|Refractors||Back to scope choices.....|
There are basically two classes of refractors - Achromatic, and Apochromatic (or APO). The APO refractor is the more expensive of the already high-priced category. Generally speaking, refractors cost more per inch of aperture than any of the other types. This is because the available technology to manufacture quality lenses is very expensive compared to grinding and figuring mirrors. One advantage of refractors is that good ones allow smaller apertures to be used in order to get the same image quality of larger reflectors and SCT's.
Today's refractor is a love child derived from 17th century DNA. The tubes for the earliest ones were made of paper in four or five segments. Later they were made of wood; then brass. The optics were simply a spectacle (eyeglass) lens at each end. Except for the way in which tubes are manufactured and lenses are ground today, the designs and principles of refractors have changed very little.
Unfortunately the telescope preceded good methods in glass making and grinding by a great many years. In fact, even the best early examples (including those utilized by the pioneers of astronomy!) were victims of optics so poor, that by today's standards would qualify as nothing more than junk. They do have great collector value however, so if you happen to find one - even in poor condition - it may be worth a fortune!
Each type of telescope has its own advantages and disadvantages. Good modern refractors are known for their high contrast images. A good refractor may out-perform a larger Newtonian reflector under certain conditions, and will usually blow a Newtonian or SCT of equal size out of the water. However, the biggest drawback of refractor optics is a dreaded anomaly called "Chromatic aberration".
Chromatic aberration is the manifestation of a bluish or purple ring (or halo) around bright objects - the moon, planets, and bright stars like Sirius. This can be bothersome for some people. Others just learn to live with it.* Chromatic anomalies will not be an issue with deep space objects - only bright things like planets and the moon. Also, the longer the focal length of the telescope, the less chromatic aberration is noticeable.
The achromatic lens was invented by Chester Hall in 1733. Essentially, an achromatic lens consists of a two-piece assembly of crown glass and flint glass. It is meant to greatly reduce chromatic aberration. In 1757, Peter Dollond developed the triplet lens which is a sandwich of flint glass between two layers of crown glass. This reduced chromatic aberration even more, but still did not eliminate it.
If you can tolerate the minor chromatic aberration in today's better 'achromats', and you're a "refractor freak", (like some of us) then you might want to consider a good refractor with a 90mm aperture (or larger) like those offered by Meade (the top two big manufacturers of consumer telescopes). Very good achromats in the 90mm to 120mm size cost between $300 and $600. But if you can't tolerate chromatic anomalies, and you can't afford the high prices for an APO, then you should consider one of the other telescope types. However, if you can afford the price of an APO refractor, then you're in for a treat.
*[NOTE: "Close" double stars cannot be split with an 85mm aperture. Larger apertures are required to view double star components which are closer than about 1.5 arc seconds]
The practical limit in aperture size for commercially available refractors seems to be six and seven inches. Anything above that resides in the realm of "Observatory" status, and approaches prohibitive means in the handling and convenience departments, not to mention almost 6-figure prices. A good refractor over seven inches will almost certainly be too long and heavy to be "transportable", and so is definitely regarded as a "Dome scope".
|Learn about another type of Telescope:|
What type of telescope should you buy?
Nomenclature - the typical Dob
What's an f/number? - Fast vs slow
What you can see... and what you WON'T see
The Cost of Amateur Astronomy
Finderscopes, Telrads, etc.
What is "GO-TO"?
Recommendations - GO-TO Systems
How things REALLY look in the eyepiece