Continuum Observing. Continuum Emission and Single Dishes

Transcription

1 July 11, 2005 NAIC/NRAO Single-dish Summer School Continuum Observing Jim Condon Continuum Emission and Single Dishes Continuum sources produce steady, broadband noise So do receiver noise and drift, atmospheric emission, ground pickup, confusion, etc. Single dishes can image large areas and low-brightness sources Single dishes can use array feeds, wide bandwidths, and incoherent detectors Astronomical Continuum Sources Synchrotron (dash-dot curve), free-free (dashes), and dust (dots) emission typical of spiral galaxies 408 MHz continuum emission, galactic coordinates Haslam et al. 1982, A&AS, 47, 1 (see Chris Salter) Orion Nebula HII region at 8.4 GHz 1

2 The Galactic dark cloud G in absorption at 8 microns (left) and in emission at 850 microns (right) SCUBA on the JCMT 850 micron SCUBA/JCMT image of the HDFN Dust emission at high redshifts Strong continuum point sources as observing tools A cross scan yields the pointing offset, beamwidth, and antenna temperature Sanity checks Pointing, focus, and surface setting Intensity and polarization calibration Offset pointing corrections 2

3 Distributions of normalized gain G resulting from Gaussian pointing errors with rms = ε HPBW Pointing errors: Setting Gravitational Thermal Refraction Differential expansion from solar heating bent the 300-foot telescope away from the Sun (near α = 13h 20m). This dense grid of pointing calibrators can be used to reduce tracking errors north of δ = -40. Axial Focus GBT focal ellipsoid axial FWHM 4λ Telescope setting by OOF holography Telescope surface-error maps: before: σ = 370 µm after: σ = 210µm 3

4 ν Flux-density and polarization calibrators: Baars, J. W. M., Genzel, R., Pauliny-Toth, I. I. K., & Witzel, A. 1977, A&A, 61, 99 Ott, M. et al. 1994, A&A, 284, ftp://ftp.atnf.csiro.au/pub/atnfdocs/guides/at.cat Single-dish position calibrators: Condon, J. J., & Yin, Q. F. 2001, PASP, 113, 362 ftp://ftp.cv.nrao.edu/nrao-staff/jcondon/pcals3.3 Single-dish continuum observations of faint and/or extended sources Thermal noise 1/f noise (receiver and atmosphere) Confusion Baselines RFI Radiometer equation for a real receiver Postdetection power spectrum showing 1/f noise at low frequencies and refrigerator microphonics near 1.2 Hz. k Use τ < 1/(2π ν k ): on-the-fly mapping, Dicke switching, Confusion Profile plot of 45 deg² near the NGP imaged with 12 arcmin resolution at 1.4 GHz. The strongest source shown has S 1.5 Jy. NVSS (45 arcsec FWHM) grayscale under GB (12 arcmin FWHM) contours illustrates source blending 4

5 The confusion amplitude P(D) distribution [for n(s) = ks -2.1 ] RMS confusion D = image brightness (e.g., Jy/beam) D = 0 mean is not a good baseline; use running median instead The 5σ extragalactic confusion limits for Arecibo (d = 220 m) and the GBT (d = 100 m). Baselines The total-power output from one receiver channel during a 4.85 GHz scan. The system temperature T 60 K includes receiver noise, atmospheric emission, spillover, leakage through the reflector mesh, etc. Running-median baselines have removed atmospheric emission, spillover, 1/f noise, and extended sources. Beam-switching The SNR 3C 10 imaged at 10.7 GHz by horizontal scans only (top) and by dual-beam basketweaving (bottom). Basketweaving references: Sieber et al. 1979, A&A, 74, 361 Emerson & Grave 1988, A&A, 190, 353 (see Chris Salter) 5

6 Future single-dish continuum projects Penn Array on GBT 64-element λ = 3 mm bolometer camera, 8 arcsec resolution can map 15 arcmin X 15 arcmin with σ = 0.2 mjy in 1 hour circumstellar disks, S-Z clusters, protogalaxies, ALFA on Arecibo 7-element λ = 21 cm coherent array, 3 arcmin resolution full-stokes large-area continuum survey Galactic loops, Faraday tomography, transient sources 6