of a series of publications, we present the scientific goalsof the GWAC-N as well as the hardware, the software and the strategy setup to achieve the scientific objectives. Thestructure, the technical design, the implementation and performances of the AOM system will be also described indetails. In the end, we summarize the current status of the GWAC-N and prospect for the development plan in thenear future.
Corresponding author: Yujie [email protected] author: PinPin [email protected] a r X i v : . [ a s t r o - ph . I M ] F e b Han et al.
Keywords: automated telescopes, observation management system (AOM) he Automatic Observation Management System: I INTRODUCTIONIn the last decade, a new type of network emerged. Thanks to the modern computing and communication technolo-gies, these telescopes are designed to form a general-purpose observation network, such as Las Cumbres ObservatoryGlobal Telescope (LCOGT, Brown et al. 2013), the Global Relay of Observatories Watching Transients Happen(GROWTH, Kasliwal et al. 2019), the All-Sky Automated Survey for Supernovae (ASAS-SN, Shappee et al. 2014),the Robotic Optical Transient Search Experiment (ROTSE, Akerlof et al. 2003), the Pan-STARRS Survey (Chamberset al. 2016), the Rapid Action Telescope for Transient Objects (TAROT, Bo¨ e r et al. 1999), and the Master-Net(Lipunov et al. 2010)... Such inclusions of individual facilities into a global interconnected network is a key to largelyenhance the discovery potentials and take up the challenge of the multi-messenger astronomy of the next decade.However, netting telescopes, organizing and scheduling for the general-purpose network are the common problems inmodern observational astronomy, since these facilities with limited resource are designed only for given purposes, whichrequires different size, photometry parameters and controlling technics. A huge human intervention is still involved inthe schedule process for most modern observation networks (Mora & Solar 2010).Under the framework of the Chinese-French Space Variable Object Monitor (SVOM) mission, an array consistedof a set of 9 Ground-based Wide-Angle Cameras (GWAC-A, hereafter) is designed to simultaneously search for theoptical prompt emission of Gamma Ray Bursts (GRBs) detected by the SVOM on-board gamma-ray instruments(ECLAIRs and GRM, Cordier et al. 2015; Wei et al. 2016). Furthermore, several robotic, multi-band, small Fieldof View (FoV) telescopes are also deployed for automatically validating and following up candidates detected bythe GWAC-A. In fact, combining these wide FoV telescopes and fast-slewing, multi-band, small FoV telescopes in awell organized network can permits to obtain a better observational coverage and detection performances useful formultiple tasks such as, large-sample surveys, periodic and quasi-periodic objects, transient targets, moving objects.But successfully performing these observations depends not only on instrument properties but also on a networkcombining robotic telescopes, communication, observation scheduling, observation controlling and data processing.Besides, in order to catch the nature of different scientific targets, different optimized observation strategies mustbe implemented. The optimization of observation strategy should balance between the fruitful scientific returns andthe limited telescope resource. Therefore, we develop an automatic observation management system to integratethe facilities and software into a network named as the GWAC network (GWAC-N hereafter), since the GWAC-A,is the majority of this network. The automatic observation management system contains functions of observationtarget management, fully-automated dynamic observation scheduling and autonomous telescope dispatching, datamanagement. The system enhances the efficiency of uses of the GWAC-N to a great level by automatized carryingout multi-target, multi-telescope, simultaneous joint observations, all the routine observations for each telescope, andkeeping the manual observing function. With standard datalink, this system can be easily adapted to other similarsubjects in time-domain astronomy and extended to the collaborative telescopes.In 2014, 12 mini-GWAC, the pathfinder telescopes of the GWAC-A started operation in the GWAC dome at Xinglongobservatory (Huang et al. 2015). Two 60-cm follow-up telescopes (GWAC-F60A/B) were installed in 2015 and achievedfirst light in the same year. The first GWAC mount equipped with 4 Joint Field of View (JFoV) cameras and 1 Full Fieldof View (FFoV) camera were installed and tested in 2017. In 2018, 2 fully equipped GWAC mounts, 2 GWAC-F60A/B,1 GWAC-F30 were in operation. Figure 1 shows the dome and telescopes of the GWAC-N. Although the telescopeswere in place, they were still not connected as a network. The telescopes were operated separately and manuallycontrolled by two observation assistants during night observations. Responding speed and observation efficiency werelow. Observation capability for scientific targets was limited. Paving a small part of sky with the GWAC-A andmonitoring several targets with the GWAC-F60A/B and the GWAC-F30 were the pattens of the routine observationsat this stage. Thus, the automatic observation management (AOM) system had been developed in 2019 to integrate thehardware and software of the GWAC-N to fulfill the scientific requirements described in the Section 4. In the late 2019and early 2020, the Tsinghua-NAOC (National Astronomical Observatories of China) Telescope (TNT) at XinglongObservatory, and the Chinese Ground Follow-up Telescope (CGFT) at Jilin Observatory started to work collaborativelywith the GWAC-N as external partners by taking advantage of the ToO alert processing and managing capability ofAOM. The GWAC-N can functionally perform the observation tasks to meet all the scientific requirements of thenetwork by adopting the AOM as of the date of this writing (December 2020). A complete GWAC-N will comprise 9mounts equipped with 36 JFoV and 9 FFoV cameras and several associated follow-up telescopes. Two world-wide sitesand advanced CMOS detectors are foreseen to be applied to the GWAC-N in a near future. The development timeline Han et al.
Figure 1.
Top: The dome of the GWAC. The roof can be opened along the rails. Bottom left: the telescopes of the GWAC-A,the GWAC-F60A/B and the GWAC-F30. Bottom right: two types of cameras (JFoV and FFoV camera) mounted on oneGWAC mount. depends on the future funding and maturity of new technology. Since the GWAC-N is still under development andevolution, this paper describes the structure of network based on the current stages.In this paper, we present the GWAC-N’s telescopes, the AOM system and the opportunities / science outputs fromthe GWAC-N. The remainder of the paper is organized as follows. Section 2 describes the system structure of theGWAC-N and the instruments of the GWAC-N, including internal telescopes and the extend partners. In Section 3We then present the AOM of the GWAC-N that we developed for performing for the multi-purpose, flexible, highlyefficient observations. We will describe the scientific opportunities of the GWAC-N and achievement of the networkin Section 4. In Section 5, we summarize the current status of the GWAC-N and describe network prospects for thenear future. SYSTEM STRUCTURE OF THE GWAC-NThe whole system of the GWAC-N (shown in the Figure 2) comprises three main parts: the target input interfaces,the AOM system and the telescopes. In this section, we describe the target input interfaces and the telescopes. TheAOM system is described in the Section 3. 2.1.
Target Input Interfaces
The GWAC-N provides multiple external interfaces connected with a variety of alert streams, survey/catalogueplanners, the GWAC-A self-detected transient validation system (Xu et al. 2020A) and scientists. All automatic ormanual observation requests are inserted into the system via these external interfaces. he Automatic Observation Management System: I GCNLIGO/Virgo GW Alert GCNGRB, Neutrino AlertChinese Multi-Messenger Server & DatabaseGravitation Wave Skymap Processor (LAL)
Automated Observation Management System of GWAC-N (Multiple independent projects run in the AOMs)Manual Target Submission Tool
Proposals
Routine Observation Planner
Objects/Surveys
Validating/Revisiting Observation of Transient Candidates Detected in SurveysGWAC TelescopesGWAC-F60A/B Telescopes GWAC-F30 Telescopes GWAC Real-time Automatic Transient Validation System (RAVS) TNT and CGFT Telescopes A l e r t A l e r t Q u e u e O b s e r v a t i o n S e q u e n ce s T a r g e t L i s t s P o i n t i n g L i s t s T il e L i s t G a l a x y L i s t Galaxy ListTile List T a r g e t s O b s e r v a t i o n P l a n s O b s e r v a t i o n P l a n s O b s e r v a t i o n P l a n s O b s e r v a t i o n P l a n s O p t i ca l T r a n s i e n t C a n d i d a t e s O b s e r v a t i o n R e q u e s t s ToO Alert Interface
GW, GRB, Neuturino
Alert T a r g e t L i s t P o i n t i n g L i s t GWAC-N O b s e r v a t i o n S t a t u s O b s e r v a t i o n S t a t u s O b s e r v a t i o n S t a t u s Figure 2.
The GWAC-N is composed of interfaces for the target inputing, the multiple telescopes and the AOM system. TheToO alert interface can receive the alerts of GW, GRB and neutrinos from the external CMM server . The TNT and the CGFTtelescopes are connected with the GWAC-N as external partners.
During the O3 run of LIGO/Virgo, the SVOM team develops the Gravitation Wave Skymap Processor (GWSP) atIr` e ne Joliot-Curie laboratory (IJCLab at CNRS/IN2P3), France. The GWSP digests the GW skymap and optimizesthe tiling observation strategy based on the telescope parameters for the GWAC-A, the GWAC-F30 and the CGFT.Using the Mangrove galaxy catalog (Ducoin et al. 2020A), the GWSP can create optimized galaxy lists for small FoVtelescopes like the GWAC-F60A/B. The format of the tiling coordinates and galaxy lists are standard, therefore, itcan also be applied to other telescopes, i.e. the GRANDMA (the Global Rapid Advanced Network Devoted to theMulti-messenger Addicts) network (Antier et al. 2020). The tiling and galaxy lists are sent to the Chinese Multi-Messenger (CMM) server using the VOEvent protocol via brokers. The CMM Service can receive the GRB or Neutrinoalert streams from GCN public access by using the pygcn code (Leo Singer ). The GWAC-N provides an interface toautomatically receive the GW alerts from the CMM in real time.Several observation planning codes are running to create target/pointing list for all telescopes to perform routineobservations. Each planner can insert the target/pointing list into the AOM using a client provided by the GWAC-N.The GWAC-N also accepts observation applications from scientists. A tool allows the scientists to customize theobservational parameters and to generate complex observation programs. The GWAC-N has another type of targets,the self-detected transient candidates of the GWAC-A validated by the Real-time Automatic transient ValidationSystem of the GWAC-N (RAVS, Xu et al. 2020A). The target needs to be quickly identified and followed-up by theGWAC-F60A/B. Therefore, an interface has been developed for real-time communications between the RAVS and theAOM. 2.2. The telescopes
The GWAC-A telescopes are the main instruments of the GWAC-N. Two GWAC-A telescopes are being operated(two more are under testings) at the Xinglong Observatory (lat = 40 ◦ ◦ https://github.com/lpsinger/pygcn/ Han et al.
Figure 3.
The sky, in Equatorial coordinates, is partitioned into 148 grids of equal area fitting the mount’s FoV. The GWAC-Atelescope points to the center of a grid (blue dot), so it can cover the sky field (red square) with its wide FoV. • the Joint Field of View (JFoV) camera, a refractive lens with an aperture of 180 mm, is equipped with 4k x 4kCCD camera. The FoV of a JFoV camera is ∼ ◦ x 12.8 ◦ . The CCD camera is composed with a 4K E2V chip anda customized liquid cooler system, which allows the CCD works in -50 ◦ Celsius with respect to the local environmenttemperature. 4 cameras are installed on a connection frame with angle adjustment mechanism. By carefully adjustingthe pointing angles of the JFoV, the four JFoVs cameras are paved in a square sky field. The joint field of view forone mount (four JFoV cameras) reaches about 25 ◦ x 25 ◦ . The limiting magnitude of the JFoV camera reaches R ∼
16 magnitude for a single image (10 seconds of exposure) in a dark night without cloud. From the stacking images, atypical limiting magnitude of R ∼
18 magnitude is obtained. • the Full Field of View (FFoV) camera, a SIGMA 50mm F1.4 lens with aperture of 3.5 cm, is equipped with anApogee U9000X 3k x 3k CCD camera. The FoV of a FFoV camera is ∼ ◦ x 30 ◦ , which covers the approximatelysame sky field of the joint FoV of the four JFoV cameras.The FFoV carries out guiding and extending the optical fluxcoverage to R ∼ ∼ degree/second slewing speed and 18’x18’ of FoV with the 2Kx2K Andor iKon-L 936 CCDs. The GWAC-F30 with aFoV of 1.8 ◦ x1.8 ◦ can complete the gaps of flux coverage and the FoV between the GWAC-A and the GWAC-F60A/B.All the three telescopes are equipped with the Johnson UBVRI filters. With remote controlling and real-time dataprocessing, they can be integrated into the GWAC-N. As a whole, the GWAC-N obtains the capabilities for multipleobjectives from survey, queue observations to follow-up observations for many types of targets. The parameters ofeach type of telescope are summarized in Table 1.By using customized datalink, the GWAC-N can collaborate with external telescopes to extend the network. Theexternal network currently includes two telescopes: the 80-cm Cassegrain reflecting TNT telescope located at theXinglong Observatory of NAOC and the 1.2-meter CGFT at the Jilin Observatory of NAOC. The parameters of theTNT can be found in Zheng et al. (2008), Huang et al. (2012). The parameters of the CGFT is being tested, as thetelescope is under hardware updating. AOM SYSTEMFor a highly efficient telescope network, a strong and smart observation management is a key factor. The automaticobservation management is the only way to integrate tens of telescopes into a complete network, the GWAC-N. he Automatic Observation Management System: I Table 1.
Telescope parameters telescope number aperature FoV filter limiting magnitude number of cameras(cm) (single/stack)GWAC 2 18 (JFOV) 12.5 ◦ *12.5 ◦ . Clear 16/18 83.5 (FFOV) 25 ◦ *25 ◦ . Clear 12 2F60A/B 2 60 18’*18’ Clear, UBVRI 18/19 2F30 1 30 1.8 ◦ *1.8 ◦ . Clear, UBVRI 16.5/17 1The limiting magnitude is measured in a 10-second, R band, single image and stacked images.The figures of above parameters are up to date as of Dec. 2020. Other interfaces GWAC-N Internal TelescopesExternal TelescopesToO Alert Interface
GW, GRB, Neuturino
AOM System Architecture
All Targets Database Communication ServerCurrent Listed Targets/Pointings Database InternalDispatcherObservation Status Monitor Server Observation Status Monitor ClientDynamic Scheduler ToO Follow-up System ExternalDispatcher Server Telescope ControlorTarget Managing Telescope Controlor
Figure 4.
All the sub-systems or modules of the AOM system are shown in the red color. The ToO follow-up sub-systemobtains the alerts of the ToO from the CMM database as input targets. The target management sub-system receives all othertypes of observation requests, and converts them as input targets. For the internal telescopes of the GWAC-N, a client runningin the telescope side can monitor the observation and data statues, and transmit them back to the AOM. For the externaltelescopes, the datalink is customized for different telescopes. No client is installed in the telescope side in current stage.
Otherwise, the huge workload during observations is unacceptable for our scientists on duty, not to mention the slowresponse and inefficient observation. Therefore, we developed the AOM system for the GWAC-N to manages all inputtargets, distributes them to all telescopes and organizes observations with all types of strategies. The system consists ofthe following sub-systems: the ToO follow-up, the target management, the scheduler, the dispatcher sub-systems andthe communication center. The architecture of the AOM is shown in the Figure 4. The functions of each sub-systemis given in the following sub-sections. 3.1.
ToO follow-up sub-system
The ToO follow-up sub-system monitors the CMM database for the newly arriving alerts. Currently, three typesof events, including the LIGO/Virgo GW, the Swift GRB and the Fermi GRB, are selected by the ToO follow-upsub-system. The sub-system generates a target or a sequence of targets with observation parameters for the alert thatmeets the alert selection criteria. The observation parameters (such as instrument, observation mode, exposure, etc. )are set based the observation strategies dedicated to different cases. The alert selection criteria, observation strategiesare defined regarding the physical variable behavior of the target and the telescope detection capability to increase thechance of detecting the optical counterpart of the ToO. The details of the selection criteria and observation strategieswill be described in another paper (Han et al. in preparation). For the external partners of the GWAC-N, the datalinkis customized for a dedicated telescope. A dispatcher sends a target or target sequence to the TNT and the CGFT
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Target Management Workflow
Target format checking. Is correct? Manual Correction
Automatic Target Submission Interface Manual Target Submission Interface
Action type checking. Duplicate target checking. Is a duplicate target? Sending Command to Communication Server
IncorrectCorrectedCorrectAdding targetsNo Updating or Deleting targetsInterface Target management system
Figure 5.
The workflow of target management starts from the target inputting via automatic/manual submission interfaces.The format validation, manual correction, adding, updating, deleting of targets and duplicate target checking are supported bythe target management sub-system. regarding an alert of the Swift GRB or the LIGO/Virgo GW event. By design, no feedback is returned to the AOMfrom external partners in current stage. A two-side datalink is planned to be implemented between the GWAC-N andthe CGFT in the next stage. 3.2.
Target management sub-system
The target management sub-system is to manage the inputs from all interfaces to prevent conflicts or duplicationprocesses during the target inputting. The workflow of the target management sub-system is shown in the Figure5. The sub-system automatically checks the format of the inputs and allows scientists and operators to make thecorrections. A target input message can be either adding new target or updating, deleting target from the sub-system.During observation and testing, different interfaces or different users could attempt to repeatedly input targets into thesub-system. These duplicated inputs will be rejected by the sub-system to avoid the waste of the telescope resources.On the other hand, the sub-system allows the users to perform the repeated observations for a target by adopting aspecific observation mode. 3.3.
Dynamic scheduling sub-system
The goal of the scheduler of the GWAC-N is to dynamically generate observation plans for all the telescopes. Theprinciple of scheduling of the GWAC-N is the prioritization of targets. The scheduler satisfies the observation requestsfor targets with the highest priorities as much as the telescope resources allow to do. The targets with higher prioritiescan interrupt observations of targets with lower priorities. Multiple levels of grading standards are pre-defined todeal with the complex relation among the target, the observation mode and the telescope. The top level of themis the observation mode, such as, the manual observation, the automatic ToO follow-up, the calibration etc. In thislevel, each mode of observation is given a range of scores based on its importance. The standard of grading can bechanged from telescope to telescope. For example, if a telescope is preferred for a certain observation mode, the scoreof this mode will be increased for this telescope. Basically, the grading is following the standard definition shown he Automatic Observation Management System: I Table 2.
The grades of observation modes
Observation mode Priority Telescope Noteroutine mode 10-19 GWAC including surveys with GWACnormal target mode 20-29 GWAC-F60, GWAC-F30 including automatic and manual monitoringof targets and supernova surveynormal queue mode 20-29 GWAC-F60, GWAC-F30 including queue observation for periodic objectsautomatic validation mode 40-49 GWAC-F60, GWAC-F30 including automatic validation observationsof the self-detected targets of GWACmanual validation mode 50-59 GWAC-F60, GWAC-F30 including manual validation observationsof the self-detected targets of GWACautomatic ToO follow-up mode 60-69 GWAC, GWAC-F60, GWAC-F30 including automatic follow-up observationsfor GW, GRB and neutrinomanual ToO follow-up mode 70-79 GWAC, GWAC-F60, GWAC-F30 including manual follow-up observationsfor GW, GRB and neutrinorevisit ToO follow-up mode 20-29,30-39,80-89 GWAC-F60 including revisit observation for interesting targetscalibration mode 90-99 GWAC, GWAC-F60, GWAC-F30 calibration observation for instrumentsmanual mode 100+ GWAC, GWAC-F60, GWAC-F30 including manual controled observationsfor all telescopes in Table 2. We give each mode a range of priority numbers for different cases. For instance, in the automatic ToOfollow-up mode, the priority of updated GRB alert is higher than the priority of the initial alert. We define the secondand/or third level priorities to indicate the sequence of all targets with the same priority in the top level. For differentobservation modes, the second and third levels can refer to different parameters. Here, we describe two strategies todefine the second and third level priorities as examples. For the ToO follow-up observations mode, the rankings needto be adopted in both tiling and galaxy targeting strategies. In our system, the probabilities of tiles and galaxies aredefined as the second level priority, while the altitude angle is the third level. For the validation mode, the trigger time(receiving time of target in the system) is the second level priority. No third level is needed in this mode. Using thesemethods, we can generate observation plans with many complex strategies. The scheduler makes a re-sorting processfor target list for each telescope based on the priorities, the observability, the status of observation and the telescope,when any update for the target is made in the database. The tables in the Figure 6 demonstrates the sorting sequencefor the target list during observations. The most important target is listed in the top of the table in the right side.All targets observable in time after the re-sorting are shown in the green cells. The yellow row shows the target beingobservable later on. Other targets including the ones already completed or no observation time window in that nightare not scheduled, which are shown in grey. After re-sorting, the observation plans are refreshed with new order oftarget list and probable new observation parameters. Each time, the dispatcher picks up the first target in the listfor a given telescope. The priority, observability, status of observation and telescope are constantly updated by othersub-systems, which makes the scheduling dynamically.3.4.
Dispatching sub-system
The scheduler does not send any observation command to the telescope controller. A dispatching system does that.Two types of dispatchers are developed for the telescopes inside of the GWAC-N and for the external partners. Thetechnologies used for the external dispatchers depend on the interfaces of the external telescopes.The internal dispatcher is dealing with the telescopes inside of the GWAC-N. The observation commands are sent tothe telescope controller via a one-way link. The observation status is obtained through a link between the monitor serverand a client. The dispatcher starts multiple threadings to different telescopes. The work flow is shown in the Figure 7.The dispatcher gets an observation plan from scheduler and then it will check the availability of the assigned telescope.If the telescope is available, the dispatcher will check the observability of the target. If yes, an observation commandwill be sent to the telescope controller. For another case, when the telescope is under observation, the dispatcher0
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Target Sorting before sorting(table left) after sorting(table right)Target ID Priority level 1 level 2 level 3 Observability Obs. status1 62 0.5 0.2 yes observing scheduled complete scheduled scheduled scheduled scheduled scheduled scheduled
10 63 0.4 0.3 yes scheduled
11 33 0.4 — no droped … … … … … … Target ID Priority level 1 level 2 level 3 Observability Obs. status10 63 0.4 0.3 yes scheduled scheduled scheduled scheduled scheduled scheduled scheduled scheduled complete complete
11 33 0.4 — no droped … … … … … … currently observable observablein following hourstarget underobservation Figure 6.
The original target list is shown in the left table. The sorted target list is shown in the right table. The first targetin the right table enters the observation procedure. The target under observation is in the red row. The targets observable inthe moment of scheduling are in the green rows. The targets that are currently not observable, but will be observable in thefollowing hours of the night, are in the rows in the light yellow. compares the priorities of targets. The new target with higher priority can interrupt the on-going observation of anold target. The dispatcher constantly monitors the observation status feedbacked from the observation status monitor.The actions of the dispatcher are based on that real time status.The client of the observation status monitor running on the telescope side sends the status back to the server,including the command reception, observation status and completeness. An error code is also sent back to the server,which can be used for system error analysis. 3.5.
Communication center
The AOM system is composed of many sub-systems and a database. The communications are very complex andfrequent among external interfaces, sub-systems and telescopes. To avoid the conflicts and sequential confusion in thecommunications, a sequential control mechanism is crucial for the AOM system. In the earlier version, all sub-systemsare directly communicating to the database. When a large number of concurrency entries occur in the database, aprotecting mechanism will be triggered in the databases from preventing damages of data. These chance faults are he Automatic Observation Management System: I AOM Dispatching: Workflow
Is the telescope observing? Does current observation reach to the end?Is the priority of new target/pointing higher than the current one?Scheduler Is observation time window of new target/pointing suitable?Starting dispatch for new target/pointingSending Observation Command
Yes No YesNo Yes NoNo Yes
Monitoring observation status. Is observation finished?
Yes Yes
AOM Dispatching: Observation Status Monitoring
Is OC received by telescope?Is observation complete or what is the completeness?Is the observation started?Observation Command (OC) is sent Re-scheduling
Yes NoYes
Analizing error code
NoNo
Returning error information and observation statusReturning observation complete status
Yes
Figure 7.
Top: The workflow of the AOM dispatching procedure. The tasks of dispatcher are drawn in red color, whilescheduler and observation controller sub-system are marked in green. Bottom: The workflow of the AOM observation statusmonitoring procedure, which is drawn in red. rare but fatal to our system. Another key point is for the scheduling. Unlike the multi-instance dispatcher, onlyone instance of scheduler can be run at time, because to deal with the dynamic information by multiple schedulerseasily causes the information confusion. The AOM system must ensure that the scheduling is well organized insuch a complex situation. A sequential controller can solve those communication issues. Therefore, we developed acommunication center (CC) combined with communicating and sequential controlling functions and a communicationclient deployed on each sub-system. The CC runs a server and many instances (communication modules). An instanceis launched when a connect request is created by a server or a client running on a sub-system. All messages betweenthe server and the clients are marked with flags to indicate different types of the messages. In the server side, themessages will be classified and distributed to the dedicated clients in the proper orders. The procedures of observationscheduling and dispatching depend on the ordering of the messages. In the client side, each message will be treated asan independent message, and be processed only in order of arrival. In this paper, we simulate four scenarios to showhow the observational procedures are executed smoothly in the GWAC-N. These scenarios are the most typical casesof communication time sequences during the scheduling and dispatching (see the Figure 8):2
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Case 1 . Communication time sequence for normal observation procedure. In the normal case, the procedure startsat the point, which a target is added into the target list. The next steps are the scheduling, the dispatching, and theobservation. The final step is a re-scheduling process. To be specific, after a new target is put into the target list froman interface (TC1), the target management client will process it, format it and send a message (TM1) with observationparameters of the target to the CC. The message will be added to a message list organized by a sequencer in the CC.The target message is instantaneously sent to an instance of the scheduler (SC1) that will start to make an observationplan. The scheduler generates the observation plans not only for that new added target but also for all observabletargets in the target list. After the scheduling is done, a message with a status of scheduling (SM1) is returned backto the CC, then a command of dispatching will start an instance of dispatcher client (DC1). The dispatcher clientdecides to choose a target with top priority from the target list for the next observation or to wait the completeness ofthe current observation. There are multiple instances of dispatcher clients running simultaneously to control differenttelescopes. The client of the dispatcher sends messages (DM1) to inform the CC when the observation is started andfinished. After the observation is done, the scheduler client receives a command from the CC to start re-scheduling toupdate the observation plans. The instance of dispatcher client is closed then. The procedure ends at this point.
Case 2 . When a target is added into the target list, the scheduler will firstly compute the observational time window.The one without the observational time window from TC2 will not be scheduled. The instance of the scheduler (SC2)will still communicate to the CC for the scheduling status (SM2) to inform the dispatcher (DC2) that the update oftarget list. The procedure ends at the dispatcher (DC2).
Case 3 . Multiple telescopes are needed to observe one target. This situation usually occurs when synchronizedmulti-band photometry is performed for the target. In the Figure 8, we assume that two telescopes are used in thatscenario. After receiving the target information from an interface (TC3), the instance of the scheduler (SC3) generatestwo observation plans for two telescopes respectively. Then the CC starts the first instance of a dispatcher client(DC3), while the second instance of a dispatcher client (DC4) will not be started, until the CC gets the feedbackmessage (DM3, the starting status of observation) from the DC3. Then the DC4 sends an observation command tothe second telescope and observation status message (DM4, the starting status of observation) to the CC. When theDC3 obtains the complete status of the observation, the SC3 will start re-scheduling process. In the meantime, theDC4 obtains the status of the second observation, but the message transmission (from DM4 to an instance of SC4)will be put on hold until the observation plans are refreshed by the SC3. Then the SC4 is started. The procedure isfinished when the SM4 is received.
Case 4 . In this case, dozens or even hundreds of targets/pointings are added into the system at nearly same time.This situation happens frequently during the Multi-Messenger follow-up observations. We simulate the scenario whentwo targets (TC5 and TC6) are inserted in the same time. An instance of scheduler (SC5) is started immediately whenthe message of target (TM5) is transmitted. The SC5 and a dispatcher client (DC5) are executed successively. Themessage (TM6 ) for the second target (TC6) will be transferred after the status message (DM5, the starting status ofobservation for the TC5) is received. Then the second instance of scheduler (SC6) and a dispatcher client (DC6) areexecuted for the TC6. When the observations and re-scheduling are finished for the TC5 and the TC6, the proceduresend.In the procedure of above cases, both actions of scheduling and actions of dispatching are triggered by the dedicatedmessages. The sequential controller can organize the messages in proper orders, which prevents the confliction andsequential confusion during observations. 3.6.
AOM system workflow
Combining with above sub-systems, the overall workflow of the AOM system is described as follows. The targetsare manually/automatically inserted into the system by using interfaces provided by the AOM system (all observationrequest are treated as targets). All the targets are processed and classified by the target management sub-system,then are inserted into the target database. Some targets are sent by the external dispatcher to trigger the follow-upobservations with external telescopes. The targets for the GWAC-N will be initially scheduled in order to computethe observation time windows and to make the initial observation plans. The targets having the observation timewindows are added in the daily target list and are stored in the database. This daily target list contains all thetargets to be observed in a given night. This list is kept updated during the night, since new targets come in, theobservation parameters and status of targets are updated, and some targets are removed from the list. Triggered bythe CC, the dynamic scheduler makes observation plans for the target in the target list, and the dispatcher selects a he Automatic Observation Management System: I Communication Time Sequence
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Figure 8.
The figure demonstrates the four typical scenarios of the message sequences in the AOM system. A functioningcommunication center (CC) consists of a message sequential controller, multiple instances that one of each is launched andcommunicates with a given connected client. The message sequential controlling of the CC is shown in the grey box. Theincoming and outputting message flows for the controller are drawn in red and green colors respectively. To clarify the messagesequencing procedure, we use yellow, red and blue colors to mark the actions of target management (shorten with TC and TM),scheduling (SC and SM) and dispatching (DC and DM). A green circle and a black circle represent the starting point and theending point of a message flow. target to observation command based on the observation plan, status of observation and the status of telescopes. Theobservation status monitor keeps observation status updated in the target list, so the scheduling and dispatching canbe fully dynamically. The system completes a closed control loop (shown in the Figure 9)3.7.
Performance of the AOM system
With the AOM, the GWAC-N integrates 5 telescopes and collaborates with 2 external telescopes. By using thestandard link provide by the AOM, the telescopes can easily join the network. The AOM also provides the customizedlink for external telescopes, which is very useful for those telescopes willing to join the ToO follow-up campaigns withoutdeveloping their own follow-up system. The AOM can perform complex observations with currently 10 observationmodes and 175 strategies. To add or modify the observation modes or strategies, the users only need to edit theconfiguration file rather than change the code of the system. During the operations, the routine survey, the targetmonitoring, the GWAC OT validation and the ToO follow-up observations are done automatically. The operators4
Han et al.
External TelescopesTargets/Observation Requests
AOM System Workflow
Current Target/Pointing list Initiating Dynamic InternalDispatchingTarget Managing ExternalDispatchingDynamic Scheduling (1) Initial SchedulingCurrent Target/Pointing list Updating (2) Internal TelescopesCurrent Target/Pointing list Updating (3) (1) Re-scheduling after any update in the current target/pointing list.(2) Adding new pointings or re-sequencing the pointing list.(3) Updating observation status of pointingsObservation Status Monitoring
Figure 9.
The workflow of a complete observation management sequence controlled by the AOM system that is drawn in redcolor. The target management sub-system receives all the targets from the interfaces (in yellow). The external dispatcher dealswith the external telescopes (in the grey box) of the GWAC-N. The system fully controls the internal telescopes (in the greenbox) with the scheduling, dispatching and status monitoring sub-systems. are needed for monitoring the operation status and manually importing the targets with the special observationrequirements into the system.The communication mechanism and system structure of the AOM ensure the stability of the system, which is anotherkey factor for a robotic telescope network. The AOM has been implemented in the GWAC-N since August 2019. Duringthe observation season of 2019-2020 (from October to April, weathers are better and night time is longer comparingwith the rest of year), the AOM is working with a high duty cycle and a stable behavior. The AOM produced 622observation plans per clear night on average (in December 2020), with failure-free. On the 7th of December, the AOMproduced 1064 observation plans, which is the highest working load in the month.The efficiency of the AOM can be valued by the time delay between the target inserting and the observation commandsending. Shortening the time delay is very important for a ToO orientated observation network. The time delay isusually caused by the communication delays, the scheduling process and the observation status monitoring process. Inthe AOM, the main time consuming is from the scheduling process, which depends mainly on the number of targets inthe target list. Because once an observation is finished, the AOM will start a re-scheduling process for all the targets inthe list and send an observation command to a telescope. During December 2020, the list contains an average numberof 1500 targets in a night. On the 7th of December 2020, the AOM handled 3955 targets, which is the largest numberfor a night among the month. We tested the time delay of each re-scheduling process during the night. The longesttime delay is less than 2 seconds, which is negligible comparing with the time delay in the telescope side betweenstopping the previous exposure and starting new observation. We simulated the extreme scenario of observing 10000 he Automatic Observation Management System: I SCIENTIFIC OPPORTUNITIES AND OUTPUT TO THE GWAC-NThe primary goal of the GWAC-N is to observe the prompt emission of GRBs in optical bands. We emphasized therole of the telescopes of the GWAC-A because of their key features of large sky coverage, high time resolution and realtime transient detection capability. These features allow the GWAC-A to independently search for optical transientswith a high cadence. Furthermore, the GWAC-A can be also used for follow-up observation of multi-messenger event.The associated multi-band small FoV telescopes in the GWAC-N are originally designed for the real-time automaticvalidation for the optical transients detected by the GWAC-A. These telescopes can be also used for other purposes,such as photometry of variable object, galaxy targeting observation for multi-messenger events and supernova survey.4.1.
Gamma-Ray Burst
The prompt emission of GRB in optical bands is difficult to be observed, since its very fast temporal decay. Toobtain the prompt emission, the speed of response of telescopes to a GRB alert is highly desired. The idea behindthe design of the GWAC-A is to eliminate the response time to a GRB alert. The total sky coverage of full GWAC-Ais as large as 5000 square degrees, which can cover the same sky area being monitored by the ECLAIRs telescope,the main GRB detector of SVOM (Wei et al. 2016). This extreme large sky coverage guarantees that the GWAC-Asimultaneously discovers the optical counterparts for about 30% SVOM detected GRBs at tigger time (T0). It canalso make the GWAC-A to be a suitable instrument to follow up the GRBs detected by other gamma-ray instruments,which cannot provide accurate localizations (the Fermi Gamma-ray Space Telescope and SVOM/GRM, etc.).The two GWAC-F60A/B telescopes and the GWAC-F30 telescope in the GWAC-N robotically follow-up the GRBsdetected not only by SVOM but also by the Swift satellite. Since 2016, these 3 telescopes manually followed up 6Swift GRBs (Xin et al. 2016, Xin et al. 2017A, Han et al. 2018A, Xin et al. 2019A, Xin et al. 2019B, Xin et al.2019C). Since 2020, the AOM automatically followed up 3 Swift GRBs by using GWAC-A, GWAC-F60 and TNTtelescopes (Xin et al. 2020B, Xin et al. 2020C, Xin et al. 2020D, Xin et al. 2021) . For GRB 201223A, the opticalcounterpart was detected in a GWAC-A image taken at 2 seconds after the burst. The GWAC-F60A started thefollow-up observations for the counterpart 23 seconds after receiving the alert of the burst and 44 seconds after theburst trigger. These observations can provide consecutive lightcurve from the prompt emission phase to the afterglowphase (Xin et al. 2020C, Xin et al. 2020D).4.2.
Multi-Messenger Target of Opportunities astronomy (gravitational wave, neutrino)
The poor localization of the Multi-Messenger Target of Opportunities (ToO-MM) alert is a great challenge for allthe optical follow-up facilities. To quickly search for the optical counterparts in a large sky area, two observationstrategies are widely used by most of the optical telescopes for the ToO-MM follow-ups, which are either by tiling thelarge localization regions or by performing galaxy-targeted observations. By using all the telescopes by the AOM, theGWAC-N can conduct efficient follow-up observations with both strategies. Taking advantage of the wide field of viewof telescopes, GWAC-A can cover a significant portion of the ToO-MM localization regions in a very short amount oftime by using the tiling strategy. In the meanwhile, the GWAC-F60A/B and GWAC-F30 carry out galaxy targetingobservations. As a group, three telescopes can search ∼
500 galaxies in a clear night. During the O2 and O3 GW run,the pathfinder telescopes mini-GWAC array and the GWAC-A performed follow-ups of large sky covering for 25 ofGW events (8 in O2 and 17 in O3, Dornic et al. 2019, Ducoin et al. 2020B, Ducoin et al. 2020C, Gotz et al. 2019,Han et al. 2019, Lachaud et al. 2019, Leroy et al. 2017, Mao et al. 2020, Turpin et al. 2019A, Turpin et al. 2019B,Turpin et al. 2019C, Turpin et al. 2019D, Turpin et al. 2020A, Wang et al. 2019, Wang et al. 2020A, Wang et al.2020B, Wei et al. 2017A, Wei et al. 2017B, Wei et al. 2017C, Wei et al. 2017D, Wei et al. 2017E, Wei et al. 2017F,Wei et al. 2019A, Wei et al. 2019B, Wei et al. 2019C, Wei et al. 2019D, Wei et al. 2020, Wu et al. 2019, Xin et al.2017B, Xin et al. 2019D, Xin et al. 2019E, Xin et al. 2020A).4.3.
Optical transient target (supernova, flare)
Thanks to the large sky coverage of the GWAC-A, the fast follow-up capability of the GWAC-F60A/B and GWAC-F30 and the dedicated online data processing pipeline of each telescope, the GWAC-N is not only capable to inde-pendently detect optical transients in the sky but also to identify the types of the candidates in realtime. Since 2018,6
Han et al.
Figure 10.
Some moving object images of the GWAC-A. The top part of images are original images. The bottom part ofimages are residual images. (a) is meteor candidate, (b-f) are other types of moving objects including giant planets, minorplanets and artificial satellites. the GWAC-N has detected several super stellar flares (Han et al. 2018B, Wang et al. 2020C, Xin et al. 2020E). TheGWAC 181229A, a supper flare with an amplitude of ∆ R ∼ . ∼ ∼ R ∼
16 in asingle image and ∼
200 clear nights per year at the GWAC site, we estimate that the GWAC-A is capable to detectabout 30 bright, nearby supernovae per year by using a dedicated pipeline.4.4.
Variable and Periodic object
The most of sky coverages of the GWAC-A’s survey are consistent in successive observation nights, which means onegiven sky area can be monitored for days or dozens of days. With a high cadence observation mode (15 seconds perimage), the GWAC-A can monitor the variables or the periodic objects in the sky area and obtain their variation. Theonline data processing pipeline of the GWAC-A can measure the photometric features for all sources in the images.Using neural network mechanism, researchers analyze the massive data of the GWAC-A to detect and to classifyvariable and periodic sources (Qiu et al. 2018, Turpin et al. 2020B).4.5.
Moving object
With its large field of view and high cadence, the GWAC-A can monitor hundreds asteroids on an observation night.The GWAC-A also has the capability to detect the decameter asteroids and meteors (Shugarov 2019, Xu et al. 2020B).They are valuable for the researchers in these fields. Our team works on the algorithm and database to recognize andmorphology analyze them from the GWAC data. The Figure 10 shows some moving objects automatically detectedin the GWAC-A images by an algorithm for selecting moving objects. The accuracy over 85% can be reached for themeteor candidate selections by using the algorithm (Xu et al. 2020B). SUMMARY AND PERSPECTIVEThe GWAC-N is currently composed of two GWAC telescopes, two GWAC-F60 telescopes and one GWAC-F30telescope. It is also collaborating with two external telescopes: the CGFT and the TNT telescope. By implementingthe AOM, those telescopes can work as a network smoothly. Besides of routine observations, the GWAC-N performedfollow-up observations of the LIGO/Virgo GW, the Fermi GRB, the Swift GRB events. During the LIGO/Virgo O3campaign, using the AOM system described in this paper, the GWAC-N observed 17 GW events and published 23GCN circulars. he Automatic Observation Management System: I forpublic download.The AOM is not only used to manage the operations of telescopes but also to manage the data. A server named theData Center (DC) is installed inside of the AOM. In the past, the massive data taken from the GWAC-N telescopesbring huge workload to the scientists and operators to collect and find images for certain observations. Currently,the data of the ToO follow-up observations taken by the GWAC-F60, the GWAC-F30 are automatically collectedand uploaded to the DC by the AOM. It allows we centralized the data processing in the DC rather than the dataprocessing distributed in the telescope side. In the future, we plan to integrate the data processing pipeline in theAOM system, as well as the data product release. ACKNOWLEDGEMENTThe GWAC team at the NAOC is grateful for financial assistance from the National K&D Program of China (grantNo. 2020YFE0202100) and the National Natural Science Foundation of China (Grant No. 11533003, 11973055,U1831207, 11863007). This work is supported by the Strategic Pioneer Program on Space Science, Chinese Academyof Sciences, grant Nos. XDA15052600 & XDA15016500 and by the Strategic Priority Research Program of the ChineseAcademy of Sciences, Grant No.XDB23040000. Damien Turpin acknowledges the financial support of the CNES post-doctoral program. We thank the staffs of the Xinglong and the Jilin observatories at which the TNT and the CGFTtelescopes are operated. We would like to thank Sarah Antier, David Corre, Jean-Gr´egoire Ducoin for their veryhelpful discussions during the developments of the AOM system.REFERENCES