CCD MOSAIC IMAGES OF THE SUPERNOVA REMNANT 3C 400.2

We have constructed CCD mosaic images of the old Galactic supernova remnant 3C 400.2 in tines of Halpha + [N II], [S II], and [O III], plus a continuum band. These are the first CCD images covering the full extent of this remnant, and they reveal significantly more nebulosity than the deepest photographic plates. Comparison with radio and X-ray images indicates dramatically different morphology in the three regimes. The optical images both in Halpha + [N II] and in [S II] show an almost complete, irregular shell of emission, with a diameter of about 16', little over half that of the radio shell, while the X-ray structure is a centrally peaked ellipsoid. Underestimates of the true diameters of SNRs through optical measurements could be partially responsible for an excess of small-diameter remnants in extragalactic samples. Approximate values for optical line fluxes are obtained; we estimate L(Halpha) almost-equal-to 3 x 10(35) ergs s-1, roughly 3 times the X-ray luminosity. We also report a previously uncataloged planetary nebula southwest of 3C 400.2. The imaging data were obtained with a TI CCD on the 0.6 m Burrell Schmidt at KPNO. This combination provides a wide (19') field and excellent sensitivity to diffuse emission, while the small aperture ''suppresses'' stars in the crowded Galactic field. With larger chips and mosaic techniques, CCD imaging of virtually all Galactic supernova remnants will become practicable for the first time.