Well, here's my take on it:
Newtonians CAN deliver extremely good performance, but you can't buy an off the shelf unit that does (to my knowledge). All the economy ones have serious design flaws that limit their performance, most notably in thermal issues. No fans, too tight tubes, thick mirrors, etc. Equatorial mounted ones have no rotating rings and undersized mounts, too tall piers or tripods, etc.
On the other hand, off the shelf apochromats often deliver close to their theoretical performance right out of the box. This creates a real-world situation in which a 4" ED is often very close in actual planetary performance to a 8" newtonian and often beat a 6" quite clearly, at least in the first couple of hours of observing and most people often only have a few hours early in the evening and have no option for letting the reflector cool down first. Under these circumstances, a newtonian is at a severe disadvantage to a refractor, despite the theoretically better performance.
If the observer is not aware of the many potential issues of the newtonian, and let's be honest here, very few observers are, then the newtonian is very often not performing as well as it can, while the small apochromat nearby is running at close to full potential. I've seen this in practice a lot at star parties and during my own comparisons.
An example: I have a 6" f/8 Sky-Watcher newtonian with a 1.25" secondary. It has very good optics. Not perfect, but very good. But the tube is too narrow, 7" in diameter, and too short. It's also metal. And black. It rapidly undercools and this creates a lot of tube currents. The tube is too short, so that warm air from my breath flows in through the light path. Wrapping it in a blanket and extending the tube with a SCT dewcap helps tremendously, but not completely. The cell is too tight, so there's no room for air to move around it and adding a fan to the rear of the scope has no benefits.
Once it is cooled and properly insulated, the optics shine and it outperforms my 85mm Zeiss very easily. But the 85mm has shown better images for at least the first hour and a half by that time and for many people, depending on the time of the year, by then it's often time to go to bed.
So, does the situation HAVE to be this way? No, but if I want to change it, I *must* build my own scope, since there's NO ONE building a properly designed and manufactured, off the shelf 6" f/8 newtonian for a reasonable amount of money. And building scopes is not for everyone. Most don't even have the option.
In my opinion, the newtonian is a completely misunderstood scope. People want it to be cheap, because it can be, so they demand the lowest cost possible and buy very cheaply made scopes, but the design actually have so many potential issues, that you HAVE to spend some real money and attention to it, to get a properly working scope, at least when we're talking planetary observing. And when you take the time to adress the issues, you suddenly end up with a scope that is not so cheap any more.
It really is strange. People spend lots of money on refractors and maksutovs. Those are considered high-end designs. The newtonian is capable of running neck to neck with them, but is not considered a high-end design, so people don't want to spend money on it.
Clear skies!
Thomas, Denmark
