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When it comes to using APO refractors for visual (imaging is a different situation) they really fall into two classes (and I have one of each):
- small refractors up to about 4" either on alt-az or light EQ mounts, they are very portable and easy to set up, no collimation hassles, cool very quickly and give the best views of any scope of their aperature. You get pleasing views of the moon, star fields and bright DSO's with very sharp and contrasty views on a flat field. Expensive for the aperature. Look sexy.
- larger refractors from 5" up, need a heavy duty EQ mount (EQ-6 and up), a power source, are quite heavy and take a while to set up and polar align. No collimation hassles and cool quickly. Have enough aperature to show larger scale images of the Moon and planets. Best images of any scope in their aperature range. Because of the high degree of sharpness and contrast they often show more pleasing (albeit not brighter) images than larger reflectors. Very expensive for the aperature as the cost of refractors goes up exponentially (at least) with aperature increases. Look incredibly sexy.
Good quality achro refractors have almost all the qualities of an APO, save that on bright objects they will show colour and the contrast will not be as good. However, on star fields and DSO's they offer performance almost as good for a fraction of the price. Achros are generally not suitable for imaging though.
For imaging, f/ratio, flatness of field and colour correction across a broader range of the spectrum becomes much more important than it is for visual. A smaller very high quality APO may well be a better choice for imaging than a larger ED-type APO that would be excellent for visual.
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