Cerro Tololo InterAmerican Observatory Victor M Blanco 4
Cerro Tololo Inter-American Observatory, Victor M. Blanco 4 -m Telescope: an upgrade to the telescope control system Timothy M. C. Abbott, German Schumacher, Michael Warner, Eduardo Mondaca, Ricardo E. Schmidt, Rolando Cantarutti, Manuel Martinez, Omar Estay, Francisco Delgado, Alistair Walker, and for the Dark Energy Survey Collaboration National Optical Astronomy Observatories, 950 N. Cherry Ave. , Tucson, AZ, USA 85719 Contact: tabbott@ctio. noao. edu ABSTRACT The CTIO V. M. Blanco 4 -m telescope is to be the host facility for the Dark Energy Survey (DES), a large area optical survey intended to measure the dark energy equation of state parameter, w. The survey is expected to use ~30% of the telescope time over 5 years and use a new 520 megapixel CCD prime focus imaging system: the Dark Energy Camera (DECam). The Blanco telescope will also be the southern hemisphere platform for NEWFIRM, a large area infrared imager currently being commissioned at the Mayall Telescope at KPNO. As part of its normal cycle of continuing upgrades and in preparation for the arrival of these new instruments, the Blanco telescope control system (TCS) will be upgraded to provide a modern platform for observations and maximize the efficiency of survey operations. The upgraded TCS will be based on that used at the SOAR telescope and will be a prototype of the TCS to be used by LSST. It will be optimized for programmed and queued survey observations, will provide extended real-time telemetry of site and facility characteristics, and will incorporate a distributed observer interface allowing for on- and off-site observations and real time quality control. Hardware modifications will include the use of absolute tape encoders and a modern servo control and power driver systems. Scope, goals and specification • Replace all telescope control components except drive motors and drive trains • Decouple telescope, dome and instrument control • Modularize and distribute control • Where possible, use off-the-shelf components with an anticipated market lifetime of at least 5 years • Optimize accelerations and slew trajectories • Extend and record telescope and environment telemetry • Provide modern human and machine interfaces • Upgrade must proceed without interrupting normal observations. Specifications • Slew specification: 2° in less than 17 seconds • Slew, project goal: 3° in 20 seconds, track-to-track • Tracking drift <0. 5”/min at airmasses < 1. 5 • Tracking jitter: 0. 1” r. m. s. • Accept 1 Hz guiding error signals • Operational deadline: September 2010 • Programmed completion date: January 2010 Upgraded TCS schematic design. Blue boxes are Linux boxes with PXI/PCI bus, yellow boxes are the telescope and associated systems, Cass systems are shown in green (communications remain RS-485, but will be upgraded to Ethernet as time permits), instruments and cameras are the grey boxes at the top. Hardware choices • PCX/PCI bus Linux boxes • Ethernet, TCP/IP • Delta-Tau power drivers • Single Heidenhain tape encoder per axis Software choices: • SCADA design principles • Rutherford Appleton Lab (Wallace) TCS Kernel TCSpk • Linux with Real Time Application Interface extensions • Lab. View v 8, C/C++, RTI Data Distribution Service, • My. SQL database, Sub. Version version control Each telescope axis is driven by two custom-built, counter-torqued motors, these will not be replaced. Currently, the motor controller selects motor drive rates (above, left). Feedback derived from tachometers is applied through a hard-wired rate loop. The motor controller receives feedback only from the encoders. In the upgraded TCS (above, right), the rate loop and motor preload logic will be integrated into the motor controller. It will receive feedback from both tachometers and encoders and calculate the torque to command of the power driver. The power driver will not receive external feedback. This arrangement permits more flexible control of the drive, allowing real-time calculation of optimum slew trajectories to minimize acceleration energies at critical telescope resonances. We employ a setpoint/status control principle in which a control application computes a set point which it sends to a device process. The device process then attempts to achieve that set point and reports its status back to the control application. An example control application is the TCS application which computes appropriate set points, e. g. telescope orientation coordinates, and sends them to the process running in the motor controller. This device process then drives the telescope to position, closing time-critical loops locally, and reports back to the TCS application when it succeeds. A telescope lumped-mass model and sample slew trajectories have been developed to ensure that the upgraded system will meet our requirements. Slew trajectories are calculated to minimize any spectral components in the acceleration that could excite the telescope structure during a slew. A step acceleration is convolved with a Chebyshev window (right), producing a low pass frequency cutoff. The figure at left shows the telescope trajectory obtained. The RA encoder mounting surface is readily provided by the lower oil bearing clear of the pads, requiring only mounting arms with alignment jigs for the read heads (right). Above are the displacements of the head with respect to the tape when in operation, which experience show to be acceptable. DEC encoder mounting surface requires a new, custom built assembly between the horseshoe and telescope.
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