IntroductionThis document provides a brief description of the Combined Array for Research in Millimeter-wave Astronomy (CARMA), suitable for planning observations and submitting an observing proposal. CARMA is a 23-antenna aperture synthesis telescope which operates at wavelengths of 1cm (27-35 GHz), 3 mm (85-116 GHz) and 1 mm (215-270 GHz). The array is located at Cedar Flat (elevation 2200 m; latitude 37.3°; longitude -118.1°) in the Inyo Mountains of eastern California. The CARMA operations base is 25 minutes' drive away at the Owens Valley Radio Observatory near Big Pine, California.CARMA is operated by the Universities of California (Berkeley), Chicago, Illinois, and Maryland, and the California Institute of Technology, under a cooperative agreement with the University Radio Observatory program of the National Science Foundation. Telescopes and Array ConfigurationsCARMA presently operates for much of the year as two arrays. One array consists of eight antennas 6.1 meters in diameter and six that are 10.4 meters in diameter, and operates at 3 mm and 1 mm. The measured surface accuracies of the antennas are typically 40 microns rms (lambda/75 at 3 mm wavelength) or better. The halfpower beamwidth is 100"/60" (6/10-m antennas) at 115 GHz, and the typical radio pointing accuracy is 5" rms. The antennas may be located at various stations distributed over an area 2 km in diameter, with spacings from 7 m to 2 km. Normally the antennas are deployed in one of 5 standard configurations (A, B, C, D, E), providing angular resolutions of roughly 0.3", 0.8", 2", 5", or 10" at 100 GHz. For the 2010B observing period (June 2010 - November 2010), the C, D, and E antenna configurations will be offered, with baselines of 26-370m, 11-148m, and 8.5-66m, respectively. The A and B antenna configurations will be offered in the proposal call for the 2011a semester.The second array consists of eight 3.5 meter antennas (the SZA), and operates at 1 cm and 3 mm. The halfpower beamwidths are 11'/3.6' at 31/95 GHz and the typical pointing accuracy is 15". The 8 telescopes are arranged in a hybrid configuration, with 6 in a compact configuration (SH) providing baselines between 4.5 and 11.5 m and 2 outriggers providing 56 to 78 m baselines. An analogous configuration (SL) optimized for low-declination (<-10deg) sources is also available, and will be scheduled depending on proposal demand. This semester, proposals will be accepted for SZA observations at 1 cm and 3 mm. In winter, the arrays combine for observations at the longest baselines - the Paired Antenna Calibration System (PACS). ReceiversThe 10.4 and 6.1-meter antennas are outfitted with 3 mm (85-116 GHz) and 1 mm (215-270 GHz) SIS receivers. One linear polarization is received. Both sidebands of the first local oscillator are received, and are separated in the correlator by phase-switching the local oscillators. The intermediate frequency (IF) band is 1-9 GHz at 3 mm and 1-4.5 GHz at 1 mm.The 3.5-meter antennas are equipped with single-sideband 26-36 GHz and 85-115 GHz receivers based on HEMT and MMIC amplifiers. The 1cm (3mm) receivers receive one circular (linear) polarization. The tuning is presently fixed for these receivers. The 1-9 GHz IF band corresponds to sky frequencies of 27-35 GHz for the 1cm band and 91-99 GHz for the 3mm band.
The Correlator SpectrometerThe digital correlator normally used for the 10.4 and 6.1-m antennas provides multiple bands (or "windows"). In cross-correlation mode, each band appears symmetrically in the upper and lower sidebands of the first local oscillator. By setting the frequencies of the second local oscillators these bands may be positioned independently within the I.F. bandwidth. The bands may be in any order, and may be overlapped. It is possible to observe several widely separated spectral lines simultaneously, or with different velocity resolutions. The best way to understand the capabilities is to use the Correlator configuration tool.By default, weighting (such as hanning smoothing) is not automatically applied to the data. For many projects this may be desirable either in the on-line processing or during data reduction, for example to reduce ringing in adjacent channels from strong spectral lines. For semester 2010B, eight bands will be available, yielding a maximum bandwidth of 4GHz per sideband. Each band may be configured in one of the following modes: |
| Nom.Bandwidth | Channels | Chan.width | dV[3mm] | Vtot[3mm] | dV[1mm] | Vtot[1mm] |
| (MHz) | (per sideband) | (MHz) | (km/s) | (km/s) | (km/s) | (km/s) |
| 500 | 129 | 3.88 | 12 | 1500 | 3.9 | 500 |
| 250 | 193 | 1.30 | 3.9 | 750 | 1.3 | 250 |
| 125 | 289 | 0.433 | 1.3 | 375 | 0.43 | 125 |
| 62 | 385 | 0.161 | 0.49 | 188 | 0.16 | 62.5 |
| 31 | 385 | 0.081 | 0.24 | 93.8 | 0.081 | 31.2 |
| 8 | 385 | 0.021 | 0.061 | 23.4 | 0.020 | 7.81 |
| 2 | 385 | 0.0052 | 0.015 | 5.86 | 0.005 | 1.95 |
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An analog filter attenuates the edge channels of the 62-MHz band. An
overlap of about 6 channels is recommended if multiple 62-MHz bands
are to be used to cover a single line. The signals from the 3.5-meter antennas are processed by a separate digital correlator. The correlator divides the 8 GHz IF bandwidth into sixteen 500 MHz bands; each of these is composed of 15 channels of 31.25 MHz.
Field of View; Sensitivity to extended Structures; MosaicingThe antenna half-power beamwidth is 100" x (115 GHz/freq) for the 6 m antennas, 60" x (115 GHz/freq) for the 10 m antennas, and 3.6' x (95 GHz/freq) for the 3.5 m antennas. Sources anywhere within the primary beam can be imaged, but the sensitivity decreases and pointing errors become more critical beyond the half-power points. The sensitivity to large scale structure is determined by the spacing of the sampled points in the aperture u-v plane. Typically the array has little sensitivity to structures which are larger than about 10 times the synthesized beamwidth.In order to image objects larger than about 1 arcminute in size, it is necessary to make observations at multiple pointing centers (mosaic) with the interferometer. The mosaic observations should cover the object to be imaged, and be spaced by approximately one half the beamwidth (no more than lambda/(2 x the 10m antenna diameter) for 6m/10m observations), in order to sample the short u-v spacings. The mosaiced interferometer observations may be combined with single dish data. The single dish observations should ideally be obtained with an antenna which is at least twice the diameter of the smallers antenna in the interferometer, as these antennas will typically have the shortest baselines. It is important that the single dish map be sampled at the Nyquist rate - that is, at no more than half-beamwidth spacings. Since the visibility function of an extended object is strongly peaked at the origin, the short u-v spacings contain most of the information about the large scale structure, and sparser sampling further out in the u-v plane mostly results in loss in sensitivity, rather than distortions in the image. Map SensitivityThe CARMA Sensitivity Calculator applet is the most convenient way of computing the expected rms noise for a CARMA observation. As inputs, you will give the array configuration, the observing frequency, the source declination, mosaic pattern, and the spectral resolution; the applet returns the synthesized beamwidth and the expected noise in mJy/beam and in Kelvin for observations of a single track. At 3mm a sensitivity of better than 0.5 mJy/beam rms (5 K in 1 km/s for a 1" synthesized beam) can be obtained in good weather at 95 GHz with an 8 hour track (70% time on source).For some projects, it will be necessary to request more time than can be allocated for a single track. In these cases, proposers should specify the total number of hours requested for the source. In general, such sources will be observed above an elevation limit of 30°, with this limit decreasing to 15° gradually for projects below 30° declination. In E configuration the elevation limits are higher to minimize shadowing. If other elevation limits are required they must be requested and justified in the proposal. Sensitivity-limited projects can often be observed in smaller pieces without regard to hour angle. An increase in the time allocated will be made to compensate for any reduction in sensitivity from that expected from longer tracks. CalibrationThe instrumental gain phase is calibrated by observing a nearby quasar every 15 - 30 minutes. A few minutes' observation of a 1 Jy quasar with ~1 GHz bandwidth provides a good calibration. Any systematic errors are minimized by selecting a quasar close to the source, but since the array geometry is determined to an accuracy of about lambda/10 it is usually better to select a stronger quasar within 30 degrees, than a weaker quasar which is closer to the source. Typically, a suitable calibrator can be found within 20 degrees of a source.The flux density scale (Jy/K) is determined from planet observations. Since the quasars are usually time variable it is necessary to calibrate the quasar flux from observations of planets at short baselines where the planets are not resolved. A history of calibrator fluxes is maintained within the MIRIAD reduction software package. The passband calibration is done in two parts. The correlator IF passband is measured with high signal-to-noise using injected noise sources. The RF part of the passband must be measured from observations of an unresolved source, usually 3C84 or 3C273, or, at short baselines, the planets. Preparing Observing FilesCARMA is operated by running observing scripts which have been prepared and checked in advance. The PI will be notified following the selection of an observing proposal, and must then submit a script which contains the complete observing sequence of source and calibration observations as well as tuning and correlator settings.Further InformationInformation for proposersThe current call for proposals Observing preparation tools, including the CARMA Sensitivity Calculator applet |