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Useful and Interesting Facts about the HET

OVERVIEW
  •  First public presentation about what became the HET concept was a talk by Dan Weedman of PSU in 1984
  •  Telescope budget:  $13.5M US
  •  Ground breaking:  March 25, 1994
  •  First Light:  December 10, 1996
  •  First spectrum (using 21 mirrors):  early September 1997
  •  HET is operated by on-site staff of 4 Engineers, 3 Telescope Operators, 3 Observer/Astronomers, Facility Manager, Administrative Assistant, and 3-4 technical staff of McDonald Observatory
  •  25 computers to run telescope and instrumentation
  •  Telescope Operator responsible for telescope safety, driving, and make call on closing
  •  Resident Astronomer responsible for navigation what/where/when and quality control
  •  Total weight of telescope (main structure, mirror truss, mirrors, tracker) = 80 tons

DOME
  •  Diameter = 86 feet
  •  Total Weight = 65 tons
  •  Shutter Weight = 6.9 tons
  •  Reflective covering (aluminum) = 7 tons
  •  Rotation drive is two rubber wheels pushing against a steel ring

TELESCOPE STRUCTURE
  •  Weight = 48 tons
  •  "Floats" on 8 air bearings, each with a capacity of 10 tons
  •  Each air bearing has 25-30 psi
  •  Rotation can reposition the azimuth to about 0.002 degrees
  •  Travel Range: ± 230°
  •  Velocity: 3.0°/sec
  •  Acceleration: 0.5°/sec2
  •  Position Resolution: ± 1 arc sec
  •  Position Measurement Accuracy: 2.5 arc sec (0.00068°)
  •  Display Resolution: 0.001°
  •  Feedback Resolution: 0.00017°
  •  Operating Temperature: -20° to +125°F (-30° to 50°C)
  •  Number of Motors: 2
  •  Motor Horsepower, HP: 2.7
  •  Motor Base Speed, RPM: 1500

PRIMARY TRUSS
  •  1747 struts
  •  389 nodes
  •  Weight = 12 tons
  •  52 temperature sensors

TELESCOPE TRACKER
  •  Weight = about 8 tons
  •  Payload = 2,200 lbs
  •  It takes simultaneous motions of 10 motors to follow a star
  •  6 degrees of motion (x,y,z, phi, rho, theta)
  •  +/- 8 degrees range of movement
  •  Instrument adapter (Prime Focus Instrument Platform = PFIP) includes an optical corrector made up of four mirrors, so light from stars actually uses 5 mirrors

MIRRORS
  •  91 identical 1 meter hexagonal segments made of Schott "Zerodur" (Coefficient of expansion 1.5E-07 /degreeC)
  •  5 spare mirror segments used during re-coating of original segments
  •  Fixed elevation angle = 55 degrees
  •  Radius of curvature = 26.165 meters; all mirrors match this to 0.5mm
  •  Each segment is 1000mm across (flat-to-flat), 52mm thick
  •  Each mirror weighs about 250 pounds
  •  Weight of 91 mirrors plus support frames = 13 tons
  •  Primary Mirror array = 11.1 meters (36.4 ft) by 9.8 meters (32.2 ft)
  •  Total area of the primary array = 78 square meters
  •  Gaps between mirrors lose 2.04% of potential reflecting area
  •  Mirror steps of about 1/3 wavelength of light (193 Angstroms or .0193 microns)
  •  Focal length of mirror: 13 meters or F 1.3
  •  Mirror segments moved by actuators (stepping motors)
  •  3 actuators/mirror = 273 total
  •  Actuators: screw shafts with 35 threads/inch; ratio of actuators is
       485 turns of motor = 1 turn of shaft; 1 step = 280nm
  •  Cleaning primary mirrors: once per month with CO2
  •  Original mirror coating is being stripped and surface aluminized

MIRROR ALIGNMENT
    SAMS (Segment Alignment Maintenance System)
  •  12 sensors/ mirror except edge segments – 480 sensors total
  •  Active sensor on one segment, passive sensor on neighboring segment and a resonant frequency is created between segments
  •  Segment shift is measured by quantifying change in resonance between segments
  •  SAMS segment corrections once per minute

    MARS (Mirror Alignment Recovery System)
  •  Shack/Hartmann detector measures focus of one segment and then sets sphere on that reference mirror
  •  Laser sent to entire array to measure positions and make corrections
  •  Used once at beginning of night operations

SPECTROGRAPHS
  •  Able to view wavelengths from .4nm to 1 micron
  •  Wavefront = 500nm
  •  LRS (Low Resolution) 23rd limiting magnitude, 3Kx1K CCD chip
  •  MRS (Medium Resolution) not completed yet 4Kx4K CCD chip
  •  HRS (High Resolution) 19th limiting magnitude, 4Kx4K CCD chip

Created:   16-Dec-1998
Last updated: 28-Jan-2004

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Copyright 1998, 1999, 2000, 2001, 2002, 2003  The University of Texas at Austin, McDonald Observatory