Yes, I read that, but I disagree with the author's understanding of what is and isn't visible for a particular aperture.
What this gets to is an understanding of how to create a "visibility index" of what is visible and what is not visible in a particular aperture. This has been discussed extensively over the years by many people here on Cloudy Nights. And the conclusions we came to was that it was nearly impossible to do so. A ranking by total integrated magnitude didn't make sense because some faint objects ranked higher than some bright ones. A ranking by surface brightness didn't make sense because it favored small objects over large ones even if the large one was more visible (or even visible to the naked eye!).
So how did the authors decide what was visible in a particular aperture? By viewing all 15,000 objects? I doubt it.
Here is my discussion of how ranking objects by visibility fails:
(see Extended Information" for the article).
So it is my guess they used integrated magnitude to decide what was and wasn't visible in a particular aperture, and this doesn't even come close for galaxies alone and works very poorly for any form of extended nebula.
Last is the understanding of just exactly what is the limit of a particular aperture.
For instance, for stars, a 4" under absolutely perfect conditions may reach magnitude 15.0. Under bad conditions, as low as magnitude 10.5. Needless to say, the deep sky objects that can be viewed in the first set of conditions will be radically different than in the second set of conditions.
The author of the atlas used a rule to decide what was and wasn't going to be visible in a particular aperture of scope and, like the authors of the Millenium Sky Atlas, chose to be quite conservative. I would argue that a star atlas for visual use should contain all the objects possibly visible in perfect conditions
for whatever aperture at which the atlas is aimed.
In my experience in the field over 50 years of observing, I think the authors erred on the side of conservatism in their choice of objects.
Uranometria has >31,000 objects shown on the charts and I am continually discovering that objects not on the charts are visible in my 12.5". When I had an 8", I noticed the same thing when using other atlases. When I got rid of my 8", I had 9300 objects in my log and had barely scratched the surface of the UMA-CVN-CBR-VIR area. I also noticed that a lot of objects were visible in the 8" that shouldn't have been, based on magnitude. So I stopped paying attention to visibility estimates and assumed I could see an object unless proven otherwise.
There isn't a problem with having an object listed that isn't visible in a particular aperture in poor conditions. It just pushes the observer to look for the object when conditions are better or when they have more experience in viewing objects at the limit. It's very upsetting to an observer to continually see objects that aren't in his charts, however. Then, tracking down the identification of the object becomes a chore--a pleasant chore, perhaps, but a chore nonetheless. Better to show more objects than can be seen at first, but which can be ferreted out by observing longer or returning to the site under better conditions.
Hence, I think this will be a great atlas for the 4" to 8" scope owner. For the 12" and larger scope, though? Not so great, in my opinion.