Bill,

I forgot:

(a) to thank you for the suggestion of deletion, which worked.

(b) to mention that I had several zero-sized aperture circles in line 26
-- probably a consequence of mucking about and forgetting to enter an
aperture size! I've tried to replicate the error by introducing
zero-sized circles, but to no avail, but it may give you a lead.
--
Stephen Tonkin