Hubble Space Telescope Expansion Parallaxes of the Planetary Nebulae NGC   6578, NGC 6884, NGC 6891, and IC 2448
  We have combined two epochs of Hubble Space Telescope WFPC2 imaging data with ground-based expansion velocities to determine distances to three planetary nebulae (NGC 6578, NGC 6884, and IC 2448). We used two variants of the expansion parallax technique--a gradient method and a magnification method--to determine the distances. The results from the two methods agree to within the errors. A fourth nebula was included in the study (NGC 6891), but the expansion was too small to determine the distance, and only a lower limit was obtained. This is the first paper in a series which will examine at least 24 nebulae in total. 
Stacy Palen, Bruce Balick, Arsen R. Hajian, Yervant Terzian, Howard   Bond, Nino Panagia