Measurements of visual double stars with PISCO2 at the Nice 76-cm refractor in 2016-2017

We present relative astrometric and photometric measurements of visual double stars made in 2016-2017, with PISCO2 installed at the 76-cm refractor of Cote d'Azur Observatory in Nice (France). Our observing list contains orbital couples as well as double stars whose motion is still uncertain. Three different techniques were used for obtaining measurements: Lucky Imaging, Speckle Interferometry and the direct vector autocorrelation (DVA) method. From our observations of 1,510 multiple stars, we obtained 2,918 new measurements with angular separations in the range 0 ''.1-20 '' and an average accuracy of 0.019. The mean error on the position angles is 1 degrees.1. Most of the position angles were determined without the usual 180 degrees ambiguity with the application of the DVA technique and/or by inspection of the Lucky images or the long integration files. We managed to routinely monitor faint systems (m(V) approximate to 9 - 12) with large magnitude differences (up to Delta(V) approximate to 4.5). We have thus been able to measure six systems containing red dwarf stars that had been poorly monitored since their discovery, from which we estimated the stellar masses thanks to Gaia measurements. We also measured the magnitude difference of the two components of 593 double stars with a mean error of 0.1 mag. Thanks to good seeing images and with the use of high-contrast numerical filters, we have also been able to obtain 46 measurements with an angular separation smaller than the diffraction limit of our instrumentation. Except for a few objects that have been discussed, our measurements are in good agreement with the ephemerides computed with published orbital elements, even for the double stars whose separation is smaller than the diffraction limit. We also report the measurements of 205 new double stars that we found in the files obtained during the observations.