  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.
