  Deep optical emission-line images are presented for nine known plus three new
Galactic supernova remnants (SNRs), all but one having at least one angular
dimension greater than one degree. Wide-field images taken in H$\alpha$ and [O
III] $\lambda$5007 reveal many new and surprising remnant structures including
large remnant shock extensions and `breakout' features not seen in published
optical or radio data. These images represent over 12,000 individual images
totaling more than 1000 hours of exposure time taken over the last two years
mainly using small aperture telescopes which detected fainter nebular line
emissions than published emission-line images. During the course of this
imaging program, we discovered three new SNRs, namely G107.5-5.1 (the Nereides
Nebula), G209.9-8.2, and G210.5+1.3, two of which have diameters >1.5 degrees.
Besides offering greater structural detail on the nine already known SNRs, a
key finding of this study is the importance of [O III] emission-line imaging
for mapping the complete shock emissions of Galactic SNRs.
