Speckle interferometry at the OAN-SPM México: astrometry of double stars measured in 2011

We present speckle interferometric measurements of binary stars performed during 2011 February and April with the 1.5-m telescope and during 2011 July and November with the 2.1-m telescope of the Observatorio Astronomico Nacional, San Pedro Martir, Mexico, focusing on objects from the Washington Double Star Catalog with separations less than 1 arcsec. Among these objects, we have been interested in performing a follow-up observation of new double stars discovered by Hipparcos. For these observations, we developed a new detector, which is a combination of CCD Watec 120N with a third generation image intensifier. This image intensifier allows us to perform near-infrared speckle interferometric observations for the first time. In this paper, we report 761 astrometric measurements of 478 pairs, with angular separations ranging from 0.09 to 2.61 arcsec. We found that 722 of our measured separations are smaller than 1 arcsec. We estimated a mean error in separation of 16 mas and 1.29 degrees in position angle. In order to overcome the usual 180 degrees ambiguity inherent to speckle measurements, we created a shift-and-add reconstructed image of each source, to establish the true quadrant of the secondary star. We confirmed 40 double stars discovered by Hipparcos and found 4 field stars resolved as interferometric pairs for the first time, with separations smaller than 0.60 arcsec.