A PSF-based approach to Kepler/K2 data - I. Variability within the K2 Campaign 0 star clusters M 35 and NGC 2158

Kepler and K2 data analysis reported in the literature is mostly based on aperture photometry. Because of Kepler's large, undersampled pixels and the presence of nearby sources, aperture photometry is not always the ideal way to obtain high-precision photometry, and, because of this, the data set has not been fully exploited so far. We present a new method that builds on our experience with undersampled HST images. The method involves a point-spread function (PSF) neighbour-subtraction and was specifically developed to exploit the huge potential offered by the K2 'super-stamps' covering the core of dense star clusters. Our test-bed targets were the NGC 2158 and M 35 regions observed during the K2 Campaign 0. We present our PSF modelling and demonstrate that, by using a high-angular-resolution input star list from the Asiago Schmidt telescope as the basis for PSF neighbour subtraction, we are able to reach magnitudes as faint as K-P similar or equal to 24 with a photometric precision of 10 per cent over 6.5 h, even in the densest regions. At the bright end, our photometric precision reaches similar to 30 parts per million. Our method leads to a considerable level of improvement at the faint magnitudes (KP greater than or similar to 15.5) with respect to the classical aperture photometry. This improvement is more significant in crowded regions. We also extracted raw light curves of similar to 60 000 stars and detrended them for systematic effects induced by spacecraft motion and other artefacts that harms K2 photometric precision. We present a list of 2133 variables.