Eric W. Greisen

Representations of celestial coordinates in FITS

In Paper I, Greisen & Calabretta (2002) describe a generalized method for assigning physical coordinates to FITS image pixels. This paper implements this method for all spherical map projections likely to be of interest in astronomy. The new methods encompass existing informal FITS spherical coordinate conventions and translations from them are described. Detailed examples of header interpretation and construction are given.

AIPS, the VLA, and the VLBA

At this writing, the AIPS package has been in active development and use for over 23 years. It is still the software of choice for all phases of data reduction for the Very Large Array, the most productive groundbased telescope in the world. It is the primary reduction system for most Very Long Baseline Interferometry including the VLBA and has been used to reduce data from other radio interferometers and single-dish telescopes as well as data taken at other wavelengths. The history and general structure of this software package are reviewed and a number of the scientific achievements for which it has been used are summarized.

Representations of world coordinates in FITS

The initial descriptions of the FITS format provided a simplified method for describing the physical coordinate values of the image pixels, but deliberately did not specify any of the detailed conventions required to convey the complexities of actual image coordinates. Building on conventions in wide use within astronomy, this paper proposes general extensions to the original methods for describing the world coordinates of FITS data. In subsequent papers, we apply these general conventions to the methods by which spherical coordinates may be projected onto a two-dimensional plane and to frequency/wavelength/velocity coordinates.

The On The Fly Imaging Technique

The On-The-Fly (OTF) imaging technique enables single-dish radio telescopes to construct images of small areas of the sky with greater efficiency and accuracy. This paper describes the practical application of the OTF imaging technique. By way of example the implementation of the OTF imaging technique at the NRAO 12 Meter Telescope is described. Specific requirements for data sampling, image formation, and Doppler correction are discussed.

Representations of Spectral Coordinates in FITS

Greisen & Calabretta (2002) describe a generalized method for specifying the coordinates of FITS data samples. Following that general method, Calabretta & Greisen (2002) describe detailed conventions for dening celestial coordinates as they are projected onto a two-dimensional plane. The present paper extends the discussion to the spectral coordinates of wavelength, frequency, and velocity. World coordinate functions are dened for spectral axes sampled linearly in wavelength, frequency, or velocity, linearly in the logarithm of wavelength or frequency, as projected by ideal dispersing elements, and as specied by a lookup table.

APERTURE SYNTHESIS OBSERVATIONS OF THE NEARBY SPIRAL NGC 6503: MODELING THE THIN AND THICK H I DISKS

We present sensitive aperture synthesis observations of the nearby, late-type spiral galaxy NGC 6503, and produce H I maps of considerably higher quality than previous observations by van Moorsel and Wells. We find that the velocity field, while remarkably regular, contains clear evidence for irregularities. The H I is distributed over an area much larger than the optical image of the galaxy, with spiral features in the outer parts and localized holes within the H I distribution. The absence of absorption toward the nearby quasar 1748+700 yields an upper limit of 5 × 1017 cm–2 for the column density of cold H I gas along a line of sight which should intersect the disk at a radius of 29 kpc. This suggests that the radial extent of the H I disk is not much larger than that which we trace in H I emission (23 kpc). The observed H I distribution is inconsistent with models of a single thin or thick disk. Instead, the data require a model containing a thin disk plus a thicker low column-density H I layer that rotates more slowly than the thin disk and that extends only to approximately the optical radius. This suggests that the presence of extraplanar gas in this galaxy is largely the result of star formation in the disk rather than cold gas accretion. Improved techniques for interferometric imaging including multi-scale Clean that were used in this work are also described.

VLA synthesis of H I absorption toward W43 (G30.8+0.0)

We used the VLA to synthesize the continuum structure of the main component of W43, a very large H II region located near the subcentral point at l=30.8°, b=0.00°, and we mapped the 21 cm H I absorption across it at resolution 12.5″ and 1.29 km s −1 . The H I occurs in features which strikingly show the influence of spiral structure on low-latitude galactic kinematics. The observations can be understood in terms of the two-arm spiral-shock model of Roberts & Burton [Topics in Interstellar Matter, edited by H. van Woerden (Reidel, Dordrecht), p. 195 (1977)], with the additional requirement that the mean H I spin temperature be a strong function of spiral phase

FITS: A Remarkable Achievement in Information Exchange

The FITS format is a remarkable achievement in information handling and sharing. Astronomy is alone among the sciences in having an international data interchange format that is used by virtually all scientists and institutions in the field. The technical and sociological reasons for this success are discussed and a few of the many remarkable scientific results made possible by this information handling are described.

The Nrao VLA Sky Survey: Lessons Applied

The NRAO VLA Sky Survey (NVSS) is a 1.4 GHz continuum survey covering the sky visible from the VLA with 217,446 partially overlapping “snapshots” made with the D and DnC configurations. About 98% of the sky with J2000 δ > -40° has been observed successfully, and we hope to fill the gaps with new observations at the end of 1997. The final data products will be (1) a set of 2326 4° × 4° continuum “cubes” with three planes containing Stokes I, Q, and U images plus (2) a catalog of almost 2 × 106 discrete sources brighter than 2.5 mJy per beam solid angle. Prior to this meeting, 1281 of the 2326 cubes had been produced and just over 106 radio sources cataloged. We expect to complete the rest during the summer of 1997 and make minor improvements thereafter. The images have θ = 45″ FWHM resolution and nearly uniform sensitivity—their rms brightness fluctuations are σ ≈ 0.45 mJy beam-1 ≈ 0.14 K (Stokes I) and σ ≈ 0.29 mJy beam-1 ≈ 0.09 K (Stokes Q and U). The rms uncertainties in right ascension and declination vary from < 1″ for the N ≈ 4 × 105 sources stronger than 15 mJy to 5″ at the survey limit.

DIVISION XII / COMMISSION 5 / WORKING GROUP FITS DATA FORMAT

The Working Group FITS (WG-FITS) is the international control authority for the Flexible Image Transport System (FITS) data format. The WG-FITS was formed in 1988 by a formal resolution of the IAU XX General Assembly in Baltimore (MD, USA), 1988, to maintain the existing FITS standards and to approve future extensions to FITS.