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General MAT informational brochure
- Paper:
A measurement of the angular power spectrum made from the Chilean Andes
- Paper:
A measurement of the power spectrum of the CMB
The cosmic microwave background (CMB) is the photon remnant from the Big Bang. By tracing fluctuations in the light intensity, the fluctuations in the matter density of the universe at roughly 100,000 years after the big bang can be inferred. Understanding these fluctuations may allow measurements of fundamental cosmological parameters that can enlighten the state of the universe in the past, present, and future. Because of the importance of the CMB, satellites are being designed to map it over the whole sky, but significant progress can be made before they are launched.
The MAT proposal was to modify an existing high altitude balloon borne telescope for ground-based operations. Test measurements indicated the site in the Chilean Andes will allow such a high quality data that nearly 90% could be used in the final analysis, as compared to 20% at a similar ground-based instrument in Saskatchewan. This is due to the site's altitude and dry atmosphere, which reduce the total atmospheric emission by two-thirds.
The called MAT (Mobile Anisotropy Telescope) was a modification of the QMAP experiment, which flew on high altitude balloons. The portable instrument allowed the most sensitive measurement then of the power spectrum of the CMB. We observed 3,400 deg2 in a ring of 10 degrees width around the entire sky at a declination of -15 degrees.
The Science:
The anisotropy in the CMB is quantified by plotting the fluctuation of the temperature or power as a function of the angular scale. The results may then be compared to predictions. It is still not clear where the spectrum peaks, if the fluctuations are Gaussian, or how large the fluctuations are. The calibration uncertainty for all measurements is between 10% and 20%.
The Technology:
The principal detector technologies for the sub-millimeter and microwave regimes are HEMTs, SIS mixers and bolometers. SIS mixers are exceptional detectors, the most sensative of which are in use at a number of radio observatories. They operate well at 4.2 K, are intrinsically single-moded, and are rugged. Most importantly, they are very fast detectors and thus allow large sample rates (upwards of 100 kHz).
The Site:
The site chosen is on Cerro Chajnantor, near the town of San Pedro de Atacama, Chile, which is at a latitude of 23° South and a longitude of 67° West. The site is on a high plateau which slopes up to an altitude of 5,500 m, with some surrounding mountains at slightly higher elevations. To minimize disturbance from the surrounding peaks, the middle of the plateau at an altitude of 5,200 m was selected.
Detailed site measurements by NRAO for the Millimeter Array indicate that it is one of the best sites in the world for taking millimeter measurements. With two months of observations, same as at Saskatoon, the net sensitivity is expected to be 2.5-3 times better at the Chilean site. This is due to the fact that nearly every day the overall sky opacity and its stability at the Chilean site is better than during the best days in Saskatoon, making one observing season in Chile worth 6-9 in Saskatoon.
The Instrument:
Optics: The optics in use here are the same as those from the three years in the experiment at Saskatoon. Multiple cooled corrugated feeds under-illuminate a large chopping flat. Because the chopper is closest to the sky, the illumination is stationary on the parabola, eliminating the synchronous pickup from its emission. The beam profiles were computed to a few percent accuracy before they were manufactured and the side lobes have been measured to follow the model predictions. The telescope is designed so that as the beam sweeps, the angular separation between the optical axis and the ground screen is constant.
The chopping flat is a 5' by 4' oval of 0.5" thick aluminum honeycomb panel. It resonantly chops the beam sinusoidally at 4.6 Hz with an amplitude of 3.25°. This leads to a peak-to-peak angle on the sky of 10° (2 x 2 x 3.25° x cos 42°). The flat is attached to the telescope by two bearings on its rotation axis and by two leaf springs. The spring constants are adjusted to give the desired resonant frequency. The Q of the system has been measured to be as high as 100 resulting in a minimum amount of power required to operate (an important consideration at a remote site).
Operation at a remote location makes the use of liquid helium to keep the SIS detectors at 4.2 K impractical. In the past we have used a recycling Gifford-MacMahon refrigerator operating at 17 K. We have identified a new closed system which can maintain the cold plate at 4.2 K with 500 mW of cooling power. The secondary stage of the system is at 40 K and will act as a thermal shunt for the wires and waveguide going to the cold plate.
Mechanical Design/Support Electronics:
The telescope was designed to be small, lightweight, and portable. It can survive being dropped by a parachute from 36 km, so shipment to Chile should not pose a problem. The telescope is mounted on a surplus military radar trailer. A large geared bearing will be mounted on the bed of the trailer with a motor to control the azimuthal position. The system will be able to slew the telescope 180° in 10 seconds. The ground screens are physically attached to the telescope, so they will have the same orientation with respect to the beams regardless of where the telescope is pointed.
The telescope pointing will be determined by a 16 bit digital encoder on the axis of the main azimuthal bearing. The encoder calibration will be determined during each of the planetary observations. At night, a computer-controlled CCD camera will log the positions of the stars in the field of view and determine the encoder azimuth almost continuously for both east and west observations. The encoder position information is then used by the gear motor to point the telescope to the desired place on the sky.
Pointing and data acquisition are controlled by two separate computers synchronized with a common clock. The pointing computer records the encoder position, runs the CCD and interprets commands from a remote station (located in the nearest town approximately 100 miles from the site). The data computer simply logs the detector outputs (both AC and DC components), chopper position, and housekeeping information. From the remote station we can monitor all signals and plot any channel to quickly assess and correct any problems.
Logistics:
While we have extensive experience making these types of observations, the site in Chile will offer some unique challenges. The altitude of the site is 5,200 m, which makes it a great place to observe, but a difficult place for humans to function. This is why it is crucial to use a self-contained system such as our balloon telescope. It has already been specifically designed to operate remotely under harsh environmental conditions. We will build, test, and operate the complete system while still in North America. When the instrument is sent to Chile by boat, it will be almost completely intact. Upon arrival we will finish assembly at a lower altitude site (about 3000 m). It will then be towed to the final observation site where only a minimal amount of physical setup will be required. Once the telescope is operational, it will be visited once every two days to swap out the data storage disks and to do routine maintenance. The power requirements for the site are dominated by the compressor for the refrigerator. To supply all the power we will purchase a 20 KW diesel generator modified to run at high altitude.