Focusing on Dark Energy
rong>The HETDEX project is the first major experiment to search for dark energy in the universe using a set of spectrographs to map the three-dimensional positions of one million galaxies.
The Hobby-Eberly Telescope (HET) is located at the McDonald Observatory in West Texas.The Hobby-Eberly Telescope (left) shown with its dome shutter open. Right is a closeup view of the HET showing the primary mirror and telescope structure that supports the PFIP. Its eleven meter spherical primary mirror is hexagonally shaped and is made up of 91 identical one meter hexagonal segments. Currently, the usable aperture of the primary is 9.2 meters and the field of view on the sky is four arc-minutes. Both of these parameters are limited by the existing optics used to correct for spherical aberrations.
The HET is a spectroscopic survey telescope, which incorporates a low resolution spectrograph, which is mounted on what is called a Prime Focus Instrument Package (PFIP) along with a medium and high resolution spectrograph. The HET is going through a wide field upgrade, which will increase the field of view to 22 arc-minutes and the usable aperture to 10 meters so that it can accommodate 192 fiber-fed remote spectrographs of a new design called a Visible Integral-field Replicable Unit Spectrograph (VIRUS).
The scientific research that will be ongoing using the upgraded HET is expected to help scientists understand Dark Energy, which makes up almost three-quarters of the matter and energy in the universe, and is the mysterious force causing the universe to expand faster as it ages. A huge project, this research will explain whether the laws of gravity are correct, and reveal new details about the Big Bang.
HETDEX (the Hobby-Eberly Telescope Dark Energy Experiment) at the University of Texas at Austin McDonald Observatory will map the positions of a million galaxies 10- to 11-billion light years away in the hopes of explaining what is happening in our universe. To do this, HETDEX will combine the light-gathering power of the world’s third largest telescope with an array of new instruments for analyzing the light from distant galaxies.
HETDEX began with the VIRUS, “…an instrument looking for a project,” said principal investigator Gary Hill. Texas astronomers had already explored ways to build a large number of spectrographs for several types of astronomical projects. After attending a meeting on dark energy, astronomer Karl Gebhardt wondered how Texas could tackle the problem. “At that moment, I realized that VIRUS would actually do this project,” Hill said. “And so, essentially, that was the half-hour conversation that was the Eureka moment. Usually, when you have these ideas, within another half-hour you’ve managed to kill them as unfeasible, just with back-of-the-envelope calculations. But this time it looked good. So we started developing it, and the project gained momentum.”
TheThis multi-axis test bench is used to test PFIP control system electronics and software. The mechanical assembly to the right contains the Maxon motors, position encoders, limit switches, and inertial loads that simulate various PFIP mechanisms. PFIP rides on a tracker at the top end of the telescope and contains a wide field corrector, acquisition camera, metrology equipment, and a focal surface assembly. Overall, the PFIP is a stand-alone remote automation island that contains 12 subsystems and 24 motion axes. Motion controllers and modular IO systems are interconnected using the CANOpen messaging protocol. All communications between ground-level systems and PFIP subsystems are either point-to-point via Ethernet or through Ethernet/CAN gateways that pass CANOpen messages transparently.
Of the approximately 24 motion axes in the PFIP, 15 are motorized and require smooth, precise motion over a range of speeds, including very low speeds. The motion system had to perform several operations through different subsystems or under different situations such as smoothly and accurately follow a velocity trajectory (shutter control), move to an absolute position, and hold accurately, and follow a multi-axis position and velocity trajectory.
The motors used in the PFIP subsystems are Maxon EC series brushless DC servos, which can be configured with optional gearheads, magnetic incremental encoders, and electrically operated brakes as required. In order to achieve smooth motion at low speeds, sinusoidal commutation is used. Consequently, an optional incremental encoder is used with the standard Hall effect sensors built into the motors. The incremental encoders provide additional position feedback to motion controllers.
This photo shows the family of motors Maxon has for use in industrial applications such as the HETDEX project. Along with a line of amplifiers and controllers, Maxon is able to supply a complete motion system to its customers.All controllers are Maxon EPOS2 50/5 Positioning Control Units. In addition to providing closed loop control of current, velocity, and position, the controllers have an interpolated motion mode that enables them to follow a programmed multi-axis trajectory. They also have analog and digital
I/O that are accessible via the CANOpen interface, and a programmable capability to respond to digital inputs, such as positive/negative limits, home position, quick stop, and drive enable/disable. Even though it might be slightly less expensive to use a mix of controllers, our intention was to use a single part number throughout the PFIP and to stock a single spare.
In the PFIP application, modular I/O stations include CANOpen bus couplers that enable all of the additional I/O to be accessible directly through the CAN bus or through an Ethernet-CAN gateway that uses a simple ASCII protocol for configuration and for passing messages in both directions.
In this application, hardware devices are attached to the CAN bus and take direction from the PFIP Control Computer in a master/slave arrangement. Considering a multi-axis move as an example, the PCC would configure several motion controllers for the desired motion, and then trigger them simultaneously by sending a single CANOpen message. In preliminary testing, the measured round trip time for a message from the PCC to reach a device in the PFIP and for a response to return is about six ms using 100 Mbps Ethernet and 1Mbps CAN bus speeds. This is comparable to the scan or loop time for a PLC, which is fast enough near real-time performance.
In general, the PFIP motion control systems operate from 24 VDC power sources. For larger inertial loads like the shutter, 48 VDC is available and is compatible with the EPOS2 50/5 controllers.
To comply with the specifications for the PFIP and HET, all hardware components are required to operate at low temperatures down to -10 degrees C or better. Maxon offers a broad range of products that meet these temperature requirements and are designed for levels of quality, reliability, and ruggedness required for industrial automation systems. All items selected for use in the PFIP are either stocked in the United States or readily available from the manufacturer within a reasonable delivery time.
Finally, there is a great degree of flexibility inherent in the overall architecture. It is very easy to make significant changes by simply adding or deleting motion controllers, I/O slices, or power supplies. The components are sufficiently small and lightweight that it is possible to include spares in the initial design for use as plug-and-play replacements or for future expansion needs. By using a small number of standardized hardware components for all subsystems, the program is able to minimize the number of spares that need to be purchased and kept on hand at the observatory.