Table 3
Diameters along the minor axes of our sample objects.
| Object | Lines | Diameters | Comments | ||
|
|
|||||
| Rim | Shell | Frew* | |||
| [′′] | [′′] | [′′] | |||
|
|
|||||
| BD +30°3639 | Hα or [N ii] | 5.1 | 5.6 | Shell edge not visible | |
| IC 418 | [O iii] | 3.9 | 11.9 | 11 | Probably ionization front at shell’s edge |
| Hα and [N ii] | 12.9 | 11 | |||
| IC 2448 | Hα and [O iii] | 8.3 | 19.0 | 22 | [N ii] very faint |
| IC 4593 | Hα and [O iii] | 5.1 | 12.9 | 15 | SW edge of shell. ambiguous structure |
| NGC 2392 | Hα and [O iii] | 16.0 | 48 ± 2 | 44 | Edge of shell is amorphous |
| NGC 3132 | Hα and [O iii] | 36.2 | 51 ± 2 | 55 | Complex shell |
| NGC 3242 | Hα, [O iii], [N ii] | 17.8 | 39 ± 2 | 39 | Shell slightly larger than the CCD |
| NGC 3918 | Hα and [O iii] | 11.7 | 17 ± 3 | 17 | Complex shell |
| NGC 5882 | Hα and [O iii] | 8.5 | 12.8 | 13 | |
| NGC 6210 | Hα and [O iii] | 14.0 | 14 | Complex, filamentary rim | |
| NGC 6543 | Hα and [O iii] | 8.4 | 16.9 | 24 | Shell very irregular |
| NGC 6565 | Hα and [O iii] | 8.3 | 10.5 | No shell; the rim is probably a recomb.-reioniz. front, | |
| [N ii] | 9.4 | like IC 418; the [N ii] diameter is 13% larger | |||
| NGC 6578 | Hα and [O iii] | 6.3 | 12.1 | 11.8 | |
| NGC 6826 | Hα, [O iii], [N ii] | 9.2 | 25.1 | 24 | |
| NGC 6886 | Hα and [O iii] | 4.4 | 4.5 | No shell, probably recombined; bipolar along major axis | |
| NGC 6891 | Hα and [O iii] | 5.4 | 16.9 | 12.7 | Shell edge faint and asymmetrically smeared by CTE (Sect. 2) |
| NGC 7009 | Hα and [O iii] | 11.2 | 23.0 | 22 | Approximately bipolar along major axis |
| NGC 7027 | Hα and [N ii] | 8.4 | 12.4 | Ionized shell not indentifiable | |
| [O iii] | 7.9 | [O iii] clearly narrower; | |||
| Probably recomb. Front at the outer edge of the cavity | |||||
| NGC 7662 | Hα and [O iii] | 13.3 ± 1 | 26.8 | 28 | NW edge of rim faint, poorly defined |
Notes. Diameters measured from the 2007/8 images, with typical error of 
if not otherwise noted.
The Frew diameters (Frew 2008, his Table 7.1) refer usually to the shell. Only if no shell is observed, they are for the rim. The listed Frew radii from his Table 7.1 are measured at 10% peak brightness and pertain to the minor axis. In contrast, our radii are determined by sharp brightness gradients at the edge of a feature.
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