Re: (IAAC) limiting magnitude
I liked your idea of condensation factors for galaxies, Jeff. And in fact, some
of that is encapsulated in Dreyer's "NGC" notation: "vsmbM" (very suddenly much
brighter to the Middle) comes close to "condensation 1" on your scale, maybe.
Unfortunately, I don't know if this idea would work as well for other objects,
e.g., diffuse nebulae with both emission and reflection components.
> IC 2955 Mean Surface Brightness 12.59 mag/sq arc min
> NGC 3862 Mean Surface Brightness 14.53 mag/sq arc min (in B)
Jeff brings up yet another twist to the lemony question of extended object magnitudes: surface brightness... This is another metric which CAN be very useful, but also sometimes misleading. To get an idea what I mean, try using Jeff Bondono's 'dObjects' software to sort all Messier objects by surface brightness: the monster galaxy M31 comes out near the bottom at 101st, with a surface brightness of 13.4. Above it on the list: such fainties as M108, M99, and M51. Meanwhile, much higher on the list (above 60th) we find unremarkable (or remarkable but faint) objects like M20, M43, M23, and M76.
Again your mileage may vary... And even what other amateurs record may not provide an infallible guide to what YOU can see - but it'll probably get you closer than most commonly available brightness measures out there.
PS: I really enjoyed the series of Webb Society Quarterly Journal articles (I believe there were some by Brian Skiff and others in the recent past, as well as the intriguing one by Jose Torres). There were also a series of articles on the same subject by Brian and others in the venerable old "Deep Sky" magazine. Would anyone care to summarize the views in the WSQJ and/or DS articles for our more quantitatively-minded subscribers?
Clear skies to all!