The Discovery Channel Telescope: Construction and Design
Progress, January 2007
Thomas A. Bida, Robert L. Millis, Byron W. Smith, Edward W. Dunham, Heather Marshall
Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff, AZ, USA 86001 Telephone: 928-774-3358
Abstract
The Discovery Channel Telescope (DCT) is a 4.2m telescope under construction in northern Arizona. The DCT is located at a new site
near Happy Jack at 2361m elevation, which was selected following a lengthy site testing campaign that demonstrated DIMM-characterized
median ground-level seeing of 0.84-arcsec FWHM. The DCT science mission includes targeted studies of astrophysical and solar system
objects utilizing RC and Nasmyth-mounted imaging and spectroscopic instrumentation, and wide-field surveys of KBO’s, NEA’s, and
astrophysical objects with a 2-degree FOV prime focus camera.
The DCT facility enclosure and control buildings will be completed soon, including the telescope mount and dome supports, major
machinery infrastructure, the instrument laboratory, control and computer rooms, and the auxiliary building for the mirror coating plant.
Meanwhile, the effort for final figuring and polishing of the 4.3m ULE meniscus primary mirror blank began in August 2006 at the University
of Arizona College of Optical Sciences. The primary mirror and its design support, and the integrated telescope mount model, were finiteelement analyzed to optimize the design of the mirror and top-end support configurations. The primary mirror axial and tangential
actuators will be fabricated in early 2007 and utilized in the final figure and polish cycle. The prime focus camera design has been refined
to achieve atmospheric dispersion-compensated 0.25-arcsec images at 1-degree field radius, from B to I-band, at reduced cost through
simplification of glasses to standard types and utilization of spheres on all but two lens surfaces.
The Discovery Channel Telescope is a project of the Lowell Observatory with major financial support from Discovery Communications, Inc.
(DCI). DCI plans ongoing television programming featuring the construction of the telescope and the research ultimately conducted with
the DCT. Lowell Observatory and Discovery Communications are actively seeking additional partners in the project; interested parties
should contact R. L. Millis, Director.
DCT Development Plan and Key Science Enabled
Development
Phase
The DCT Primary Mirror Blank
The 4.3m ULE (ultra-low-expansion) meniscus primary mirror blank was accepted for
delivery from Corning Glass Works in September 2005. The photograph below left shows
DCT project manager, Byron Smith (left) and Lowell Observatory Director, Robert Millis
(right), with the completed blank. The mirror blank is now undergoing its 3yr figuring and
polishing process at the U. of Arizona College of Optical Sciences. The final polishing
specification is to meet the wavefront structure function with characteristic isoplanatic patch
size of 125cm, or system seeing contribution of 0.08arcsec. The primary mirror support will
consist of 120 axial actuators in 5 rings, 36 lateral force supports, and 3 tangential defining
links. An FEA analysis of the mirror support system by Goodrich Corporation showed
deformation in radius under gravity load of less than 24nm RMS, illustrated below right.
1. Ritchey–
Chrétien
Available
Instruments
Low and
Medium
Resolution
Spectrographs
RC HighResolution
Camera
Theme One
“What is the future and
history of our solar
system?”
Spectral survey of TransNeptunian objects.
Synoptic studies of comet
nuclei near aphelion.
Spectral Survey of NEAs
Theme Two
“How do stellar and planetary
systems function and evolve?”
Finding the Spectral
Signatures of Extra-Solar
Giant Planets
Brown Dwarf Survey
Theme Three
“How do galaxies form
and evolve?”
Evolution of Dwarf Galaxy
Building Blocks
Mass–Luminosity
Relationship of Massive Stars
Spectral Monitoring of Young
Stars
Gyrochronology
Mass Luminosity Relationship
of Massive Stars
2. Prime
Focus
Wide-Field
Camera
A Deep Survey of the
Distant Solar System
Mass–Luminosity
Relationship of Massive Stars
An Extended NEA Search
3. Nasmyth
Focus
High-Resolution
Optical
Spectrograph
Physical Processes in
Distant Comet Comae
Sub-Stellar Companion
Survey
Evolution of Dwarf Galaxy
Building Blocks
Solar Twin Activity and
Variability Survey
Patterns of Variability of
Stellar Magnetic Fields
DCT Facility
Science and Outreach
The DCT facility consists of a Telescope Enclosure Building and an Auxiliary
Building. The Telescope Enclosure Building has three levels: observing,
mezzanine, and base levels. The observing and mezzanine levels will be
ventilated naturally through the use of remotely operated louvers in the
telescope dome and mezzanine ringwall. The mezzanine level will serve as
an instrument storage area, with hook-ups at 3 locations for instrument glycol,
power, communications and closed-cycle He cooling lines. Space is provided
at the mezzanine level for future installation of a dome AC system if deemed
necessary. The base level is divided into control, computer, instrument lab,
electrical equipment, and receiving rooms. A hydraulic lift will service the
mezzanine and observing levels for instrument and equipment transport,
through a hatch on the observing level floor through which the Primary Mirror
Assembly and the Prime Focus Assembly will also be lowered via the dome
crane. The Auxiliary Building contains spaces for mirror washing and coating,
and for mechanical and electrical equipment to support the facility including
chillers, HVAC system, electrical transformers, generators, communications,
etc. A rail system is included between the two buildings to facilitate a simple
process for removing, cleaning, re-coating, and replacing the primary mirror.
The Discovery Channel Telescope holds distinct advantages for survey-based and targeted astronomical
research over both current 4m-class and larger aperture telescopes. Implementation of the DCT Prime Focus
Camera (PFC) will introduce a new regime in survey capability through étendue AΩ, representing telescope
collecting area (A, at 13.9m2) coupled with sky coverage (Ω, at 2°) in a single exposure. A relatively small user
community and flexible operations plan will enable long-term and/or frequent scheduling for programs with
precise or dense timing requirements, as well as long-duration scheduling for programs requiring significant sky
time.
The site is located near Happy Jack, AZ, 65km SE of Flagstaff, at 2361m altitude,
in the Coconino National Forest. The required National Environmental Policy Act
(NEPA) process was completed in 2004 and the special use permit received to
build the DCT at Happy Jack. The site is on a 50m-high cinder cone on the edge
of the upper Mogollon Rim, overlooking the Verde River valley. It is immediately
adjacent to a year-round paved highway, and commercial power is available less
than one mile from the site. Local topography drops 140m over 1km from ESE
through WNW. Prevailing winds are from the SW. The site features an
exceptionally dark sky.
An extended campaign to test astronomical and environmental qualities of the
Happy Jack site was conducted over 21 months total. A semi-permanent test
station was installed, consisting of a Differential Image Motion Measurement
(DIMM) system to measure seeing image quality, and a standard weather station
augmented with a 9.8m, 8-sensor wind measurement array to characterize wind
flow over the site. The seeing distribution, shown below, is consistent throughout
the year, with a median FWHM of 0.84 arcsec at 0.7µm. Wind velocity is steady,
greater than 2.2m/s 87% of the time at 9.8m height, and well-correlated above
7.3m height. Consistent sub-arcsec seeing is achieved with winds from near the
median direction, SW, 222° E of N, and when greater than 2.5m/s.
DCT astronomical research will address fundamental themes in planetary, stellar, and galactic astronomy, as
diagrammed above. Phased development of instrumentation will enable key observational projects in each
major theme. For example, a detailed second-phase conceptual design of the Prime Focus Camera (PFC) has
been completed that will meet the requirements of the advanced imaging surveys. It is expected that
instrumentation goals will be further refined with the addition of another institutional partner to the DCT project.
Through the collaboration with Discovery Communications, an ongoing series of television programs will be
produced about the DCT and the research performed with it. Discovery Communications has nearly 1 billion
subscribers in more than 150 countries.
Major Optical Specifications of the
Discovery Channel Telescope
Facility construction is consistent with modern observatory practice using steel
framed, metal clad buildings designed to equilibrate rapidly and with interior
ventilation designed to exhaust waste heat downwind of the observatory.
Within the Telescope Enclosure Building only the control room, instrument
workspace, and computer room are actively heated and cooled, and these
spaces are heavily insulated. The exterior of the building will be white to
minimize solar heating.
The formal groundbreaking took place in July 2005, with the concrete and
structural steel work completed in June 2006. The main facility exterior was
finished in December 2006, and a temporary roof installed afterwards.
Finishing of drywall, plumbing, and electrical installation are expected to be
completed by spring 2007.
DCT Site: Location and Seeing
DCT Prime Focus
Camera Configuration
Parameter
Prime Focus
Ritchey-Chrétien Focus
Clear Aperture
4.2 meters
4.2 meters
Effective f/ratio
Areal Central Obscuration
Linear Field of View
Image Scale (15µm pixel)
Image Quality (Note 1)
ADC
UV Cutoff
2.3
10%
2°
0.32"/pixel
0.38" FWHM
Included
360nm
6.2
6.6%
30' unvignetted
0.12"/pixel
0.20" FWHM
Optionally Removable
300nm (w/o ADC)
Note 1: Image quality includes all effects except free-atmosphere seeing.
Telescope Mount Configurations
The telescope mount design is currently in development at Vertex-RSI in collaboration with the DCT Project Team. The mount is an AltAz gimbal, based on designs developed for the SOAR and VISTA telescopes. The mount will support instruments at both Prime and
Cassegrain focal positions, through attachment of interchangeable top ends that incorporate either the Prime Focus Camera (PFC), or
Ritchey-Chrétien (RC) secondary mirror assemblies. Mount support for upgrade to Nasmyth instrumentation is also planned.
Preliminary optical alignment will be achieved using a carbon composite and steel truss structure, with strict requirements for
parallelism, concentricity, and perpendicularity between optical and mechanical axes. The active optics system will then be used to
obtain fine alignment by adjusting M1 tip, tilt, piston, and figure; M2 tip, tilt and piston; and/or PFC tilt. Results obtained with the SOAR
telescope mount indicate that tracking accuracy is sufficient for unguided tracking for short, 30-second exposures anticipated for the
routine survey work performed by the PFC. For longer exposures, guiding and low order wavefront sensing are anticipated.
Sensitivities of the system’s delivered optical image quality to misalignments of the major assembly optics were derived from Zemax
optical models of both the RC and PFC configurations. Tolerances derived from the sensitivities and error budget terms for optical
misalignments from gravity and wind loads are guiding FEA of the telescope structure. The FEA results are defining the material and
layout of the truss structure supporting separately the RC secondary and PFC top end assemblies. For example, truss tubes made of
wound carbon fiber with elastic modulus of 30MPSI, in an eight-leg arrangement, can meet a tight specification of approximately 0.6mm
decenter of the secondary mirror relative to the primary mirror. The comprehensive FEA model also includes the axial and tangential
primary mirror supports and predicts the effects of wind as well as gravity loads on the mirror figure.
Prime Focus Assembly
The most notable feature of the DCT is its large prime focus assembly (PFA). The
wide field of view of the PFA, illustrated below right in the image of Andromeda, will
be essential for KBO and NEO surveys. It will also be a valuable asset for several
proposed astrophysics programs. A Phase-2 design study has been completed at
Goodrich Corporation that reduced the projected cost significantly through changes
in several of the original requirements: relaxation of the image quality requirement
to that based on 10% degradation of median seeing rather than 1st quartile average
seeing; no U-band image quality requirement; modification of the ADC to a
tilt/decenter style on element L4; and repackaging of the PFA without a secondary
mirror. The new PFA design uses fused silica in 4/5 elements and standard glass
LLF6 in element 4, and utilizes spherical surfaces on all but two: an ellipsoid on L3
and an asphere on L5. The image quality requirements, including recovery of the Uband, are exceeded over the full 2o field to 75o zenith distance. The figure below left
shows the new conceptual PFA structure. From left to right, the main components
are the cable wrap, the prime focus CCD camera, the shutter and the filter changing
mechanism, the ADC assembly, and the prime focus corrector optics.
DCT 2o FOV