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| Prototype setup of major elements of HECTOSPEC Fiber Positioner robot |
One of six new scientific instruments are currently under
development at SAO for use with the converted Multiple Mirror Telescope (MMT)
at the Fred Lawrence Whipple Observatory (FLWO)
Mt.Hopkins AZ, Hectospec is a
multi-object spectrograph capable of measuring the spectra of up to three
hundred celestial objects simultaneously. The intent of these observations will
be to gather data for the study of the large-scale structure of the universe.
The instrument consists of two major components--a Fiber Positioner and
Sectrograph--plus an electronics control rack.
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The Fiber Positioner, which is mounted at the focal plane of the telescope, is a dual, six axis, pick and place robotics positioning system, capable of placing up to three hundred optical fiber carrying magnetic buttons on individual celestial objects (galaxies) on the focal plane of the telescope. These optical fibers are placed to sub-micron accuracy at the average rate of one fiber per second using servo and stepper motors. Position and velocity feedback is provided with rotary and linear encoder feedback on the large X and Y axes, LVDT feedback on the Z axis, and precision glass scale analog feedback using interpolating electronics on the Pi and Theta Gimbal axes. The gripper axis is stepper motor driven and utilizes several discrete position sensing switches. All axes have accurate home and limit sensors for homing the instrument and limiting the travel of each axis. The Fiber positioner robots are controlled by Delta Tau PMAC multi axes controllers run by a VME bus computer using a SUN workstation as Host. |
The Hectospec spectrograph is an optical table utilizing various optical
elements including a grating, various filters, a high speed shutter, and a
large-format CCD camera to image the spectra of each of the three hundred
objects under observation. These optical elements are carried on a precision
optical bench, and are positioned by stepper motors that are
controlled by a VME bus computer via SBS Greenspring Industry Pack stepper motor
controllers. Each optical element positioner has self-contained limit and home
switches and, in the case of the filter selector, a linear encoder. The CCD
camera system uses two 2K X 4K CCD devices read out through in-house-designed
electronics to a SUN workstation data reduction system.
Electrically, the Hectospec project is essentially a large systems job consisting of a complex interconnection of commercial off-the-shelf (COTS) motors and encoders combined with electronic signal conditioning systems that were designed in-house to adapt the COTS hardware for our special requirements. The motion system control and drive electronics must be located remotely from the positioner to eliminate heat sources in the telescope dome, which required significant attention to the transmission of position feedback signals over the long cable lengths required between the fiber positioner and our control room. In the case of the Gimbal axes, custom electronics were designed to pre-process the analog encoder signals for interpolation inside the fiber positioner itself. The board at right is part of this signal conditioning electronics.
The system is also required to be immune to the sometimes severe lightning storms experienced on Mt. Hopkins. This requirement is complicated by the length of the cables and the poor grounding available on a mountaintop. For this purpose, we developed a combination interface/protection panel utilizing MOVs and TVS arrays, carefully selected for voltage and signal speed compatibility with each type of signal used in the system.
The fiber positioner also contains an in-house-designed electronic subsystem for housekeeping (supply voltages and temperatures) which also carries the Gimbal Conditioning Electronics. The fiber positioning robots themselves presented an interesting problem in cable design and packaging. The design required the routing of high voltage, high power motor drives in the same bundles as sensitive analog position feedback signals through two successive energy chains for the X and Y axes. The overall robotic cable system had to have a lifetime of one million cycles at a minimum 5X bend radius, which pushed the envelope of current cable capabilities. The final solution consisted of a combination of several types of IGUS servo control cable combined with semi-custom encoder cable obtained through one of our encoder vendors. The Hectospec Fiber Positioner is connected to the Instrument Control Racks through a set of flex-rated cables designed to work continuously as the telescope rotates on its axis and the building rotates to track a given star field.
The Instrument Control Racks house all of the control and drive electronics
for the Hectospec and other MMT conversion instruments. The racks consist of a
triple bay BUD rack with a cooling tower on the high power dissipation drive
electronics bay. These racks interface to our cable set through the same type of
transient suppression panel used on the Fiber Positioner.