Solar Stereoscopy with STEREO/EUVI A and B spacecraft from small (6 deg)   to large (170 deg) spacecraft separation angles
  We performed for the first time stereoscopic triangulation of coronal loops in active regions over the entire range of spacecraft separation angles ($\alpha_{sep}\approx 6^\circ, 43^\circ, 89^\circ, 127^\circ$, and $170^\circ$). The accuracy of stereoscopic correlation depends mostly on the viewing angle with respect to the solar surface for each spacecraft, which affects the stereoscopic correspondence identification of loops in image pairs. From a simple theoretical model we predict an optimum range of $\alpha_{sep} \approx 22^\circ-125^\circ$, which is also experimentally confirmed. The best accuracy is generally obtained when an active region passes the central meridian (viewed from Earth), which yields a symmetric view for both STEREO spacecraft and causes minimum horizontal foreshortening. For the extended angular range of $\alpha_{sep}\approx 6^\circ-127^{\circ}$ we find a mean 3D misalignment angle of $\mu_{PF} \approx 21^\circ-39^\circ$ of stereoscopically triangulated loops with magnetic potential field models, and $\mu_{FFF} \approx 15^\circ-21^\circ$ for a force-free field model, which is partly caused by stereoscopic uncertainties $\mu_{SE} \approx 9^\circ$. We predict optimum conditions for solar stereoscopy during the time intervals of 2012--2014, 2016--2017, and 2021--2023. 
Markus J. Aschwanden, Jean-Pierre W\"ulser, Nariaki Nitta, and James   Lemen
astro-ph.SR