I measured about 9.60 inches diameter with a certainty of + 0.005 inches on my 1100 mount using a spring caliper. If you need closer than that (to the nearest 0.001") I can make that measurement with my vernier calipers the next time I take my mount down. Probably in another week or so.
Linwood's measurement of about 9 inches makes me wonder if Astro-Physics has changed this dimension.
