Refractors are what the average person identifies with the word "telescope", a long, thin tube where light passes in a straight line from the front objective lens directly to the eyepiece at the opposite end of the tube. Advantages: Easy to use and reliable due to the simplicity of design. Little or no maintenance. Excellent for lunar, planetary and binary star observing especially in larger apertures. Good for distant terrestrial viewing. High contrast images with no secondary mirror or diagonal obstruction. Color correction is good in achromatic designs and excellent in apochromatic, fluorite, and ED designs. Sealed optical tube reduces image degrading air currents and protects optics. Objective lens is permanently mounted and aligned.

Contrast is the difference in brightness between the bright objects in your field of view and the background compared to the background. It’s important because good contrast is needed for seeing fainter objects and discerning subtle visual details.

Maximum image contrast is desired for viewing low-contrast objects such as the moon and planets. Newtonian and catadioptric telescopes have secondary (or diagonal) mirrors that obstruct a small percentage of light from the primary mirror. This degradation is only significant if more than 25% obstruction is present.

To calculate the secondary obstruction, use the formula πR2 to calculate the area of the secondary and primary mirrors. Then divide to find the percentage of obstruction. For example, an 8" telescope with a 2.16" secondary obstruction has a 7.3% secondary obstruction:
Primary radius is 4”: πR2 = 50.27
Secondary radius is 1.08”: πR2 = 3.66
Percentage = 3.66 / 50.27 or 7.3%

For a given object, a telescope’s design (including central obstruction), coatings, optical quality, cleanliness and collimation affect contrast. Important external factors affecting image contrast with an astronomical telescope are seeing (air turbulence) and air quality.

Between myself and SWMBO there are four ED scopes at this house, so I have to say yes, they are worth the price, else we wouldn't have bought them.

Some theory first. When you refract light through a lens it breaks into a component wavelengths. The idea of a telescope is to try to focus all of the wavelengths into one point, which is achieved by using multiple lenses, or combinations of different types of glass. If the light doesn't all arrive at the same point, various aberrations occur

First lets look at the achromats. Here you have a typical two element objective that gives colour correction at two areas of the spectrum. At shorter FLs and on larger targets, this leads to Chromatic Aberration, ie false colour and fringing, usually blue or violet fuzzy edges around planets, for example. This can be overcome in two ways. These days an appropriate filter helps a lot. The older method was that achro refractors used long FLs. There is a formula for the relationship between FL and aperture for an achro doublet that I can't remember, but basically it comes down to somewhere about f/15 or longer eliminates almost all CA. This is because the further the distance from the objective to the focal point, the closer all the light will come to the same point.

But here are the drawbacks. Lets say you have a 4" achro. At f/15 the thing will be about 6' long by the time you include the focuser and dewshield, and this becomes unwieldy and demands a -more expensive mount. Drop to a shorter f ratio, say f/7 and CA becomes an issue, so you need a filter, and one worth having is not cheap, plus you are restricting some light transmission with the filter.

However, this mainly applies to the bright targets. The "short tube" achros of today were mainly developed as spotting scopes, but have found their niche in Astronomy for wide field deep space viewing, where you are collecting small points of light from a dim set of objects, and so CA is not really a significant factor, if at all. eg viewing a cluster.

The newer style ED refractors still only use a doublet objective (two element) but use one element of a special type of glass. Usually a Flourite glass. This is more expensive to make than the traditional Crown Flint and others used in achros, but allows more light to come to one point at a much shorter focal length. The word APO or Semi Apo is bandied about by various manufacturers, and really its hard to define, but IMO these scopes are not true Apos

Nonetheless, for your extra money you get quite a few benefits. A relatively compact scope with *almost* zero CA, no filter required and can be really steady on a relatively moderate mount. Also wider FOV's than the older traditional long achros. But also these days, as the market demand for the ED's has grown, manufacturers have really set up these scopes in a more premium manner. Crayford focusers, often rotating or two speed, or both for example, are now the norm on the ED scopes, and this is another big benefit that you are paying for. Also standard 2" visual backs etc. and premium quality fit and finish.

The "true" Apos IMO are the triplets. ie using a three element objective, at least one of Flourite glass. Unfortunately these are way out of the price range for the average user, although they are getting cheaper.

Really its going to come down to your budget, and what you want to use a scope for most often. There is still a place for the good old achro. (I've owned enough of them), but if you can spend the extra for an ED you won't regret it especially on planetary use.