We describe bistatic scattering measurements at 230 GHz, in the 330-490 GHz range and, at 650 GHz on various surfaces. These include a series of eight reference targets constructed from alumina grit embedded in an absorptive epoxy matrix, and a set of conventional outdoor building materials. The samples’ surface topographies were measured by focus-variation microscopy (FVM) and their autocorrelation lengths and RMS roughness levels extracted. All bistatic measurements were performed in the principal plane, at incidence angles of 25o, 45o, and 65o, in s and p polarization. The reference samples’ normalized roughness levels cover the range 0.040 ≤ σ/λ ≤ 0.60, and their normalized autocorrelation lengths cover the range 0.086 ≤ 𝐿/λ ≤ 1.14. The measurements are described in terms of bidirection reflectance distribution function (BRDF) or normalized radar cross section (nRCS), and include regimes of both diffuse scattering and specular reflectance. The reference samples’ measurements are compared to two ab initio scattering theories, the Modified Integral Equation Method (IEM-B) of A. Fung, and the Generalized Harvey-Shack (GHS) model, that have no free parameters. Although there are several individual cases where either the IEM or GHS theory (or both) provide a good match to measurement, their overall agreement across the entire dataset is poor. In addition, the diffuse BRDF in each bistatic scan has been fit to a Lambertian (constant) dependence of scattering angle, and a purely empirical model developed for the dependence of Lambertian scattering on frequency, roughness, polarization, and incidence angle. The empirical model provides the best match to measurement across the full dataset, and can be used for reliable phenomenology studies of submillimeter imaging or wireless telecommunication. Nearly all the outdoor building materials, like the roughest of the reference samples, fall in a regime where L/σ is not large, and therefore where ab initio scattering theories can’t be expected to apply.
Many groups are developing submillimeter cameras that will be used to screen human subjects for improvised explosive devices (IEDs) and other threat items hidden beneath their clothing. To interpret submillimeter camera images the scattering properties, specifically the bidirectional scattering distribution function (BSDF) must be known. This problem is not trivial because surfaces of man-made objects and human skin have topographic features comparable to the wavelength of submillimeter radiation—thus simple, theoretical scattering approximations do not apply. To address this problem we built a goniometer instrument to measure the BSDF from skin surfaces of live human subjects illuminated with a beam from a 650 GHz synthesized source. To obtain some multi-spectral information, the instrument was reconfigured with a 160 GHz source. Skin areas sampled are from the hand, interior of the forearm, abdomen, and back. The 650 GHz beam has an approximately Gaussian profile with a FWHM of approximately 1 cm. Instrument characteristics: angular resolution 2.9⍛; noise floor -45 dB/sr; dynamic range ˃ 70 dB; either s or p-polarization; 25⍛ bidirectional-scattering-angle ≤ 180⍛ ; The human scattering target skin area was placed exactly on the goniometer center of rotation with normal angle of incidence to the source beam. Scattering power increased at the higher frequency. This new work enables radiometrically correct models of humans.
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.
The eSMA ("expanded SMA") combines the SMA, JCMT and CSO into a single facility, providing enhanced sensitivity
and spatial resolution owing to the increased collecting area at the longest baselines. Until ALMA early
science observing (2011), the eSMA will be the facility capable of the highest angular resolution observations at
345 GHz. The gain in sensitivity and resolution will bring new insights in a variety of fields, such as protoplanetary/
transition disks, high-mass star formation, solar system bodies, nearby and high-z galaxies. Therefore the
eSMA is an important facility to prepare the grounds for ALMA and train scientists in the techniques.
Over the last two years, and especially since November 2006, there has been substantial progress toward
making the eSMA into a working interferometer. In particular, (i) new 345-GHz receivers, that match the
capabilities of the SMA system, were installed at the JCMT and CSO; (ii) numerous tests have been performed
for receiver, correlator and baseline calibrations in order to determine and take into account the effects arising
from the differences between the three types of antennas; (iii) First fringes at 345 GHz were obtained on August
30 2007, and the array has entered the science-verification stage.
We report on the characteristics of the eSMA and its measured performance at 230 GHz and that expected
at 345 GHz. We also present the results of the commissioning and some initial science-verification observations,
including the first absorption measurement of the C/CO ratio in a galaxy at z=0.89, located along the line of sight to the lensed quasar PKS 1830-211, and on the imaging of the vibrationally excited HCN line towards
Active surface correction of the Caltech Submillimeter Observatory (CSO) primary mirror has been accomplished. The
Dish Surface Optimization System (DSOS) has been designed and built to operate at the CSO, on Mauna Kea, Hawaii.
The DSOS is the only active optics system of its kind in the world. There are 99 steel rod standoffs that interface the
dish panels to its backing structure. Each standoff is now fitted with a heating/cooling assembly. Applying a controlled
potential to each of the 99 assemblies adjusts the surface of the dish. Heating elongates and cooling shortens the
standoffs, providing the push or pull on the primary's panel surface. The needed correction for each standoff, for a
given elevation, is determined from prior holography measurements of the dish surface. Without the DSOS the
optimum surface accuracy was 25-μm RMS, yielding a beam efficiency of 33% at the 350-μm-wavelength range. With
the DSOS on, this has been improved to 10-μm RMS. The best beam efficiency obtained is 56%, with an average beam
efficiency of 53%. The DSOS has been in operation on the CSO since February 2003. Observers using the SHARCII
(a 384 pixel submillimeter high angular resolution camera) and the 850 GHz heterodyne receiver, have been able to
detect new weak and/or distant objects including detection of an earth-massed planet in Fomalhaut with the help of this
unique active optics system.
A submillimeter Fourier Transform Spectrometer of the Martin-Puplett
type was constructed and deployed to the geographical South Pole
in 2001. The instrument operates from about 300 GHz to almost 2 THz
and was used over winter to acquire atmospheric
spectra with resolution as fine as 250 MHz.
The main motivation for constructing and deploying
this FTS was for astronomical site testing, but the obtained
spectra can have important secondary uses in atmospheric
science and transmission model validation.
Some preliminary, low spectral resolution
site testing results are presented here.