Yutaro SEKIMOTO

Heterodyne mixing performance of a waveguide SIS mixer with inhomogeneous distributed junction (DJ) array composed of 3 SIS junctions is experimentally investigated at 375-500GHz. Quantum-limited noise temperature of 3-DJ mixer is demonstrated. Besides its low noise temperature, the mixer conversion gain of 3-DJ mixer is found to be more uniform over RF band than that of a PCTJ (parallel-connected twin junctions) mixer. The FTS (Fourier transform spectrum) response indicates a broad RF bandwidth of the 3-DJ mixer that is limited by the bandwidth of waveguide probe instead of mixer's tuning circuit.

We developed a 385-500GHz balanced mixer with a waveguide quadrature hybrid coupler. The balanced mixer consists of an RF quadrature hybrid coupler, two double sideband (DSB) SIS mixers with noise temperature of ~ 60K, and an IF 180 degree hybrid coupler covering 4 - 8 GHz IF band. An RF quadrature hybrid coupler was designed and fabricated whose fabrication error was within 5μm. The noise temperatures of the balanced mixer was similar to those of two DSB mixers in spite of adding an RF quadrature hybrid and an IF coupler. The required LO power for pumping the balanced mixer was reduced by ~12dB on average compared with those for the DSB mixers and -15dB coupler. The sideband noise of the local oscillator (a quintupler + a Gunn oscillator) was measured to be 20K at offset frequency of 4 - 8 GHz, which corresponds to 70K/μW. To authors' knowledge, this is the first direct measurement of LO sideband noise at submillimeter range. If a varistor quintupler degrades the signal-to-noise by 10dB (K. Saini 2003 [1]), the sideband noise of a Gunn oscillator is 7K/μW at the offset frequency of 0.8-1.6 GHz.

Abstract-We have developed a 385-500 GHz sidebandseparating (2SB) mixer, which is based on a waveguide split-block coupler at the edge of the E-plane of the waveguide, for the Atacama Large Millimeter/submillimeter Array (ALMA). An RF/LO coupler, which contains an RF quadrature hybrid, two LO couplers, and an in-phase power divider, was designed with the issue of mechanical tolerance taken into account. The single-sideband (SSB) noise temperature of a receiver using the RF/LO coupler is 104 K at the band center, which corresponds to 5 times the quantum noise limit (hf/k) in SSB, and 280 K at the band edges. The image rejection ratio of the receiver was found to be larger than 9.5 dB and typically 15 dB in the 385-500 GHz band.

In this paper, we report on the design and experimental results of a fix-tuned Superconductor-Insulator-Superconductor (SIS) mixer for Atacama Large Millimeter/submillimeter Array (ALMA) band 8 (385-500 GHz) receivers. Nb-based SIS junctions of a current density of 10 kA/cm2 and one micrometer size (fabricated with a two-step lift-off process) are employed to accomplish the ALMA feceiver specification, which requires wide frequency coverage as well as low noise temperature. Parallel-connected twin junctions (PCTJ) are designed to resonate at the band center to tune out the junction geometric capacitance. A waveguide-microstrip probe is optimized to have nearly frequency-independent impedance at the probe's feed point, thereby making it much easier to match the low-impedance PCTJ over a wide frequency band. In addition, a superconducting magnet fixed onto the compact mixer block to provide efficient magnetic field coupling is designed. The SIS mixer demonstrates a minimum double-sideband receiver noise temperature of 108 K at the band center and temperatures of less than 167 K over the whole band (for an intermediate-frequency range of 4-8 GHz). © 2005 IEEE.

Millimeter- and submillimeter-wave heterodyne mixers based on the Superconductor-Insulator-Superconductor (SIS) junctions have used a local oscillator (LO) source which is a combination of a Gunn diode and multipliers. Since such an LO source has a mechanical complexity and poor frequency coverage especially at the submillimeter wavelength, a compact and mechanically-simple LO source with broad frequency coverage is highly required for submillimeter-wave SIS receivers in the radio telescopes. Photomixers, which generate a difference frequency of two lasers at the millimeter and submillimeter wavelength by photoconductive mixing, have been alternatively developed. Photomixers are so compact sources with broad frequency tunability that they can meet the requirement for the LO source of the SIS receivers at the millimeter and submillimeter wavelengths in the radio telescopes. We have been developing a new type of millimeter-wave oscillator, which is called a photomixer, based on non-linear optical frequency conversion, or photomixing, in a Uni-Traveling-Carrier Photodiode (UTC-PD).[1-5] We have successfully demonstrated that the photomixer can produce a millimeter wave at 100 GHz with output power as high as 2 mW, when the photodiode is optically-pumped by two 1.55-μm lasers with input power of ∼100 mW each. Schematic diagram of our photomixer is shown in Fig. 1 and a schematic drawing and a photograph of the photomixer mount as well as a photograph of a diode chip are shown in Fig. 2. We have used the photomixer as an LO source for SIS mixers at 100-GHz band. The photomixer output and an RF signal are combined by a cross-guide coupler with a coupling efficiency of ∼-25 dB placed on the 4-K stage in a dewar and then coupled into the SIS mixer. This LO coupling scheme is popularly used in low-noise receivers at millimeter wavelengths. It is demonstrated that the photomixer can provide sufficient LO power required for optimum operation of the SIS mixer in the frequency band from 85 to 110 GHz. Since it is important to know the magnitude of (amplitude) noise generated by the photomixer, it is intended to measure the noise of the photomixer in comparison to that of a Gunn-diode oscillator. We have made measurement of noise performance of SIS mixers using a Gunn-diode LO source as well as photomixer LO source. Then, we carefully compared receiver noise temperatures of the SIS mixer pumped by the photomixer with those pumped by the Gunn-diode oscillator. Measured receiver noise temperature in those two cases are plotted in Fig. 3. It is clearly shown that receiver noise temperature of the SIS mixer pumped by the photomixer is as low as that pumped by the Gunn oscillator and no significant difference of receiver noise temperatures between the two cases was found. This experimental result indicate that the noise in the photomixer is sufficiently low for supplying LO to low-noise SIS mixers. Similar comparison of noise temperatures for Gunn diodes and photomixers have been made in the SIS mixers at 150-GHz band and 490-GHz band and no significant increase of noise temperature of SIS mixers have been found in both cases. Test observation of a spectral line of CS (J=2-1) at 98 GHz with an SIS receiver using the photonic LO has been made using a SIS receiver in the 45-m telescope at Nobeyama Radio Observatory.[6] The observed spectrum was carefully compared with that obtained by a receiver with conventional Gunn-diode LO. It is clear that nearly identical spectra have been obtained for both cases as shown in Fig. 4. © 2005 IEEE.

We have developed a cartridge-type 800 GHz receiver for the ASTE telescope in Atacama. Chile. The receiver has been assembled with a cooled receiver optics. a Nb-based SIS mixer, a local oscillator (LO) optics, and IF components in a 170 mm diameter column-type cartridge. The cooled optics is composed of a single ellipsoidal Mirror to Couple between the feed horn and the subreflector of the antenna, and an LO coupler with 10% efficiency. Owing to its cartridge and cryostat structure, the mechanical vibrations of the GM cryocooler are significantly reduced, and therefore the receiver is highly stable on the telescope. The receiver noise temperature, using a Nb-based SIS mixer and a 4-8 GHz HEMT amplifier, was attained to 1300 K in DSB at an LO frequency of 815 GHz. The system noise temperature. T-sys, was typically 4000-8000 K in DSB at an LO frequency of 812 GHz during operations, which depended oil the atmospheric opacity. The typical zenith opacity at an LO frequency of 8 121 GHz was similar to 1. The half-power beam width (HPBW) of the Main beam was measured by total power scanning across the Moon, and was consistent with the diffraction limit. A spectrum of the CO J = 7-6 line (806.6518 GHz) toward Orion KL was successfully detected.

The ALMA (Atacama Large Millimeter Submillimeter Array) is an international project by Europe, North America, and Japan, which is constructing large millimeter and submillimeter array at an altitude of 〜5000 m on Atacama desert in northern Chile. The array consisted of 80 high surface accuracy antennae with 12 m or 7 m diameter extends 14 km to achieve 0.01 arc second resolution. Atmospheric windows from 30 GHz to 900 GHz are covered by 10 frequency bands. We present ALMA instruments dictated for ultimate sensitivity, especially cartridge-type superconducting low noise receivers and photonic local oscillator system. The sensitivity of ALMA is two orders of magnitude larger than those of existing millimeter arrays. The partial operation of ALMA will be started from the end of 2007 and the full operation will be started from 2012.

N2H+ observations of molecular cloud cores in Taurus with the Nobeyama 45 m radio telescope are reported. We compare cores with young stars to cores without young stars. The differences in core radius, line width, and core mass are small. Line width is dominated by thermal motions in both cases. N2H+ maps show that the intensity distribution does not differ much between cores without stars and those with stars. This is in contrast to the result previously obtained in H13CO+ toward Taurus molecular cloud cores. A larger degree of depletion of H 13CO+ in starless cores is one possible explanation for this difference. We studied the physical state of molecular cloud cores in terms of "critical pressure" for the surface (external) pressure. There is no systematic difference between starless cores and cores with stars in this analysis. Both are not far from the critical equilibrium state. We suggest that molecular cloud cores in which thermal support dominates evolve toward star formation by keeping close to the critical equilibrium state. This result is in contrast to that obtained in the intermediate-mass star-forming region OMC-2/3, where molecular cloud cores evolve by decreasing the critical pressure (dissipating turbulence). We investigate the radial distribution of the integrated intensity. Cores with stars are found to have shallow (-1.8 to -1.6) power-law density profiles.

This paper aims to introduce a submillimeter-wave horn antenna designed by the authors which possesses high gain, very low loss and very low cross-polarisation within a very wide range of frequencies. It is a well-known fact that corrugated horn antennae radiate with symmetry pattern and very low cross-polarized field [1]. It is for this reason that corrugated horn antennae have chosen to design low loss horn antennae for radio-astronomical telescopes. However, the circular horn could not perform good just created corrugations inside of their flares [2]. Especially, it have been difficult to design the feed horns for radio-astronomical telescopes because they are required to possess very low loss. This paper presents a new submillimeter-wave corrugated horn antenna designed by the authors inserting an unique mode-transform section between a feed circular waveguide and a flare of a corrugated horn antenna.

This paper introduce the two types of submillimeter-wave horn antennae designed by the authors and present the experimental results obtained by an evaluative testing system that has also been developed by the authors. Submillimeter-wave components are widely used in radio-astronomical observation apparatuses. It should be noted, however, that, because the submillimeter-waves radiating from astronomical objects are extremely weak, there is a need to minimize (1) the various losses possibly occurred in the wave-receiving unit of observation apparatus, and (2) the quantity of the unwanted electromagnetic waves mixing in. With a view to achieving this objective, therefore, the authors provide in this paper their theoretical and experimental analyses of the submillimeter-wave horn antennae as well. It is a well-known fact that a corrugated horn antenna possesses very low levels of cross-polarized field intensity and high levels of radiation gain. It is for this reason that the authors have chosen to use corrugated horn antennae or, to be more specific, two types of such antennae - one designed by the authors for use in the range of 280GHz to 360GHz frequencies and tested actually to ascertain its characteristics, and the other designed for use in the range of 385GHz to 500GHz frequencies and tested actually to ascertain its characteristics. Note that (1) the measurements of the antenna beam patterns have been found to largely correspond to those of the numerical analyses, (2) it may, therefore, be safely assumed that the corrugated horn antennae presented here are functionally as efficient as they were designed, and (3) it may well be concluded that the measuring system developed by authors for evaluating submillimeter-wave horn antennae doubtless serves its useful purposes.

The ALMA telescope will be an interferometer of 64 antennas, which will be situated in the Atacama desert in Chile. Each antenna will have receivers that cover the frequencies 30 GHz to 970 GHZ. This frequency range is divided into 10 frequency bands. All of these receiver bands are fitted on a cartridge and cooled, with bands 1 and 2 at 15K and the other 8 are SIS receivers at a temperature of 4K. Each band has a dual polarization receiver. The optics has been designed so that the maximum of the optics is cooled to minimize the noise temperature increase to the receivers. The design of the optics will be shown for each frequency bands. Test results with the method of testing on a near field amplitude and phase measurement system will be given for the first 4 frequency bands to be used, which are bands 3 (84-116 GHz), 6 (211-275GHz), 7 (275-375 GHz and 9 (600-702 GHz). These measurements will be compared with physical optics calculations.

We have developed a simple and small thermal link for cooling a cylindrical cartridge, on which device under test is mounted. It consists of a crown-like ring with an inner diameter of 170 or 140 mm and a clamping belt, which is a metal spring or nylon. A thermal conduction of the thermal link is achieved as the crown-like ring clamps the disk-like stage of the cartridge with external force of the clamping belt. This link can be applied at various temperature ranges from 2 to 100 K. The measured thermal conductance of the 170 mm link is 1.7, 5.6 and 3.3 WK-1 for 4, 12, and 80 K stages, respectively. These values are consistent with a empirical calculation within 10% errors. This link is also effective to reduce mechanical vibration to be 6 μm (peak-to-peak) in the horizontal direction of the cartridge. This link is easily fabricated and is useful to various detectors which require to be cooled down. © 2003 Published by Elsevier Ltd.

Noise at millimeter wavelengths from a photonic local oscillator (LO) is compared with that from a Gunn oscillator using a low-noise superconductor-insulator-superconductor (SIS) receiver. No significant additional noise is added to the receiver by the photonic LO in the frequency range of 96-110 GHz. [DOI: 10.1143/JJAP.42.L704].

The 3P1-3P0 fine-structure line of the neutral carbon atom ([C I]) has been mapped over the 1°.8 × 1°.3 area of the L1688 cloud in the ρ Ophiuchi region with the Mount Fuji submillimeter-wave telescope. The 3P2- 3P1 line of [C I] has also been observed toward two representative positions to evaluate the excitation temperature of the [C I] lines. The overall extent of the [C I] distribution generally resembles that of the 13CO distribution. The [C I] distribution has two major peaks; one (peak I) is at ρ Oph A, and the other (peak II) is toward the east side of the C18O core in the southern part of L1688. Peak II is located beyond the C18O core with respect to the exciting star HD 147889. The C0 column density is 5.0 × 1017 cm-2 toward peak II. The spatial distribution of the [C I] emission is compared with plane-parallel photodissociation region (PDR) models, which suggest that peak II is associated with a lower density PDR front, adjacent to the dense cloud cores observed in the C18O line emission. Alternatively, peak II is in the early stage of chemical evolution, where C0 has not been completely converted to CO. In this case, the difference in the [C I] and C 18O distributions represents an evolutionary sequence. This is consistent with a picture of a shock-compressed formation of the dense cores in this region due to influences from the Sco OB2 association.

We have developed a W-band (75-110 GHz) waveguide photomixer with a uni-traveling carrier photodiode, which can be driven by two 1.5-μm lasers. It generates an output power of 2.2 ± 0.2 mW at 100 GHz with a laser power of less than 100 mW, and its relative power variation is as small as 3 dB across the entire frequency range of the W-band. A 100-GHz superconductor-insulator-superconductor receiver driven by this photomixer shows the same noise temperature around 26 K as that driven by a conventional Gunn oscillator.

We developed a cryogenic system, which houses 3 cartridge-type superconductor-insulator-superconductor receivers for millimeter and submillimeter wavelengths. Since it was designed as a prototype receiver of the Atacama Large Millimeter/submillimeter Array (ALMA), high stability, accurate alignment, and easy handling were required. To meet these requirements, the cryogenic system included the following technologies: 1) a thermal link without screws for receiver cartridges; 2) a central support structure to reduce vacuum and gravitational deformation; 3) bellows structures to reduce mechanical vibration of the cryocooler; and 4) a 3-stage Gifford McMahon (GM) cryocooler with an He pot (temperature stabilizer) to reduce the thermal ripple. The cryostat and receiver cartridges are composed of three stages. The temperatures on the 4 K, 12 K, and 100 K stages of the cartridge are 3.5 K, 13.4 K, and 78.3 K, respectively. The thermal conductances of the thermal links showed high performances of 1.7 WK-1 at the 4 K stage, 5.6 WK-1 at the 12 K stage, and 3.3 WK-1 at the 100 K stage. The mechanical vibration on the 4 K stage of the cartridge was reduced to one-tenth, as small as ≈ 2 μm peak-to-peak, compared to that on the 4 K coldhead of the cryocooler, ≈ 20 μm peak-to-peak. The temperature ripple on the cartridge was reduced to as small as 2 mK peak-to-peak, which corresponds to one-seventh of the ripple on the 4 K coldhead with an He pot.

This paper aims to introduce two types of submillimeter-wave horn antennae designed by the authors and to present numerical findings obtained by an evaluative testing system that has also been developed by the authors. Submillimeter-wave components are widely used in radio-astronomical observation systems. There is a need to minimize (1) the various losses possibly incurred in the wave-receiving unit, and (2) the quantity of the unwanted electromagnetic waves mixing in. It is a well-known fact that a corrugated horn antenna possesses very low levels of cross-polarized field intensity, loss, and side lobes. It is for this reason that the authors have chosen to use corrugated horn antennae as two types of such antennae - one designed for use in the range of 280 GHz to 360 GHz frequencies and tested at 280 GHz, 332 GHz, and 360 GHz, and the other designed for use in the range of 385 GHz to 500 GHz frequencies and tested at 385 GHz, 442.5 GHz, and 500 GHz. The measurements of the antenna beam patterns have been found to largely correspond to those of the numerical analyses; it may be concluded that the antennae are functionally as efficient as they were designed and the testing system doubtless serves the desired purpose.

We present the results of experimental radioastronomical observations of an interstellar molecular spectral line with an SIS (superconductor-insulator-superconductor) receiver pumped by a photonic local oscillator (LO). A millimeter-wave LO signal was generated by feeding an optical comb signal into a W-band (75-110 GHz) photomixer using a uni-traveling-carrier photodiode (UTC-PD). A CS J = 2-1 line at 98 GHz was observed toward a high-mass star-forming region W 51 (H2O). We compared the obtained spectra with those obtained with a conventional Gunn oscillator as an LO. As a result, the line parameters, such as the width, intensity, and frequency, were consistent with each other. The results presented here show that the photonic LO is a very promising source for heterodyne receivers in millimeter- and submillimeter-wave astronomy.

A photonic millimetre-wave and sub-millimetre-wave emitter (PME) that uses a log-periodic antenna and a uni-travelling carrier photodiode has been fabricated and tested. The output power and spectra of the PME were measured with a Fourier transform spectrometer and a bolometer at 100-800 GHz in a low-power excitation condition. The extrapolated output power with 15 mA photocurrent is several tens of microWatts in the 300-600 GHz range.

The large-scale distribution of the C I (3P1- 3P0, 492 GHz) emission line from Orion A and B giant molecular clouds has been imaged with the Mount Fuji submillimeter-wave telescope. The total area observed is about 15 deg2. The overall spatial and velocity structure of the C I line is found to be similar to that of the 13CO (J = 1-0) line. The derived column density ratio N(C I)/N(CO) shows a large variation ranging from 0.2 to 2.9 toward the cloud edges, whereas it is relatively constant between 0.1 and 0.2 toward the interior region of the entire Orion clouds in both massive star-forming and dark cloud regions. This almost constant ratio suggests that C I can coexist with CO even in the deep inside of the molecular cloud.

We present observations of the 3P1-3P0 fine-structure transition of atomic carbon [C I], the J = 3-2 transition of CO, and the J = 1-0 transitions of 13CO and C18O toward DR 15, an H 11 region associated with two mid-infrared dark clouds (IRDCs). The 13CO and C18O J = 1-0 emissions closely follow the dark patches seen in optical wavelength, showing two self-gravitating molecular cores with masses of 2000 and 900 M⊙, respectively, at the positions of the cataloged IRDCs. Our data show a rough spatial correlation between [C I] and 13CO J = 1-0. Bright [C I] emission occurs in the relatively cold gas behind the molecular cores but does not occur in either highly excited gas traced by CO J = 3-2 emission or in the H II region/molecular cloud interface. These results are inconsistent with those predicted by standard photodissociation region models, suggesting an origin for interstellar atomic carbon unrelated to photodissociation processes.

We present ASCA observations of the gravitationally lensed blazar PKS 1830-211. Intensity variations of about 10% and spectral variations were detected in the eight observations made at intervals of about 5 days. The spectral variations can be described by a change of absorption column density if we represent the spectrum with a single power-law model. However, it is more likely that the spectrum consists of two spectral components with different absorptions and that their intensity ratio varies. The column densities of the two components are consistent with the column densities of the two lensed images. However, the intensity ratio is different by a factor of 7.4 from the magnification ratio of the two lensed images. We suggest that the discrepancy is most likely due to X-ray microlensing, among several other possibilities. We estimate that the size of the X-ray emission region must be smaller than ∼3 × 1014 cm in order to explain the observed microlensing magnification.

We report the detection of the gravitationally lensed BALQSO H1413+117 at z=2.56, with the 40 ks Chandra ACIS-S observation. The X-ray image contains quadruply lensed images, however the flux ratios of the four images differ from the optical values by a factor of similar to5. We obtained the luminosity of 3.9x10(44) ergs/s for the QSO, and an upperlimit of 3.7x10(43) ergs/s for the possible lens object. The X-ray energy spectrum requires a strong BAL absorption feature with a hydrogen column density of 2.4(-1.2)(+1.5) x 10(23) cm(-2) at z=2.55(-0.18)(+0.27), consistent with the QSO redshift. In addition, a strong emission line at 6.21 +/- 0.16 keV with the equivalent width of 960(-480)(+1400) eV at the QSO rest frame, suggests that most of the direct component from the central engine is blocked by the BAL flow thus making the scattered component dominant.

A 10-m submillimeter telescope designed for interferometric observations at bands from 3 to 0.3 mm has constructed at Nobeyama Radio Observatory. The telescope is an engineering model for a large millimeter and sub-millimeter array, and will be operated for developments of sub-millimeter observation techniques at a remote site. We have fabricated lightweight machined aluminum panels (15 kg m-2) that have a surface accuracy of 5 μm rms. They have a typical size of 0.8 m×0.6 m, and are supported with three motorized screws. The back-up structure is constructed of a central hub of low thermal expansion alloy, and CFRP honeycomb boards and tubes. Holography measurements will be made with a nearby transmitter at 3 mm. The overall surface accuracy is expected to be <25 μm rms; the goal being 17 μm rms. We have achieved an accuracy of 0.03 inch rms for angle encoders. The drive and control system is designed to achieve a pointing error of 1 inch rms with no wind and at night. Under a wind velocity of 7 m s-1, the pointing error increases to 2 inches rms. An optical telescope of 10-cm diameter mounted on the center hub will be used to characterize pointing and tracking accuracy. Thermal effects on the pointing and surface accuracy will be investigated using temperature measurements and FEM analyses. The fast position switching capability is also demanded to cancel atmospheric fluctuations. The antenna is able to drive both axes at a maximum velocity of 3 deg s-2 with a maximum acceleration of 6 deg. s-2. The telescope is currently equipped with SIS receivers for 100, 150, 230, and 345 GHz and a continuum backend and an FX-type digital autocorrelator with an instantaneous bandwidth of 512 MHz and 1024 channel outputs.

The Mt. Fuji submillimeter-wave telescope has been operated since November 1998 to survey neutral atomic carbon (CI) toward the Milky Way. It has a 1.2 m main reflector with a surface accuracy of 10 μm in rms. A dual polarization superconductor-insulator-superconductor (SIS) mixer receiver mounted on the Nasmyth focus receives 810/492/345 GHz bands in DSB simultaneously. An acousto-optical spectrometer (AOS) has 1024 channels for 0.8 GHz bandwidth. The telescope was installed with a helicopter and bulldozers at the summit of Mt. Fuji (alt. 3725 m) in July 1998 after a test operation at Nobeyama for a year. It has been remotely operated via a satellite communication from Tokyo or Nobeyama. Atmospheric opacity at Mt. Fuji was 0.4 - 1.0 at 492 GHz in 30% of time and 0.07 - 0.5 at 345 GHz in 60% of time during winter five months. The system noise temperature was typically 1200 K (SSB) at 492 GHz and 500 K (DSB) at 345 GHz. The beam size was measured to be 2.′2 and 3.′1 at 492 and 345 GHz, respectively. We have conducted a large-scale survey of the CI (492 GHz) and CO (3-2:345 GHz) emission from nearby molecular clouds with a total area of 10 square degrees. We describe the telescope system and report the performance obtained in the 1998 winter.

We present a plan of heterodyne receivers for Atacama Submillimeter Telescope Experiment (ASTE), which is one of Japanese R&D project of Large Millimeter Submillimeter Array (LMSA) and Atacama Large Millimeter Array (ALMA). A new 10 m submillimeter-wave telescope has been pre-installed at Nobeyama since February 2000 and will be installed at Pampa la Bola (el. 4800 m) in northern Chile. The telescope has four receiver layouts: (1) A shaped Cassegrain optics was designed for the Nobeyama operation to achieve high beam-efficiency at millimeter-wave bands. (2) Normal gaussian optics will be replaced for the Chile operation to optimize submillimeter-wave bands up to 850 GHz. (3) It is possible to install an ALMA prototype receiver at the focus of secondary reflector. (4) an optics for submillimeter SIS photon camera. We describe the 350 GHz receiver which noise temperature was around 55 K in the frequency band of 330 - 360 GHz. The temperature ripple at the 4 K stage of two stages Gifford-McMahon refrigerator has been reduced to be less than 10 mK by employing a He-pot temperature stabilizer.

Large-scale mapping observations of the P-3(1)-P-3(0) fine-structure transition of atomic carbon (C I, 492 GHz) and the J = 3-2 transition of CO (346 GHz) toward the Orion A molecular cloud have been carried out with the Mount Fuji submillimeter-wave telescope. The observations cover 9 deg(2) and include the Orion Nebula M42 and the L1641 dark cloud complex. The C I emission extends over almost the entire region of the Orion A cloud and is surprisingly similar to that of (CO)-C-13 (J = 1-0). The CO (J = 3-2) emission shows a more featureless and extended distribution than C I. The C I/CO (J = 3-2) integrated intensity ratio shows a spatial gradient running from the north (0.10) to the south (1.2) of the Orion A cloud, which we interpret as a consequence of the temperature gradient. On the other hand, the C I/(CO)-C-13 (J = 1-0) intensity ratio shows no systematic gradient. We have found a good correlation between the C I and (CO)-C-13 (J = 1-0) intensities over the Orion A cloud. This result is discussed on the basis of photodissociation region models.

A distribution of the neutral carbon atom (C I) in Heiles cloud 2 (HCL2) has been investigated with the Mount Fuji submillimeter-wave telescope. A region of 1.2 deg 2 covering a whole region of HCL2 has been mapped with the 3P1-3P0 fine-structure line (492 GHz) of C I. The global extent of the C I emission is similar to that of 13CO, extending from southeast to northwest. However, the C I intensity is found to be rather weak in dense cores traced by the J = 1-0 line of C18O. On the other hand, strong C I emission is observed in a south part of HCL2 in which the C18O intensity is fairly weak. The C I/CO abundance ratio is greater than 0.8 for the C I peak, whereas it is 0.1 for the dense cores such as the cyanopolyyne peak. The C I-rich cloud found in the south part may be in the early evolutionary stage of dense core formation where C I has not yet been converted completely into CO. This result implies that formation of dense cores is taking place from north to south in HCL2.

We have developed a transportable 492 GHz tipping radiometer to measure the atmospheric opacity at potential sites for future ground-based astronomical observations in the submillimeter-wave band. With this radiometer, we measured the atmospheric opacity at two sites in northern Chile, Pampa la Bola (elevation 4800 m) and Rio Frio (elevation 4100 m), each for a few days. The 492 GHz opacity mostly ranged from 0.5 to 1.5 during the measurements. The 220 GHz opacity was also measured at the same time. The 492 GHz opacity correlates well with the 220 GHz opacity, the ratio between the 492 and 220 GHz opacities being 21.2±0.4. This result supports the standard atmospheric model, and can be used to evaluate the observable fraction of time for submillimeter-wave observations on the basis of the long-term 220 GHz opacity data.