To me one of the best sources for sky images. SkyView is a Virtual Observatory on the Net generating images of any part of the sky at wavelengths in all regimes from Radio to Gamma-Ray.
The PPM can be considered a replacement of two preceding catalogs: AGK3 and the SAO Catalog. The PPM Star Catalog effectively replaced these catalogs by providing more precise astrometric data for more stars on the J2000/FK5 coordinate system.
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