  Light echoes are interesting because of the wealth of information they offer
about their progenitors and the reflecting dust. Due to their faint surface
brightnesses, difference imaging is necessary to separate most light echoes
from the sky background. However, difference images reveal only the relative
fluxes between two epochs. Obtaining absolute fluxes for individual epochs has
traditionally relied on a single template image that is free of light echoes.
Since such an observation is normally unavailable, a light echo-free template
must be constructed by a complicated and usually subjective process. Here we
present an application of the NN2 method of Barris et al. to extract the
relative fluxes of light echoes across a range of epochs directly from a series
of difference images. This method requires no privileged image and makes
maximal use of the observational data. Statistical methods to estimate the
zero-flux level and thus the absolute flux are also presented. The efficacy of
the technique is demonstrated by an application to the light echoes around SN
1987A. The resulting images reveal new detail and faint light echo structures.
This method can be adapted and applied to other extended variable light
sources, such as stellar outflows and supernova remnants.
