Relative Positions of Faint Stars from Sky Surveys

Vast amounts of untapped data on wide pairs lie in the various sky surveys taken with the world's largest Schmidt telescopes at ESO, Siding Spring and Palomar Mountain. What is more the data now encompass several wavelength bands and epochs. A determined individual, such as Domenico Gellera of Lodi, Italy, who has built and used his own measuring machine3 can make substantial contributions because many of the pairs on these charts are not only unmeasured but uncatalogued. Sr Gellera has shown that it is possible to measure pairs as close as 5'' using a microscope fixed to a two-axis measuring machine. He has made over a thousand measures of the pairs of Pourteau and in most cases these are the first and only measures since the original catalogue was compiled from Astrographic zone plates.4, 5 This work was done from photographic prints of the Palomar Schmidt survey and a single print typically contains hundreds of pairs. In collaboration with Willem Luyten he used his measuring machine to measure the relative positions of pairs of white dwarfs.6

It is not even necessary to have a measuring machine to extract data from the Sky Surveys. The USNO have created a number of large catalogues the biggest of which (the A2.0 catalogue) is the result of scanning Schmidt plates using the PMM machine at Flagstaff Station in Arizona. The result is a catalogue with 526 million stars down to magnitude 19 or so and distributed on 10 CD-ROMs. A smaller alternative is the SA2.0, with 55 million stars now only available by ftp from the USNO site.7 UCAC1 is a more recent and more accurate catalogue based on Tycho-2 and USNA2.0 which contains 27 million stars between magnitudes 8 and 16 in the southern hemisphere. Pairs and multiple stars closer than 3'' are not listed. It is not quite complete covering about 80% of the southern sky. The UCAC project is continuing with the astro-graph used being relocated in the northern hemisphere. The sky has now been observed as far north as +45° and the results will appear in UCAC2 in 2003 or so. UCAC1 gives positions good to 0.02'' between magnitudes 9 and 14 and 0.07'' at magnitude 16. The mean epoch is between 1998.0 and 1999.9. The data is made available in a form suitable for Unix/Linux, MAC or MS Windows.

An alternative is to use facilities such as ALADIN on the SIMBAD website. A description on how to use this facility is given by West.8

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