The Discovery Channel Telescope a Wide Field Telescope in Northern Arizona

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The Discovery Channel Telescopea Wide Field
Telescope in Northern Arizona
Thomas A. Sebring Project Manager
Edward Dunham Project Scientist
Robert L. Millis Director, Lowell Observatory

• Lowell Observatory
• 1400 W. Mars Hill Rd.
• Flagstaff, Arizona 86001
• sebring_at_lowell.edu

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Origins

• Under Consideration by Lowell Observatory Since
  1995
• Originally Titled the Next Generation Lowell
  Telescope
• Initial Studies by Electro-Optical Systems
  (Blanco et al 2003)
• Additional Study by Harland Epps (Epps
  DeVittorio 2003)
• Discovery Communications Became Major Partner in
  2003
• Project Office Staffing Begun in March 2003
• Configured to Support Lowell Observatories
  Dedication to Broad Spectrum Astronomical Science

Special Kudos to Bob Millis and Bill Putnam For
the Vision to Make DCT a Reality
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The DCT Configuration

• Altitude over Azimuth
• Dual Prime/Cassegrain Focus via Top Flipping
  Mechanism
• Upper Hexapod for Coarse Alignment
• Wide Field Imager at Prime Focus Part of Project
• RC Focus Can Mount Single Large or Multiple
  Instruments

A Truly Unique World-Class Telescope
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Optical Specifications

• Its BIG

• Its FAST

Parameter Prime Focus Ritchey-Chrétien Focus
Clear Aperture 4.2 meters 4.2 meters
Effective f/ratio 2.3 6.2
Central obscuration 10 10
Linear Field of View 2 degrees 30 unvignetted
Image scale (15 µ pixel) 0.32 "/pixel 0.12 "/pixel
Image Quality 0.27" FWHM 0.20" FWHM
ADC Included Optionally removable
UV cutoff 330 nm 300 nm (without ADC)

• WIDE Angle

• Its SHARP

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Mechanical Specifications
Parameter Specification
Operating azimuth range 270
Operating zenith angle range 0.4 lt Z lt 85
Maximum slew rate 3/sec
Retargeting Time lt 6 sec for 2 move
Pointing error lt 2" rms
Pointing stability lt 0.1" jitter with 0.1"/min drift
Non-sidereal object track rates gt 5"/sec
Guiders Non-sidereal capability
Prime/RC focus selection Tumbling top end
Nominal RC focus payload 5000 lb
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Happy Jack Site

• 40 Miles SE of Flagstaff
• Good Paved Road
• Power lt 1 mile Away
• 7700 feet
• Very Dark Site

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Site Testing

• Operating nightly
• Seeing Data
• Weather logging
• Encouraging results
• 0.8 median seeing
• 0.6 first quartile
• Other Issues
• Boundary layer _at_ 30 feet
• Geotechnical Survey Completed
• Sky Brightness Measurements

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Telescope Site Works

• Existing Road Improvement
• 1 mile
• Underground Powerline 1 mile
• Water Tank for Fire Grey Water
• Bottled Drinking Water
• RF Ethernet Link to Lowell Observatory

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Telescope Facility

• Minimal Heated Area
• Downwind Ventilated
• 6550 feet2 Area
• Steel Framed Building
• Key Spaces
• Mech/Elec
• Computer Room
• Control Room
• Instrument Work
• High Bay/Rcving
• Equipment
• Coating

Prevailing Wind
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Observing Level
Prevailing Wind

• Equipment Lift
• Floor Level with Telescope Yoke
• Hatch for PM Removal
• 60 foot Man Lift
• 2640 feet2

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The Dome

• Follows SOAR Practice
• Original Design SOAR Team w M3
• Detailed Design Mfg by Equatorial Sistemas
  (Brazil)
• Steel Frame w Crane
• Fiberglass Panels
• Excellent Mechanisms
• Dome for DCT Will be 72 Feet Diameter at
  EquatorSOAR was 66 Feet

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Seeing Control

• Trade Natural vs Downdraft Dome Ventilation
• CDF Modeling by TFD, Capetown S.A.
• Dome Interior Air Conditioned During Day
• Mount Features Circulating Fans for Day Use
• Mount is Insulated for Minimum Heat Flow
• All Motors are Insulated and Glycol Cooled

vml--gt                
Analysis by Sarel Venter
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Telescope Mount

• Concept Development by Vertex RSI, Richardson,
  Texas
• Rolling Element Bearings
• Azimuth With Recirculating Oil
• 4 Helical Gear Drives in Azimuth
• Direct Drive Torque Motors in El
• 150 Tons Total Mass (Preliminary)
• Anticipate Full Insulation
• Interior Circulation System for Daytime
  Equilibration
• Heidenhain Encoders on All Axes
• Field Rotation at Cassegrain

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Hexapod Top Ring

• Uses Hexapod Structure for Coarse Alignment
• /- 20 µ Accuracy
• /- 2 µ Resolution
• 6 Degrees of Freedom
• Stiff Enough
• 30k per Actuator
• Allows Alignment of Either Prime Focus or
  Cassegrain
• Fine Tip/Tilt and Focus Achieved with Active
  Primary Mirror

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Mount Modes

• 1st is Twisting While Pointed at Horizon(top
  view) 7.41 Hz
• 2nd is Side Sway at Zenith 8.26 Hz
• 3rd is Lowest Elevation Mode at Zenith 11.20 Hz
• All Modes Will Improve as Mount is Optimized

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Optical Design

• Standard RC Configuration
• Conventional Corrector Design
• Meets Image Quality Requirements
• May be Further Optimized When Instrumentation
  Plans are Finalized

Optical Design Work by Malcolm J.
MacFarlane Goodrich Electro-Optical Systems
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Prime Focus

• 9 Optics
• 6 Fused Silica
• 3 Other
• Alignment Tolerable
• Residual Error at Edge of 2 degree FOV is
  Challenging

Optical Design Work by Malcolm J.
MacFarlane Goodrich Electro-Optical Systems
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Prime Focus Instrument

• Invar Structure
• Optimized for Acceptable Structural Performance
• Mechanisms
• ADC
• Filter Changer (5)
• Shutter
• Field Rotation at Camera
• Lightweighted ULE Secondary Mirror

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Parts of the Prime Focus Assembly
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Structural Analysis

• All Invar Frame Cells
• Preliminary Gravitational and Thermal Analysis
  Complete
• Alignment Tolerances Acceptable
• Mechanisms Concept Design Complete
• ADC
• Shutter
• Filter Changer
• Field Rotator

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PFA 1st Three Modes

• Mode 1 28.23 Hz
• Mode 2 40.44 Hz
• Mode 3 50.68 Hz

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Mosaic Focal Plane

• Contract with e2v to Design Entire Array
• Uses 40 CCD44-82 2k x 4k Standard e2v CCDs
• Requires Focal Plane Flatness to 10 µ
• Anticipate Acceptance Test at Cryo of Fully
  Populated Array Prior to Delivery
• Design, Analysis, Modeling In Process
• Outboard CCDs Used for Wavefront Sensing Guiding

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Primary Mirror System

• 127 force actuators
• Closed Loop About Sensors
• Laminar Flow Seeing Control
• Step Motors w Harmonic Drive Leadscrews
• 100mm x 4.3 m ULE Facesheet
• 0.03µ RMS Optical Quality

Concept Baseline is Electro-Mechanically Actuated
Active PM
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Optical Coating Chamber

• Contract with Astronomy Technology Centre
  (Edinburgh)
• Provide VISTA Design IPR for Use
• 2nd Generation Magnetron Chamber Design
• Single Magnetron for DCT
• High Deposition Energetics
• Single Pass Coating
• Magnetron Covers Entire Radius of PM
• ATC Support in Review of Design Modifications

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Control System

• Negotiating to Use SOAR Control System
• PC Platforms
• Fast Ethernet
• Linux Windows OS
• National Instruments LabVIEW Software
• Rutherford Appleton Labs (P. Wallace) Pointing
  Kernel and Mount Model
• Additions by Lowell for Non-Sidereal Tracking
  Capability

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Project Team to Date
Oliver Wiecha Electrical Eng. Mgr From
SOAR Project
DCT Headquarters Lowell Observatory
Byron Smith Mechanical Eng. Mgr From
Vertex RSI
Mariana deKock Civil Engineering Mgr
From SALT Project
E. (Ted) Dunham Project Scientist
From MIT/NASA
Kim Westcott Administrative Mgr From
Grand Canyon Foundation
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Project Schedule
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A Unique Telescope
Telescope Diameter Collecting Area Solid Angle A?
UH 2.2-m/PFCam 2.2 0.8 0.25 0.2
Palomar/Quest 1.2 1.1 16.60 18.3
CFHT/Megacam 3.6 10.0 1.00 10.0
MMT/one-degree camera 6.5 33.2 1.00 33.2
Subaru/Suprimecam 8.0 50.2 0.25 12.6
Discovery Channel Telescope 4.2 13.9 3.10 43.1
Pan-STARRS 3.6 10.0 7.00 70.0
LSST 8.3 54.0 7.00 378.0
And Thats Just the Prime Focus Capability5000
lbs at RC!!!
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Opportunities

• DCT Is a Unique Telescope
• Remote Observing Makes It Virtually Accessible to
  All Partners
• Data Archiving Supports Multiple Mining Research
  Threads
• Instrumentation for Cassegrain is Wide Open

Our Dance Card Is Not Yet Full Contact T.
Sebring sebring_at_lowell.edu R. Millis
rlm_at_lowell.edu T. Dunham dunham_at_lowell.edu