Chris Peterson wrote:
Okay. I'll note there is a substantial difference between that clarification and your original statement, " I cannot convincingly detect a point source dimmer than vmag 15."
Quite right Chris. I now wish I had never mentioned my own case, as I am limited to subs of ~30s due to field rotation, due to my (poor) choice of alt-az mount for (not very) deep sky photography. Initially, I thought that by setting the ISO so high, I was bringing the image to the limit of sky glow faster. I now know (thanks mainly to the education you've given me over the last couple of months) that while I am detecting more objects this way, I am also increasing the readout noise and reducing the S/N ratio. I'm probably still a fair way from the limit of my sky glow.
My original point -- however poorly expressed originally -- was that very few amateur deep sky photographers, shooting through the atmosphere, seem to produce narrow-field
images, no doubt due to the extra time and money it requires. It is hard to compete with technology like the HST.
It would have been better if my original statement said something like "the vast majority of amateurs would struggle to produce a satisfactory narrow-field image of Hercules A (or more interesting targets comparable in apparent size and magnitude)". This is slightly different from Roger's implication (as I perceived it) that amateurs leave objects like this alone because they can only achieve wide-field images.
Chris Peterson wrote:
Aesthetically, we tend to like high surface brightness images, which means you'll need a longer exposure time. But the same information is present in the shorter exposure image made at a long focal length.
I take your point, but if my eyes can't detect an object in an image, I would argue (perhaps semantically) it is more than just a lack of aesthetics. I assume you mean that "the same information is present in the shorter exposure image made at a long focal length" [or short focal length].
My original point could just as easily have not been made, as it wasn't a particularly important point. But I'm glad I made it, and that I had the opportunity to clarify it, because it opened up a sideline of discussion from which I have learnt a few things. Many thanks.