Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Arecibo Observatory wikipedia , lookup
Lovell Telescope wikipedia , lookup
Spitzer Space Telescope wikipedia , lookup
James Webb Space Telescope wikipedia , lookup
International Ultraviolet Explorer wikipedia , lookup
Optical telescope wikipedia , lookup
CfA 1.2 m Millimeter-Wave Telescope wikipedia , lookup
Very Large Telescope wikipedia , lookup
Bolometer Arrays For Mm/Submm Astronomy E. Kreysa, H.-P. Gemiind, A. Raccanelli, L.A. Reichertz, and G. Siringo Max-Planck-Institut fur Radioastronomie, Bonn, Germany Abstract. Arrays consisting of large numbers of sensitive bolometers have become powerful tools for Mm/Submm Astronomy. On large ground-based telescopes for example they were essential in the discovery of a population of faint, highly redshifted point sources which provide important clues to the star-formation history of the universe. The Bolometer group at the Max-Planck-Institut fur Radioastronomie has been developing bolometer arrays since 1980. This paper is meant to give an overview of the state and future of this effort. INTRODUCTION The Bolometer group at The Max-Planck-Institut fur Radioastronomie (MPflR) in Bonn has an active program of developing arrays of bolometers for ground-based Mm/Submm Astronomy. The purpose of this contribution is to present those arrays that are currently in operation at different telescopes and look at future prospects, especially also at new facilities. Due to space limitations, only especially interesting features of the arrays will be discussed here, as more complete descriptions will be the subject of future publications. MAMBO MAMBO stands for Max-Planck Millimeter Bolometer Array, and refers an array of 37 bolometers operating at 300 mK with an effective wavelength of 1.2 mm. It has operated since 1997 at the 30 m Millimetre Radio Telescope (MRT) of IRAM (Institut fur Radioastronomie im Millimeterbereich), situated on Pico Veleta, Sierra Nevada, in Spain. MAMBO has been used by a large user community with good success. Its sensitivity at the 30m MRT is strongly weather dependent, but can reach 20 mJy Hz-1/2, which is within a factor of two of the thermal background limit. For our group at MPIfR, a particularly interesting astronomical result of MAMBO has been the discovery of highly redshifted point sources in surveys of "empty" fields. These so called MAMBO sources provide important clues concerning the formation of stars in the early universe. The rate of discovery of these sources would increase dramatically if one could map large areas with high efficiency. Because this requires large format arrays it provides enough motivation to go on using the present, proven technology and to explore the limits of array size. Horn-Coupled Arrays The architecture of MAMBO is typical for the present MPIfR bolometer arrays: the thermal, electrical and mechanical structure of the bolometers is produced microlithographically with high precision on a Silicon Wafer. Millimeter radiation is coupled to the bolometers through an array of conical horns, the antenna properties of which are well known. In front of the horn array there are filters at different temperatures for defining the bandpass, and for rejecting high-frequency thermal background. The transformation of the focal ratio of the horns to that of the telescope is done with a room temperature optical system of lenses and mirrors. A small cryostat is therefore sufficient to accommodate even fairly large arrays. CP616, Experimental Cosmology at Millimetre Wavelengths, 2K1BC Workshop, edited by M. De Petris and M. Gervasi © 2002 American Institute of Physics 0-7354-0062-8/02/$ 19.00 262 Bolometer Design The MAMBO bolometers are of composite type, which allows the separate optimisation of critical bolometer parameters, like thermal conductivity, heat capacity and absorptivity. Freestanding membranes of Silicon Nitride provide low thermal conductivity for the bolometers by virtue of their amorphous structure. The membranes are manufactured by standard LPCVD techniques and, with a thickness of about 1 micron and a size of 3.4 mm square, are very strong. Therefore the only manual manufacturing step, namely the attachment of the Neutron Transmutation Doped (NTD) Germanium thermistors, can safely be done on a freestanding membrane. Utilising the wide bandwidth available in the 1 mm atmospheric window, the thermal background on a ground-based telescope is so high that the thermal conductivity of the unstructured membrane is sufficiently low already [1], The phonons created by absorption of photons are collected by a "split ring" of Gold, with a thickness of 200 nm, in the center of the membrane, achieving spatially uniform absorption within the ring (Figure 2.). NTD Germanium thermistors with electrical contacts on one side ("flatpacks") are then Indium-soldered across the one gap of the Gold ring. Electrical connection with negligible thermal conductivity is provided by 80 nm thick sputtered Niobium wires. If lower thermal conductivity should be required, the membrane can be structured as indicated in figure 2. The fact that the time constant is about 4 ms shows that the contribution made by the Gold and the thermistor to the heat capacity is not excessive. 1 1 I J 1 1 —— ET 1 1 I1 1 1 IL ^1 • r _, i 1 1 1 1 L-^l i L FIGURE 1. Bolometer layout. The large square, marked by a thick dashed line is the Silicon Nitride membrane, 3.4 x 3.4 mm in size. The NTD Germanium thermistor is shown across one of the gaps of the split Gold ring. Gold and Niobium layers are drawn in thick and thin continuous outlines respectively. The thin dashed lines mark openings in the membrane that would lower the thermal conductivity if this should be necessary. Electromagnetic Modelling The properties of bolometer absorbers consisting of one layer of dielectric with a metal film on one or both sides, have been described in the literature in the approximation of planar incident waves and layers of infinite lateral extent [2]. These approximations are no longer valid for small absorbers placed close to the exit of the circular waveguide. Three-dimensional numerical simulations on the basis of a finite difference program package were carried out in order to maximise the wideband absorption. The first result was that absorbers based on a dielectric with high refractive index were just as effective behind the waveguide as in free space. For example, a thin Silicon layer, a quarter wave thick (90 (im @ 1.2 mm wavelength) with a metal film on the back side, with maximum absorption at 1.2 mm, was entirely satisfactory and was therefore used in some early arrays. However, the fabrication would be much easier if one could do without the extra Silicon layer of about 90 jim thickness. Simulations with the metal film just on the Silicon Nitride membrane and a quarter wave reflector behind it led to the conclusion that satisfactory absorption could only be achieved if the waveguide was flared into a small horn. Further simulations showed that the metal ring around the absorber had no adverse effect. 263 FIGURE 2. Detail of the horn bolometer interface. Shown are from left to right: the vertex of the conical horn with the circular waveguide and the small flared horn, the Silicon Nitride membrane (thick vertical line) on the Silicon wafer and the quarter wave reflector. Horn Design Nearly all MPIfR bolometers are designed to detect a single mode of the radiation field in the focal plane. The full spatial resolution of the telescope will therefore be preserved and the sensitivity to point sources maximized. In this case the same considerations with respect to aperture and beam efficiency apply as for coherent receivers. For example, for general purpose observations, one is led to the same goal of about -13dB edge-taper of the illumination on the telescope primary. Each bolometer or array is designed for one frequency and can therefore be optimized for that frequency without any compromise. The feedhorns are corrugated or smooth-walled conical horns. For arrays they are combined in a closepacked hexagonal grid. Each horn feeds into a circular waveguide, which is about two diameters long. The waveguide acts as a mode filter and at the same time as a highpass filter, taking advantage of the steep cutoff of the fundamental waveguide mode (Hll). An additional lowpass filter in front of the horn array restricts the bandwidth to that of the fundamental mode of the circular waveguide of about 27%. Rf Shielding Ever since the time of an incident of strong radar interference on Pico Veleta, all MPIfR bolometer arrays are equipped with two layers of RF shielding. The first shield is the outer shell of the cryostat. All wires entering the cryostat are filtered, and the entrance window is covered on the inside with a specially designed inductive mesh. The bolometer-mount with the horn array presents the second shield. All signal and bias wires enter the bolometer cavity via feedthrough capacitors with a chip resistor in series. Cryostat and Preamplifier All the MPIfR arrays that are cooled to 300 mK, fit comfortably into a small, commercial He-4 cryostat with a 4" diameter cold work surface. MAMBO is equipped with one of the very compact He-3 sorption coolers, developed in France [3], with high pressure internal storage of the He-3 and designed for side-looking configurations. For the large diameter windows and filters of MAMBO2 a bottom-looking configuration is more convenient and a homemade He-3 stage with low pressure external storage was fitted in the same size He-4 cryostat. During operation the He-4 is pumped on continuously, as this will decrease significantly the thermal load on the He-3 stage. The hold time between cooling cycles is typically two days. The preamplifiers are based on junction field-effect-transistors (FETs) with low noise at low frequencies, mounted in an RF tight preamplifier box at 300K on top of the cryostat. The transistor noise at 300 K is not significantly higher than at the optimum temperature around 100 K and microphonics can be reduced to insignificant levels by careful wiring inside the cryostat. Only in the cryogenically more complex HUMBA array was it necessary to put the transistors on a heated stage in the cryostat. 264 Laboratory Tests In bolometer receivers where the spectral response can be affected by several components, a simple multiplication of the transmission curves of each individual component does not lead necessarily lead to a correct spectral system response. There could be interactions between components, and the response of the horn bolometer assembly is difficult to calculate or measure. Before a system goes on a telescope we try to characterise the system response in our lab with a Martin-Puplett interferometer. In this measurement a blackbody is used as the source and the complete array cryostat as the detector. Assuming a flat response of the Martin Puplett interferometer, the resulting spectrum should be that of the system multiplied by that of the blackbody. The angular response of MAMBO was checked recently in the feed-pattern measurement facility of the MPIfR. The facility has absorbing walls such that low level sidelobes can be detected. The source was a 230 GHz coherent source and in this setup no significant sidelobes were seen above those expected theoretically. MAMBO2 After the commissioning of MAMBO it became clear that a larger array (MAMBO2) would greatly improve the efficiency of the search for faint cosmological sources, the study of which is one of the main scientific activities of the Millimeter and Submillimeter Astronomy group of the MPIfR. If the area to be mapped is much larger than the array itself, then the time for mapping to the same depth depends as 1/n, if n is the number of array elements. As a compromise between technical risk and speed of development the number of array elements was set to around 120. MAMBO2 was planned to be a copy of the successful MAMBO, similar except for the higher number of bolometers. Wafer Layout FIGURE 3. View of MAMBO2 from the side opposite to the horns. The dark squares are the Silicon Nitride membranes, carrying the (square) Gold rings and Niobium wires. The NTD-Germanium chips are barely visible across one corner of each Gold ring. The octagonal Silicon wafer is fixed mechanically and electrically to thermal shunts on the surrounding mounting ring via ultrasonically bonded Gold wires. From the thermal shunts, wires are bonded to the center conductor of the feedthrough capacitors, which are visible on the mounting ring around the wafer. Note that two membranes are broken. 265 The layout of MAMBO2 still fits comfortably on a standard 4" silicon wafer (Figure 3.). With horn diameters of 5.5 mm and 3.4 x 3.4 mm membranes there is plenty of room for wiring between the rows of membranes. There is one common ground line in each row. The wafer is glued with wax, wiring side down, to a Sapphire wafer of the same size, then etched with KOH from the back in order to release the membranes, and diced to the desired shape. Bolometer Mount The bolometer wafer is held by 50 micron diameter Gold bonding-wires in the center of a Gold-coated Copper ring containing the feedthrough resistors (Figure 3.).The wires end on thermal shunts on the copper ring, and in this way provide robust electrical, thermal and mechanical connections between the ring and the wafer. Further bonds connect the thermal shunts to the center conductors of the feedthrough capacitors. The Copper ring with the wafer is sealed on the wiring side by the reflector plate and on the opposite side by the horn array. While this whole assembly is at 300 mK, the bias resistors are on the He-4 surface at 1.5 K. The parts at 300 mK are enclosed by a radiation shield at He-4 temperature (1.5K), which also serves as mounting surface for the filters and as attachment for Kevlar strings between the He-3 and He-4 cooled parts (Figure 4.). FIGURE 4. View of the array of horns in MAMBO2. The scale is indicated by the array grid constant of 5.5 mm. Connectors for the bolometer signals are visible on two sides of the horn array. The radiation shield at 1.5K carries the wide band filters and serves as attachment for Kevlar strings that fix the components at 0.3K relative to those at 1.5K. Optical Design In the Gaussian beam approximation, a beam can be transformed to a similar one with different beam parameters by a Gaussian beam telescope (GET). A GET is a combination of two lenses (or mirrors) with a common focus between them. A beamwaist at the front focus of the first lens is transformed into another one at the back focus of the second lens. It can also be shown that this transformation is broadband and that the waist radii will be in the ratio of the focal lengths. These results are easily derived in the thin lens approximation for beams on axis. For a large field, this condition is no longer valid, but one can start with a design that satisfies the GET condition on axis. By ray tracing, one can then optimise the image quality across the image plane within the boundary conditions of vertical incidence of off-axis bundles and a low curvature of the focal plane. For the large field of MAMBO2, a solution with a spherical mirror and an aspherical lens (made of high density polyethelene - HDPE), fitting within 266 the space available in the receiver cabin of the 30 m MRT, illustrated in Figure 5. MAMBO uses two asperical HDPE lenses, which were optimised in the same procedure, by ray tracing. FIGURE 5. Optics for matching MAMBO2 to the IRAM 30 m MRT. The radiation from the secondary is entering from the left. The spherical mirror has a diameter of 450 mm. Because of the Nasmyth focus of the 30 m MRT, the bottom-looking cryostat of MAMBO2 will stay vertical in the final focus behind the HDPE lens. MAMBO2 at the IRAM 30m MRT FIGURE 6. MAMBO2 in the receiver cabin of the IRAM 30 m MRT. The spherical mirror with a diameter of 450 mm is visible in the lower right corner, while the cryostat, HDPE-lens and flat folding mirror are in the upper left. The whole assembly is mounted on a vibration isolated optical table. 267 In February 2001, MAMBO2 was briefly installed for the first time in the receiver cabin of the 30 m MRT. The field diameter in the Nasmyth focus of the 30 m MRT is limited by the size of the Nasmyth mirrors, and the field of MAMBO2 is already close to that limit. Major mechanical changes were necessary to fit MAMBO2 with its optics into the limited space reserved for bolometers; at the same time a new backend (ABBA) for the large number of detectors had to commissioned. Alignment of the beams of an array on to secondary mirror is a slow process in a Nasmyth focus. For the central beam to hit the center of the secondary at all elevations it is necessary for this beam to propagate along the elevation axis, before being reflected by the Nasmyth mirror. After only a preliminary alignment, a beammap on Saturn was obtained as a first light observation. This observation was useful for debugging the system. Problems that were found were not of a very serious nature, so that it is likely that MAMBO2 will be online for the next winter season. HUMBA HUMBA, the "hundred millikelvin bolometer array", is a 19-element bolometer array for 2 mm wavelength and designed primarily for Sunyaev-Zeldovich studies. HUMBA is cooled by a dilution refridgerator, which can be adjusted to operate continously to any temperature between 50 and about 200 mK. Until recently HUMBA was subject to exess noise originating from the fridge, which severely limited its sensitivity. The solution of this problem is the subject of the paper by A. Raccanelli et al. in this volume. Tests of this system at the IRAM 30m are in progress. POLARIMETRY Polarimetry with arrays could be very exiting. The paper by G.Siringo et al. in this volume describes a retardation device that can be tuned to different wavelengths. The insertion of an additional polariser in front of the cryostat will transform any bolometer array into a polarimeter. Although the power in other polarisation is lost, one does not have to provide a second identical array; this represents a substantial saving of cost and effort. This is work in progress at the MPIfR. SIMBA SIMBA, the "SEST imaging bolometer array" is a joint project of the European Southern Observatory (ESO), Onsala Space Observatory, Bochum University and the MPIfR. The array and the cryostat are copies of MAMBO. In the Cassegrain focus of the 15m SEST telescope, SIMBA has to operate over the whole elevation range of 90 degrees. The same type of coupling optics as for MAMBO2, but now with a 45 degrees angle between the incident and final beams, allows the coverage of the whole elevation range, within an inclination range of the cryostat of +/45 degrees. Efficient mapping is the main purpose of using arrays and this is usually performed by scanning slowly in the chopping direction. SEST does not have a chopping secondary, therefore a new mapping mode, called "fast scanning", had to be developed. This observing mode is explained in the paper by L.A. Reichertz et al. in this volume. SIMBA was commissioned succesfully in June 2001. APEX With ESO and Onsala Space Observatory as partners, MPIfR is going to build a submm telescope of 12 m diameter, to be placed on the ALMA site in Chile. The location is at 5000 m altitude in a high region of the Chilean Atacama desert. APEX (Atacama pathfinder experiment), as the telescope is called, is a copy of an ALMA prototype telescope. It will offer unique opportunities for Submm astronomy in the southern hemisphere, making use of the exellent atmospheric conditions there. While the optical and mechanical characteristics of the main mirror will be identical to those of the ALMA telescopes, care will be taken in the design of secondary optics, to allow very large bolometer arrays take advantage of the full field of view. First light is foreseen for 2003. 268 CONCLUSIONS It seems clear that the near future will see very large bolometer arrays, with a tendency to fill the available telescope focal plane. New submm telescopes will be designed for the maximum field; this is a new development in Radioastronomy. Arrays are often compared by the numbers of their elements, which can be misleading if used as the sole characteristic of their performance. The area covered by two arrays each with the same number of elements can differ by a factor of 16 depending on whether the elements are sized for instantaneous Nyquist sampling or full efficiency with respect to point sources. A more significant figure for comparison would be the array throughput AO, where A is the effective array area. Arrays with several hundred elements can hardly be envisaged without cold multiplexers. Besides the advantage of fully lithographic fabrication, the attractiveness of superconducting bolometers [4] lies in the promise of multiplexed SQUID readout [5]. These are exiting times in this field, and the bolometer group of the MPIfR is planning to participate in these developments. ACKNOWLEDGMENTS The succes of the bolometer development at MPIfR owes a lot to the skill and dedication of our engineers and technicians, W. Esch, G. Lundershausen and B. Ufer. E.E. Haller and J. Beeman of LBNL Berkeley have always been a reliable source of excellent NTD-Ge thermistors. The group of V. Hansen at the University of Wuppertal performed the electromagnetic field simulations of the bolometer absorber structures, and developed software for calculating mesh filters. During micromachining campaigns in the microlab of UC Berkeley, E.K. enjoyed the friendly atmosphere and the helpfulness of the staff and many fellow labmembers. Special thanks go to X. Meng for the deposition and etching of the Niobium layers. The Millimeter and Submillimeter Group of the MPIfR, under the direction of K. Menten, can always be trusted to spur on the technical effort by proposing observations which make new demands. REFERENCES 1. 2. 3. 4. 5. Holmes, W., Gildemeister, J.M., Richards, P.L. and Kotsubo, V., Appl. Phys. Letters, 72, 2250-2252, (1998). Carli, B.and lorio-Fili, D., Journal Opt. Soc. Am., 71, 1020-1025, (1981). Torre, J. P. and Chanin, G., Review Sci. Inst, 56, 318-320, (1985). Gildemeister, J.M., Lee, and A.T., Richards, P.L., Appl. Phys. Letters, 77, 4040-4042, (2000). Yoon, J., Clarke, J., Gildemeister, J.M., Lee, A.T., Myers, M.J., Richards, P.L., and Skidmore, J.T., Appl. Phys. Letters, 78, 371-373, (2001). 269