The large, faint, generally circular, and limb-brightened nebular structures (called "halos") surrounding some planetary nebulae (PN) are explored using deep CCD images of NGC 40, 650-1, 1535, 2392, 6210, 6543, 6720, 6803, 6804, 6826, 6853, 6891, 6894, 7009, 7662, IC 1454, 3568, 4593, Abell 1, 2, 3, and BD +30 deg 3639. New halos have been discovered in a few objects (IC 1454, 4593, and possibly NGC 40, 6210, and 6803), and known halos have been mapped in detail in several PN (e.g., NGC 6543, 6720, 6826, 6853 and 7662). The present deep search does not reveal similar large and faint halos in NGC 1535, 2392, 6894, 7009, and IC 3568-PN whose inner regions are morphologically similar to others with easily observable halos.
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Balick et al. (1992) studied this question.