| Abstract
| - Deep CCD exposures of the peculiar supernova remnant CTB 80 in the light of the H $\alpha+[$N ii ], [S ii ], [O ii ], and [O iii ] filters have been obtained. These images reveal significant shock heated emission in the area of the remnant. An extended bright diffuse nebula in the south-east part of CTB 80 overlaps soft X-ray emission from ROSAT but it does not appear to be related to the remnant under study. New diffuse and filamentary structures are detected to the south, south-east, and north of PSR 1951+32, most likely associated with CTB 80. Especially, the sulfur line image shows emission in the north along the outer boundary of the IRAS and HI shells. The [O iii ] emission is filamentary, suggesting shock velocities greater than 100 km s -1 but its spatial extent is quite limited. Lower shock velocities are expected in the north and north-east areas of the remnant, since [O ii ] emission is present, while [O iii ] line emission is not detected. The comparison between the [O iii ] and [O ii ] line images further suggest the presence of significant inhomogeneities in the interstellar medium. The flux-calibrated images do not indicate the presence of incomplete recombination zones, and we estimate that the densities of the preshock clouds should not exceed a few atoms per cm 3. The typical projected angular widths of the observed filaments are ~30´´. Typical surface brightness values of the long [O iii ] filament in the south are ~12 10 -17 erg s -1 cm -2 arcsec -2 while the [O ii ] image is characterized by fluxes of 10-20 10 -17 erg s -1 cm -2 arcsec -2. The area covered by the optical radiation along with the radio emission at 1410 MHz suggest that CTB 80 occupies a larger angular extent than was previously known.
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