The communication architecture required to provide a bidirectional communication between a central command node and a full set of fiber positioners feeding a spectrograph is studied. Six different architectures have been analyzed in terms of communication time and power consumption. These architectures are the result of the combination of three different communication protocols: transmission control protocol/internet protocol (TCP/IP) over ethernet, interintegrated circuit (I2C), and controller area network. The design of communication architecture must prioritize between communication time and power consumption. The fastest architecture is the hybrid TCP/IP over ethernet-I2C. This architecture requires the least time to provide a full set of coordinates to every fiber positioner less than 50 ms. The most power efficient solution is the I2C—I2C with demultiplexers. This architecture solves a bidirectional communication between a central node and a full set of fiber positioners requiring only an addition of 27 mW.
The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14000 square degrees will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope will deliver light to 5000 fiber optic positioners. The fibers in turn feed ten broad-band spectrographs. We present an overview of the instrumentation, the main technical requirements and challenges, and the current status of the project.
Proc. SPIE. 9912, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II
KEYWORDS: Telescopes, Spectrographs, Spectrographs, Control systems, Time metrology, Telecommunications, Antennas, Wireless communications, Chemical elements, Multiplexers, Data communications, Data communications
This paper presents a design proposal for controlling the five thousand fiber positioners within the focal plate of the DESI instrument. Each of these positioners is a robot which allows positioning its optic fiber with a resolution within the range of few microns. The high number and density of these robots poses a challenge for handling the communication from a central control device to each of these five thousand. Furthermore, an additional restriction applies as the required time to communicate to every robot of its position must be smaller than a second. Additionally. a low energy consumption profile is also desired.
Both wireless and wired communication protocols have been evaluated, proposing single-technology-based architectures and hybrid ones (a combination of them). Among the wireless solutions, ZigBee and CyFi have been considered. Using simulation tools these wireless protocols have been discarded as they do not allow an efficient communication. The studied wired protocols comprise I2C, CAN and Ethernet.
The best solution found is a hybrid multilayer architecture combining both Ethernet and I2C. A 100 Mbps Ethernet based network is used to communicate the central control unit with ten management boards. Each of these boards is a low-cost, low-power embedded device that manages a thirty six degrees sector of the sensing plate. Each of these boards receives the positioning data for five hundred robots and communicate with each one through a fast mode plus I2C bus. This proposal allows to communicate the positioning information for all five thousand robots in 350 ms total.
The next generation of large-scale spectroscopic survey experiments such as DESI, will use thousands of fiber positioner robots packed on a focal plate. In order to maximize the observing time with this robotic system we need to move in parallel the fiber-ends of all positioners from the previous to the next target coordinates. Direct trajectories are not feasible due to collision risks that could undeniably damage the robots and impact the survey operation and performance. We have previously developed a motion planning method based on a novel decentralized navigation function for collision-free coordination of fiber positioners. The navigation function takes into account the configuration of positioners as well as their envelope constraints. The motion planning scheme has linear complexity and short motion duration (2.5 seconds with the maximum speed of 30 rpm for the positioner), which is independent of the number of positioners. These two key advantages of the decentralization designate the method as a promising solution for the collision-free motion-planning problem in the next-generation of fiber-fed spectrographs. In a framework where a centralized computer communicates with the positioner robots, communication overhead can be reduced significantly by using velocity profiles consisting of a few bits only. We present here the discretization of velocity profiles to ensure the feasibility of a real-time coordination for a large number of positioners. The modified motion planning method that generates piecewise linearized position profiles guarantees collision-free trajectories for all the robots. The velocity profiles fit few bits at the expense of higher computational costs.
In the large-scale, Dark Energy Spectroscopic Instrument (DESI), thousands of fiber positioners will be used. Those are
robotic positioners, with two axis, and having the size of a pen. They are tightly packed on the focal plane of the
telescope. Dedicated micro-robots have been developed and they use 4mm brushless DC motors. To simplify the
implementation and reduce the space occupancy, each actuator will integrate its own electronic control board. This board
will be used to communicate with the central trajectory generator, manage low level control tasks and motor current
feeding. In this context, we present a solution for a highly compact electronic. This electronic is composed of two layers.
The first is the power stage that can drive simultaneously two brushless motors. The second one consists of a fast
microcontroller and deals with different control tasks: communication, acquisition of the hall sensor signals,
commutation of the motors phases, and performing position and current regulation. A set of diagnostic functions are also
implemented to detect failure in the motors or the sensors, and to sense abnormal load change that may be the result of
two robots colliding.
XMS is a multi-channel wide-field spectrograph designed for the prime focus of the 3.5m Calar-Alto telescope. The
instrument is composed by four quadrants, each of which contains a spectrograph channel. An innovative mechanical
design -at concept/preliminary stage- has been implemented to: 1) Minimize the separation between the channels to
achieve maximal filling factor; 2) Cope with the very constraining space and mass overall requirements; 3) Achieve very
tight alignment tolerances; 4) Provide lens self-centering under large temperature excursions; 5) Provide masks including
4000 slits (edges thinner than 100μ). An overview of this very challenging mechanical design is here presented.
Fiber-fed spectrographs dedicated to observing massive portions of the sky are increasingly being more demanded
within the astronomical community. For all the fiber-fed instruments, the primordial and common problem is the
positioning of the fiber ends, which must match the position of the objects of a target field on the sky. Amongst
the different approaches found in the state of the art, actuator arrays are one of the best. Indeed, an actuator
array is able to position all the fiber heads simultaneously, thus making the reconfiguration time extremely short
and the instrument efficiency high. The SIDE group* at the Instituto de Astrofisica de Andalucia, together with
the industrial company AVS and the University of Barcelona, has been developing an actuator suitable for a large
and scalable array. A real-scale prototype has been built and tested in order to validate its innovative design
concept, as well as to verify the fulfillment of the mechanical requirements. The present article describes both
the concept design and the test procedures and conditions. The main results are shown and a full justification
of the validity of the proposed concept is provided.
Two feasibility studies for spectrographs that can deliver at least 4000 MOS slits over a 1° field at the prime focuses of
the Anglo-Australian and Calar Alto Observatories have been completed. We describe the design and science case of the
Calar Alto eXtreme Multiplex Spectrograph (XMS) for which an extended study, half way between feasibility study and
phase-A, was made. The optical design is quite similar than in the AAO study for the Next Generation 1 degree Field
(NG1dF) but the mechanical design of XMS is quite different and much more developed. In a single night, 25000 galaxy
redshifts can be measured to z~0.7 and beyond for measuring the Baryon Acoustic Oscillation (BAO) scale and many
other science goals. This may provide a low-cost alternative to WFMOS for example and other large fibre spectrographs.
The design features four cloned spectrographs which gives a smaller total weight and length than a unique spectrograph
to makes it placable at prime focus. The clones use a transparent design including a grism in which all optics are about
the size or smaller than the clone rectangular subfield so that they can be tightly packed with little gaps between
subfields. Only low cost glasses are used; the variations in chromatic aberrations between bands are compensated by
changing a box containing the grism and two adjacent lenses. Three bands cover the 420nm to 920nm wavelength range
at 10A resolution while another cover the Calcium triplet at 3A. An optional box does imaging. We however also studied
different innovative methods for acquisition without imaging. A special mask changing mechanism was also designed to
compensate for the lack of space around the focal plane. Conceptual designs for larger projects (AAT 2º field, CFHT,
VISTA) have also been done.
SIDE (Super Ifu Deployable Experiment) is proposed as second-generation, common-user instrument for the GTC. It
will be a low and intermediate resolution fiber fed spectrograph, highly efficient in multi-object and 3D spectroscopy.
The low resolution part (R = 1500, 4000) is called Dual VIS-NIR because it will observe in the VIS and NIR bands (0.4
~V 1.7 microns) simultaneously. Because of the large number of fibers, a set of ~10 identical spectrographs is needed,
each with a mirror collimator, a dichroic and two refractive cameras. The cameras are optimized for 0.4 - 0.95 microns
(VIS) and 0.95 - 1.7 microns (NIR) respectively.
SIDE (Super Ifu Deployable Experiment) will be a second-generation,common-user instrument for the Grantecan (GTC)
on La Palma (Canary Islands, Spain). It is being proposed as a spectrograph of low and intermediate resolution, highly
efficient in multi-object spectroscopy and 3D spectroscopy. SIDE will feature the unique possibility of performing
simultaneous visible and IR observations for selected ranges. The SIDE project is leaded by the Instituto de Astrofsica de
Andaluca in Granada (Spain) and the SIDE Consortium is formed by a total of 10 institutions from Spain, Mexico and
USA. The feasibility study has been completed and currently the project is under revision by the GTC project office.
LIRIS is a near-infrared (1-2.5 microns) intermediate resolution spectrograph (R=1000-3000) with added capabilities for multi-slit, imaging, coronography, and polarimetry, built by the IAC to be a common instrument for the WHT (La Palma). Here we report the results of the two commissioning periods. The image quality was checked, obtaining a FWHM of 0".5 in the Ks band over the whole field of view (4'.2 x 4'.2). Zero points and sky brightness were measured, and very low values were found in the latter. The long slit spectra obtained matched the expected spectral resolution (2.6 pixels for a 0".65-wide slit). Flexure tests were carried out with good results. Several science targets were observed, the most note-worthy result being the detection of the CIV 154.9 nm line in the most distant qso at z=6.41.
LIRIS is a near-infrared (0.9 - 2.4 microns) intermediate resolution spectrograph (R = 1000-3000) conceived as a common user instrument for the (WHT) at the Observatorio del Roque de los Muchachos (ORM) La Palma. LIRIS is now being assembled, integrated and virified at the Instituto Astrofisico de Canarias (IAC). LIRIS will have imaging, long-slit and multi-object spectroscopy working modes. Coronography and polarimetry capabilities will eventually be added. Image capability will allow easy target acquisition.
LIRIS is a near-IR intermediate resolution spectrograph with added capabilities for multi-object, imaging, coronography, and polarimetry. This instrument is now being constructed at the IAC, and upon complexion will be installed on the 4.2m William Herschel Telescope at the Observatorio del Roque de Los Muchachos. The optical system uses lenses and is based on a classical collimator/camera design. Grisms are used as the dispersion elements. The plate scale matches the median seeing at the ORM. The detector is a Hawaii 1024 X 1024 HgCdTe array operating at 60K.
The Instituto de Astrofisica de Canarias (IAC) is undertaking the design and construction of a common-user near IR spectrograph (LIRIS) for the Cassegrain focus of the 4.2 m William Herschel Telescope sited at the Observatorio del Roque de Los Muchachos. LIRIS will be a near IR intermediate-resolution spectrograph designed to operate over a spectral resolution range between 1000 and 5000, with added capabilities for coronographic, multiproject and polarimetric observations. The instrument allows the combination of an adequate spatial resolution with a large useful field of view across the slit, thanks to the use of the new 1024 X 1024 pixel HgCdTe Hawaii detector manufactured by Rockwell. All the optics and mechanisms situated inside the cryostat will be cooled to below 100 K. The detector will operate at 77 K. Calibration and tracking will be made with the existing Cassegrain A and G Box, into which a near IR calibration system will be incorporated.