Quantitative Method for the Optimal Subtraction of Continuum Emission
  from Narrow-band Images : Skewness Transition Analysis
  We present an objective method to remove the stellar continuum emission from
narrow-band images to derive emission-line images. The method is based on the
skewness of the pixel histogram of the residual images. Specifically, we
exploit a transition in the skewness of the signal in the continuum-subtracted
image, which appears when the image changes from being under-subtracted to
over-subtracted. Tests on one-dimensional artificial images demonstrate that
the transition identifies the optimal scaling factor {\mu} to be used on the
broad-band image IB in order to produce the optimal line-emission image IE,
i.e., IE =IN - {\mu} IB, with IN the original (un-subtracted) narrow-band
image. The advantage of this method is that it uses all information-bearing
pixels in the final image, and not just a sub-set of those pixels (the latter
being common in many traditional approaches to stellar continuum removal from
narrow-band images). We apply our method to actual images, both from
ground-based and space facilities, in particular to WFPC2 and ACS images from
the Hubble Space Telescope, and we show that it is successful irrespective of
the nature of the sources (point-like or extended). We also discuss the impact
on the accuracy of the method of non-optimal images, such as those containing
saturated sources or non-uniform background, and present `workarounds' for
those problems.
