Sky & Telescope March 1982 [antikvár]

Radío Maps of the SkyGlyn Haslam and Richard Wielebinski, Max Planck Institute for Radio Astronomy and Wolfgang Priester, Astronomical Institute of the University, Bonn, West GermanyMaps allow us to fmd our way around the Earth. They give us information about the continents and show us the positions of the highest mountains and the deepest seas. Detailed maps allow us to find roads, rivers, hills, valleys and places of interest.Sky maps have been made since prehistoric times. At first, using the unaided eye. the brightest stars were grouped into constellations. With the invention of the telescope the number of observable stars increased dramatically. The Bonner Durchmuste-rung, compiled by Friedrich Wilhelm Arge-lander more than a century ago, lists some 450.000 stars. The invention of the wide-field telescope allowed the accurate photography of large areas of the sky in one picture. The National Geographic Society-Palomar Observatory Sky Survey and its southern Science Research Council and European Southern Observatory counterparts give us a high-resolution view of the optical splendor of the heavens.Radio waves from our galaxy were detected by Karl Jansky only SO years ago while researching ways to provide better radiotelephone service. The receiving equipment was crude by present-day standards: his antenna revolved on wheels from a Model T Ford and the angular resolution was only about 30°. Still, this early result showed that the most intense radio waves came from the direction of the constellation Sagittarius, from what we now know to be the center of our galaxy.The first radio map of the northern sky was made by Grote Reber some 40 years ago. Using a homemade paraboloidal dish antenna, this pioneer of radio astronomy surveyed the sky seen from Wheaton, Illinois, with 7 resolution. His rough map of the radio intensity distribution showed a peak in the direction of the galactic center, intense emission along the galactic plane, and a secondary peak in the direction of Cygnus X (the tangential view along the local spiral arm).Radio astronomy developed rapidly after 1945. Surveys of the sky at numerous radio frequencies were made in Australia, England. and Holland. Angular resolution improved quickly with the construction of ever-larger telescopes, while sensitivity was also pushed to new frontiers by the intense development of electronic receiving devices and associated techniques. At first, data reduction posed a great problem. To make a map of the whole sky with satisfactory detail, a large number of observations must be taken, carefully calibrated, and transferred into a map grid in celestial coordinates. Thistime-consuming bottleneck was solved by computers and their application to contour mapping and picture processing.An all-sky radio map not only allows us to get an overall view of the Milky Way as seen from near the Sun but also provides a crucial tool in our attempts to understand the energy balance of the processes within our galaxy. The first all-sky map was made by Franz Dröge and Wolfgang Priester only 25 years ago. They used a simple dipole-array antenna near Kiel, West Germany, and surveyed the northern sky at 200 megahertz (MHz). The southern data was originally published by the Australian radio astronomers Clay Allen and Colin Gum. In this map, produced by many hours of careful hand analysis, the previously known pattern of radio distribution was enriched by the appearance of "spurs" radio contours starting from the galactic plane and following a small circle on the celestial sphere.The next combination of northern and southern data was made in 1970 by Thomas Landecker and Richard Wielebinski working in Australia. The 150-MHz southern survey made at Parkes was joined with observations made by Tony Turtle and John Baldwin in Cambridge, England, at 178 MHz. The angular resolution was 3.5 and many structural details away from the galactic plane could be seen. Careful calibration made it possible to use the 150-MHz all-sky map for many years as a standard for comparison with other work. This was also the first survey to employ digital computation methods for much of the data processing.An all-sky survey with an angular resolution of better than 1 ° requires a lot of careful planning. Radio telescopes, like old opticaltelescopes, are still instruments with a very limited field of view. One beam must scan the sky repeatedly, in raster fashion, to record a number of sweeps or scans from which a map can be made in much the same way that a television picture is formed. Since the received signals are very weak and the data is collected over many separate observing runs, there are problems with scan baselines, the joining of adjacent map fields, and absolute