The Elusive Implication of Surface Brightness

I saw the following question on CloudyNights.com regarding the visibility of extended objects and surface brightness:

"What I am interested in is finding some way to calibrate surface brightness to the chances of seeing the object in my scope."

This seems to be a popular feature for planning software. I find it a fascinating topic and I have studied it myself. My general feeling is that the ideas described in Roger Clark's Visual Astronomy of the Deep Sky help to address the question, but these alone are insufficient.

I was delighted to see this response to the question above from Sky & Telescope's Tony Flanders:

"Nobody can give you a formula that will accurately predict an object's visibility -- thank heavens!"

He also explains:

"Another way of looking at it is that the aperture of your telescope places a limit on the total magnitude of the objects you can see. And the quality of your skies places a limit on the surface brightness of the objects you can see."

I like Flanders' analogy. Clark's ideas describe the importance of the difference in contrast between the sky and the extended object. My own study has also identified several additional metrics: sky conditions, weather, personal visual acuity and sensitivity. It is indeed hard to fit all of these elements into a mathematical model that accurately predicts whether an object can be observed.

A visibility prediction feature will find its way into Deep-Sky Planner, though I am still studying the problem now. When it is released, I will state plainly that the prediction is only a model that invites further improvement over time. I truly doubt that a completely accurate model will ever be devised, so an adaptable model is probably the best we can do. That is the current requirement in the Deep-Sky Planner development plan.