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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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