1. The Southern Sky as seen with the Double Astrograph photography in the SPM4 catalog. The dark band in the image is the Milky Way disk, while the two small dark spots at the bottom are the Large and Small Magellanic Clouds. Understanding our Galaxy The structure of our Milky Way Galaxy has been the main research objective of the Yale Southern Observatory (YSO) since the founding of the Cesco Observatory in 1962. Nearly five decades have been devoted to improving and renovating the telescope, photographing the sky, collecting and analyzing the data. We are now seeing the fruits of our labors with the publication of the Yale-San Juan Southern Proper Motion catalog (SPM4) of 103 million absolute proper motions of stars and galaxies in the Southern Sky. The SPM4 is recognized as the premier astrometric survey dedicated to the study of faint stars in our Galaxy and it will remain so until the results from the next generation of space satellites are available in approximately 2023. From Telescope to Catalog The SPM4 catalog is based on observations made with the YSO Double Astrograph, the principal telescope of Cesco Observatory. The astrograph consists of two lenses, each 51-cm in diameter; one designed to image blue light and the other yellow light. From 1965 the lenses focused light separately onto two photographic plates with dimensions of 43 x 43 cm, and since 2000 onto two CCD cameras. The photographic plates were purchased in the US, shipped to El Leoncito, where they were exposed in the telescope, developed and then shipped back to the US for measurement with the precision measuring machines at Yale and at the US Naval Observatory. The CCD data were also analyzed at Yale, and combined with the photographic measures to produce the catalog. The 51-cm Double Astrograph (below); Yale astronomer, Dr. Terry Girard (top); and YSO President Dr. William van Altena (lower right) during the installation of the CCD cameras in 2000. UNSJ and OAFA Argentine astronomer Lic. Carlos López with YSO Chilean observer Danilo Castillo and Venezuelan astronomer and SPM collaborator Dr. Katherine Vieira. SPM Future After upgrades to the astrograph’s camera system in early 2011, SPM observations continued with the objective of extending the sky coverage of the SPM4 catalog as well as improving its precision for targets of special interest. The broad goals of the SPM are to improve our understanding of the kinematical and spatial structure of our Milky Way Galaxy through observations made with the Double Astrograph. Yale-UNSJ Collaboration The SPM4 is the result of a 49-year collaboration between the YSO and the Universidad Nacional de San Juan (UNSJ) – the longest agreement between universities in Argentina and the United States. More than 50 scientific, technical, administrative and support personnel from both countries as well as others have participted in the research. The YSO has obtained $18.2 million dollars (2011 dollars) in grant funds from various United States agencies, including the NSF, NASA, Yale University, Columbia University and Ford Foundation to build and operate the Cesco Observatory and to fund the SPM program. Likewise, the UNSJ has contributed to the SPM through the provision of scientific, technical and operations personnel as well as funds to help support the Double Astrograph and the Cesco Observatory. To date, more than 100 research papers documenting the results of the SPM have been published in international research journals during the course of the program. The SPM Program and the Yale Southern Observatory Yale astronomer, Dr. Dana Casetti prepares one of the SPM photographic plates for measurement using the Yale PDS microdensitometer.