  We aim to construct an exceptionally deep (V ~< 27) catalog of variable
objects in selected Galactic and extragalactic fields visited multiple times by
the Hubble Space Telescope (HST). While HST observations of some of these
fields were searched for specific types of variables before (most notably, the
extragalactic Cepheids), we attempt a systematic study of the population of
variable objects of all types at the magnitude range not easily accessible with
ground-based telescopes. The variability timescales that can be probed range
from hours to years depending on how often a particular field has been visited.
For source extraction and cross-matching of sources between visits we rely on
the Hubble Source Catalog which includes 10^7 objects detected with WFPC2, ACS,
and WFC3 HST instruments. The lightcurves extracted from the HSC are corrected
for systematic effects by applying local zero-point corrections and are
screened for bad measurements. For each lightcurve we compute variability
indices sensitive to a broad range of variability types. The indices
characterize the overall lightcurve scatter and smoothness. Candidate variables
are selected as having variability index values significantly higher than
expected for objects of similar brightness in the given set of observations.
The Hubble Catalog of Variables will be released in 2018.
