I think that, quite literally, the simplest definition is “the photography of objects residing beyond Earth’s atmosphere”, a wide enough definition that includes artificial satellites like the ISS.
Hence, if we ask what Deep Space Objects (DSO) astrophotography is, the simplest and most comprehensive answer is “the photography of objects in space placed outside of the Solar System”.
In neither of these definitions the words “equatorial mount” or “autoguide” are present.
But, usually, on forums or when chatting with seasoned astroimagers, altazimuth astrophotography is never considered a “proper” form of astrophotography, at least as far as DSO astrophotography is involved.
Well, certainly there remains a certain prejudice, due to the novelty of the technique and to the long established lore that “proper” DSO astrophotography requires an equatorial mount and also (essentially, if not necessarily) autoguiding.
Now, I agree that because of field rotation, exposures with an altazimuth mount are essentially way shorter than those achievable with an equatorial mount, particularly when autoguiding is employed.
However, we have already seen that the difference between long and short exposures is minimal for light polluted skies and we also know that an altazimuth setup is both simpler and cheaper than one with an EQ mount plus autoguide.
A vulgarization of (indeed rather an analogy to) the Pareto Principle states that 80% of the result can be obtained with 20% of the effort.
Can we say that (at least from a urban o suburban site) we can take pictures with 80% of the quality and at 20% of the cost of those obtained with a full classical EQ+autoguide setup?
To be able to compare red and green apples (so to speak) I would need to put one of those classical setups on my balcony, under my sky and take pictures, with the same optics and cameras I use on my altaz mount, and with the same total integration time, and then make comparisons.
One day I will perhaps be able to do the test with the help of some “classical” astroimagers, but in the meantime I can guess that if it is not 80/20, it is anyway 70/30.
Besides, one should have an agreed upon definition of “quality” to properly make quantifiable comparisons.
And there comes my second, but perhaps most important, point.
Taking out of the picture (some pun intended) those who do astrometry or other types of amateur research, amateur astrophotography is essentially a matter of aesthetics and certainly aesthetics is subjective.
For instance, it happens to me to see on Astrobin and elsewhere pictures with pitch black skies and bright, glossy galaxies superposed so neatly on them, as to look like the product of some cut and paste work.
Do not take me wrong: I do not imply any wrongdoing, I am absolutely sure nobody is cutting and pasting someone else’s material.
What I mean is simply that some may like that kind of images, some don’t, exactly like some may appreciate a manifest aesthetical surgery and some may not.
I personally think that some noise contributes to make the image more “natural” and “real”.
But, again, it is a matter of points of view.
At the same time all this concentration on the aesthetics of DSO images makes me wonder what in the end astrophotography is, or should or may be all about.
Honestly, is it all just about having pinpoint stars all the way to the corners of the field?
I have realized myself the importance of imaging repeatedly the same subject multiple times through time, so that one can verify the improvement obtained through equipment and technique, but at the same time I wonder if the end of it all is just that of obtaining the “perfect” picture (whatever it may be).
Well, I have come to the conclusion that, at least to me and in my experience, DSO astrophotography is at least as much also a way to “see” what I cannot see visually, because of the site I stargaze from, because of the limits of human eyesight and so on.
When you think of it in this way, my view of astrophotography is more akin to Electronically Enhanced Astronomy (EAA), and indeed I think altazimuth DSO imaging can well be seen as something in between “classical” astrophotography and EAA.
I have practiced some EAA myself, and albeit I find it both interesting and instructive, I can say that the images I obtain with my equipment after properly stacking and post-processing them, are quite above the results allowed by pure EAA: quite simply you see more in the pictures.
So what is my astrophotography?
In aims it owes more to EAA than to classical astrophotography: to see more than you can see visually, without obsessing oneself with aesthetics, and it also uses a simpler and cheaper instrumentation than that used by mainstream DSO imagers.
On the other end, the time required to collect lightframes, stack them and post-process them, is much closer to, if not coincident with, that required by “orthodox” astrophotography, and hence much longer than the more or less “realtime” pace of EAA.
Hence, ultimately, I see altazimuth astrophotography, at least in the way I practice it, as a sort of bridge between EAA and conventional astrophotography, mixing the aims and most of the simplicity of the first with most of the techniques and procedures of the second.
“In medio stat virtus”, virtue stands in the middle, or at least so goes the ancient saying.
I do not think however that this concept (my vision) of altazimuth astrophotography has any superior virtue, but I surmise it can pretty well complement the others and may enrich the experience of amateur astronomers who want to try their hand at DSO imaging.