CCAT will be a 25 m diameter telescope operating in the 2 to 0.2 mm wavelength range. It will be located at an altitude
of 5600 m on Cerro Chajnantor in Northern Chile. The telescope will be equipped with wide-field, multi-color cameras
for surveys and multi-object spectrometers for spectroscopic follow up. Several innovations have been developed to
meet the <0.5 arcsec pointing error and 10 μm surface error requirements while keeping within the modest budget
appropriate for radio telescopes.
An imaging displacement sensor (IDS) has been developed that can measure the displacement with an accuracy of 30
nm in 0.03 s with the precision improving to 1 nm for averaging times of 100 s. The IDS consists simply of a light
emitting diode (LED) pinhole collimator and a charge-coupled device (CCD) camera chip. The position accuracy is
better than 0.05% over the few mm CCD size with deviation from linearity <140 nm. All six degrees of freedom (DoF),
three translations and three angles, can be measured with the same accuracy by combining multiple IDS with different
collimator beam orientations and knowing the nominal separation between the collimators and CCDs.
The 25-m aperture CCAT submillimeter-wave telescope will have a primary mirror that is divided into 162 individual
segments, each of which is provided with 3 positioning actuators. CCAT will be equipped with innovative Imaging
Displacement Sensors (IDS) - inexpensive optical edge sensors - capable of accurately measuring all segment relative
motions. These measurements are used in a Kalman-filter-based Optical State Estimator to estimate wavefront errors,
permitting use of a minimum-wavefront controller without direct wavefront measurement. This controller corrects the
optical impact of errors in 6 degrees of freedom per segment, including lateral translations of the segments, using only
the 3 actuated degrees of freedom per segment. The edge sensors do not measure the global motions of the Primary and
Secondary Mirrors. These are controlled using a gravity-sag look-up table. Predicted performance is illustrated by
simulated response to errors such as gravity sag.
The 25 meter aperture Cornell Caltech Atacama Telescope (CCAT) will provide an enormous increase in sensitivity in
the submillimeter bands compared to existing observatories, provided it can establish and maintain excellent image
quality. To accomplish this at a very low cost, it is necessary to conduct accurate engineering trades, including the most
effective segment and wavefront sensing and control approach, to determine the best method for continuously
maintaining wavefront quality in the operational environment. We describe an integrated structural/optical/controls
model that provides accurate performance prediction. We also detail the analysis methods used to quantify critical design
CCAT will be a 25 m diameter, submillimeter-wave telescope. It will be located on Cerro Chajnantor in the
Atacama Desert, near ALMA. CCAT will be an on-axis, Ritchey-Chrétien design with an active primary to
compensate gravitational deformations. The primary mirror will have 162 segments, each with ~0.5 × 0.5 m
reflecting tiles on a ~2×2 m, insulated, carbon-fiber-reinforced-plastic subframe. CCAT will be equipped with
wide-field, multi-color cameras and multi-object spectrometers at its Nasmyth foci. These instruments will cover
all the atmospheric windows in the λ = 0.2 to 2 mm range. The field of view at the Nasmyth foci will be 1°,
so CCAT will be able to support cameras with a few ×104 detectors (spaced 2 beamwidths) at λ = 1 mm to
a few ×106 detectors (spaced half a beamwidth) at λ = 350 μm. Single instruments of this size are probably
impractical, so we will break the field into smaller pieces, with a separate sub-field camera for each piece. The
cameras will require some relay optics to couple the fairly slow beam from the telescope to the detectors. A
reflective relay for 1° field of view is too large to be practical, so we plan to use a compact, cold, refractive relay
in each sub-field camera.
High angular resolution observations are essential to understand a variety of astrophysical phenomena. The resolution
of millimeter wave interferometers is limited by large and rapid differential atmospheric delay fluctuations.
At the Combined Array for Research in Millimeter-wave Astronomy (CARMA) we have employed a Paired Antenna
Calibration System (C-PACS) for atmospheric phase compensation in the extended array configurations
(up to 2 km baselines). We present a description of C-PACS and its application. We also present successful
atmospheric delay corrections applied to science observations with dramatic improvements in sensitivity and
CCAT will be a 25 m diameter submillimeter-wave telescope that will operate inside a dome located on Cerro
Chajnantor in the Atacama Desert. The telescope must have high aperture efficiency at a wavelength of 350 microns
and good performance out to a wavelength of 200 microns. A conceptual design for a carbon fiber reinforced plastic
(CFRP) truss and primary reflector support truss has been developed. This design yields a telescope with a net ½ wave
front error of <10 microns using a lookup table to adjust the segment actuators to compensate for gravitational
deflections. Minor corrections may be required to compensate for the expected 20 C temperature excursions. These can
be handled using a coarse lookup table.
The 25-m aperture Cornell Caltech Atacama Telescope (CCAT) will have a primary mirror that is divided into 162
individual segments, each of which is equipped with 3 positioning actuators. This paper presents a mathematical
description of the telescope, its actuators and sensors, and uses it to derive control laws for figure maintenance. A
Kalman Filter-based Optical State Estimator is used to continuously estimate the aberrations of the telescope; these are
used in a state-feedback controller to maintain image quality. This approach provides the means to correct for the optical
effects of errors that occur in un-actuated degrees of freedom, such as lateral translations of the segments. The control
laws are exercised in Monte Carlo and simulation analysis, to bound the closed-loop performance of the telescope and to
conduct control design trades.
The Cornell Caltech Atacama Telescope (CCAT) is a 25 m diameter telescope that will operate at wavelengths as short
as 200 microns. CCAT will have active surface control to correct for gravitational and thermal distortions in the
reflector support structure. The accuracy and stability of the reflector panels are critical to meeting the 10 micron
HWFE (half wave front error) for the whole system. A system analysis based upon a versatile generic panel design has
been developed and applied to numerous possible panel configurations. The error analysis includes the manufacturing
errors plus the distortions from gravity, wind and thermal environment. The system performance as a function of panel
size and construction material is presented. A compound panel approach is also described in which the reflecting surface
is provided by tiles mounted on thermally stable and stiff sub-frames. This approach separates the function of providing
an accurate reflecting surface from the requirement for a stable structure that is attached to the reflector support structure
on three computer controlled actuators. The analysis indicates that there are several compound panel configurations that
will easily meet the stringent CCAT requirements.
To meet the 10 µm RMS half wavefront error requirement for the 25 m diameter Cornell Caltech Atacama Telescope
(CCAT), active control of the approximately 200 primary mirror panels is required. The CCAT baseline design includes
carbon fiber aluminum honeycomb sandwich mirror panels. Distortions of the panels due to thermal gradients, gravity
and the mounting scheme need to be taken into consideration in the control system design. We have modeled the
primary mirror surface as both flat and curved surfaces and have investigated mirror controllability with a variety of
sensor types and positions.
To study different mirror segmentation schemes and find acceptable sensor configurations, we have created a software
package that supports multiple segment shapes and reconfigurable panel sizing and orientation. It includes extensible
sensor types and flexible positioning. Inclusion of panel and truss deformations allows modeling the effects of thermal
and gravity distortions on mirror controllability.
Flat mirrors and curved mirrors with the correct prescription give similar results for controlled modes, but show
significant differences in the unsensed flat mirror modes. Both flat and curved mirror models show that sensing
schemes that work well with rigid, thermally stable panels will not control a mirror with deformable panels. Sensors
external to the mirror surface such as absolute distance measurement systems or Shack-Hartmann type sensors are
required to deal with panel deformations. Using a combination of segment based sensors and external sensors we have
created a promising prototype control system for the CCAT telescope.
Five partners have currently joined a Consortium to develop the Cornell Caltech Atacama Telescope (CCAT.) Included
are Cornell University, the California Institute of Technology (Caltech), the University of Colorado at Boulder, the
United Kingdom as represented by the Astronomy Technology Centre (ATC), and Canada as represented by the
Universities of British Columbia and Waterloo. This consortium has continued work toward the design of the telescope
and instrumentation, pursued fund raising, and further developed the science case for CCAT. An Engineering Design
Phase is being planned for 2009-2011 with construction planned to begin shortly thereafter. CCAT continues as a wide
field (20 arc min) FOV telescope operating from a shortest wavelength of 200µ. Testing has continued near the summit
of Cerro Chajnantor in the Atacama Region of Chile above 5600 meters altitude and data indicates significantly lower
water vapor in the seeing column than measured at the ALMA site on the plateau below. Work over the past two years
has included research on manufacturing methods for optical segments, extensive study of mirror alignment sensing and
control techniques, additional concepts for major structures, and further development of instrumentation.
The Combined Array for Research in Millimeter-wave Astronomy (CARMA) comprises the millimeter-wave antennas of the Owens Valley Radio Observatory (OVRO), the Berkeley-Illinois-Maryland Association (BIMA) Array, and the new Sunyaev-Zel'dovich Array (SZA). CARMA consists of six 10.4-m, nine 6.1-m, and eventually eight 3.5-m diameter antennas on a site at elevation 2200 m in the Inyo Mountains near Bishop, California. The array will be operated by an association that includes the California Institute of Technology and the Universities of California (Berkeley), Chicago, Illinois (Urbana-Champaign), and Maryland. Observations will be supported at wavelengths of 1 cm, 3 mm, and 1.3 mm, on baselines from 5 m to 2 km. The initial correlator will use field programmable gate array (FPGA) technology to provide all single-polarization cross-correlations on two subarrays of 8 and 15 antennas with a total bandwidth of 8 GHz on the sky. The next generation correlator will correlate the full 23-antenna array in both polarizations. CARMA will support student training, technology development, and front-line astronomical research in a wide range of fields including cosmology, galaxy formation and evolution, star and planet formation, stellar evolution, chemistry of the interstellar medium, and within the Solar System, comets, planets, and the Sun. Commissioning of CARMA began in August 2005, after relocation of the antennas to the new site. The first science observations commenced in April 2006.
We present a first cut instrument design package for the proposed 25 meter Cornell-Caltech Atacama Telescope (CCAT). The primary science for CCAT can be achieved through wide field photometric imaging in the short submillimeter through millimeter (200 μm to 2 mm) telluric windows. We present strawman designs for two cameras: a 32,000 pixel short submillimeter (200 to 650 μm) camera using transition edge sensed bare bolometer arrays that Nyquist samples (@ 350 μm) a 5'×5' field of view (FoV), and a 45,000 pixel long wavelength camera (850 μm to 2 mm) that uses slot dipole antennae coupled bolometer arrays with wavelength dependent sampling that covers up to a 20' square FoV. These are our first light instruments. We also anticipate "borrowed" instruments such as direct detection and heterodyne detection spectrometers will be available at, or nearly at first light.
We report on the current progress of the water vapor radiometer (WVR) phase correction project for the Combined Array for Research in Millimeter-wave Astronomy (CARMA). CARMA is a new millimeter array that merges the Owens Valley Radio Observatory (OVRO) array, the Berkeley-Illinois-Maryland Association (BIMA) array and eventually the Sunyaev-Zel'dovich Array (SZA). WVRs are designed for phase correction by monitoring the water vapor in the atmosphere along the line of sight toward astronomical sources. In addition, we discuss the stability of the current OVRO water vapor radiometers in preparation for testing at the CARMA site. We will systematically analyze the receivers with atmospheric correlations to decouple the effects of instrumentation and atmospheric noise. Finally, we report on the status of the correlation receivers in development.
A new Combined Array for Research in Millimeter-wave Astronomy (CARMA) interferometer is being assembled from the existing Owens Valley Radio Observatory (OVRO), the Berkeley-Illinois-Maryland Association (BIMA) millimeter interferometers and the new Sunyaev?Zeldovich Array (SZA) at Cedar Flat, a site at 2,200 m altitude in the Inyo Mountains east of OVRO. The array will consist of 23 antennas of three different diameters, 3.5, 6.1 and 10.4 m, and will support observations in the 1 cm, 3 mm and 1.3 mm bands. The fist-light correlator is a flexible FPGA based system that will process up to 8 GHz of bandwidth on the sky for two subarrays consisting of 8 and 15 elements. The array configurations will offer antenna spacings from 5 m to 1.9 km allowing unprecedented high resolution and wide field imaging at millimeter wavelengths. Radiometers observing the 22 GHz water vapor emission line will be used to measure and correct for the water vapor induced path delay along the line of sight for each telescope and thereby minimize the time lost to “bad seeing”. This university based facility will emphasize technology development and student training along with leading edge astronomical research in areas ranging from Sunyaev-Zeldovich effect galaxy cluster surveys to studying protoplanetary disks.
Cornell and Caltech are undertaking a two year conceptual design study for a 25-m class sub-mm telescope. The nominal location for this facility will be the high Atacama Desert of Northern Chile. The baseline design is a segmented mirror telescope optimized for operation at wavelengths longer than 200 microns to take advantage of a low precipitable water vapor at the site. We discuss science drivers and their implications for telescope design and technical requirements, and planned technical study areas.
The Combined Array for Research in Millimeter-wave Astronomy (CARMA) requires a flexible correlator to process the data from up to 23 telescopes and up to 8GHz of receiver bandwidth. The Caltech Owens Valley Broadband Reconfigurable Array (COBRA) correlator, developed for use at the Owens Valley millimeter-wave array and being used by the Sunyaev-Zeldovich Array (SZA), will be adapted for use by CARMA. The COBRA correlator system, a hybrid analog-digital design, consisting of downconverters, digitizers and correlators will be presented in this paper.
The downconverters receive an input IF of 1-9GHz and produce a selectable output bandwidth of 62.5MHz, 125MHz, 250MHz, or 500MHz. The downconverter output is digitized at 1Gsample/s to 2-bits per sample. The digitized data is optionally digitally filtered to produce bands narrower than 62.5MHz (down to 2MHz). The digital correlator system is a lag- or XF-based system implemented using Field-Programmable Gate Arrays (FPGAs). The digital system implements delay lines, calculates the autocorrelations for each antenna,
and the cross-correlations for each baseline. The number of lags, and hence spectral channels, produced by the system is a function of the input bandwidth; with the 500MHz band having the coarsest resolution, and the narrowest bandwidths having the finest resolution.
We present results from phase correction efforts at the Owens Valley Radio Observatory millimeter array (OVRO). A brief description of the theory of phase correction is followed by a description of the water line monitors (WLMs) constructed and placed on each of the six antennas of the array. A summary of the current software in place is also included. We present examples of data corrected using this technique and the first image created using radiometric phase correction at OVRO. The phase correction system is undergoing further development and will soon be made available for general observing at the array. A brief discussion of application of the technique for future arrays (e.g. MMA, LSA, etc.) is included as a conclusion to this contribution.
The design of the Leighton telescopes and the unique techniques used in their fabrication make these telescopes particularly amenable to precise modeling and measurement of their performance. The surface is essentially a continuous membrane supported at 99 uniformly distributed nodes by a pin joint triangular grid space frame. This structure can be accurately modeled and the surface can be adjusted using low- resolution maps. Holographic measurements of the surface figure of these telescopes at the Caltech Submillimeter Observatory (CSO) and the Owens Valley Radio Observatory (OVRO) have been made over several epochs with a repeatability of 5 - 10 micrometer over the zenith angle range from 15 to 75 degrees. The measurements are consistent with the calculated gravitational distortions. Several different surface setting strategies are evaluated and the 'second order deviation from homology,' Hd, is introduced as a measure of the gravitational degradation that can be expected for an optimally adjusted surface. Hd is defined as half of the RMS difference between the deviations from homology for the telescope pointed at the extremes of its intended sky coverage range. This parameter can be used to compare the expected performance of many different types of telescopes, including off-axis reflectors and slant-axis or polar mounts as well as standard alt-az designs. Subtle asymmetries in a telescope's structure are shown to dramatically affect its performance. The RMS surface error of the Leighton telescope is improved by more than a factor of two when optimized over the positive zenith angle quadrant compared to optimization over the negative quadrant. A global surface optimization algorithm is developed to take advantage of the long term stability and understanding of the Leighton telescopes. It significantly improves the operational performance of the telescope over that obtained using a simple 'rigging angle' adjustment. The surface errors for the CSO are now less than 22 micrometer RMS over most of the zenith angle range and the aperture efficiency at 810 GHz exceeds 33%. This illustrates the usefulness of the global surface optimization procedure.