Types of Telescopes #1: The Venerable Refractor

Last time I gave a brief introduction to the two prevalent types of telescopes, refractors and reflectors. These warrant further scrutiny, so we’ll focus on the older of the two designs:the traditional refractor telescope.

Author's 150mm, f/8 refractor telescope.

Author’s 150mm, f/8 refractor telescope.

A refractor, as the name implies, bends or refracts incoming light. It utilizes at least two glass elements, or lenses, mounted at the front of the scope. This instrument is the one most people think of when the word “telescope” pops out in conversation. Until very recently, it is the type seen in movies and TV shows, as a prop in the background, against the window of some executive with a penthouse view.

Why two lens elements? When light is refracted, its constituent color elements are bent as they pass through a glass lens, and come into focus at different places on the focal plane. With the original design, in the early seventeenth century, a single glass element caused serious chromatic aberration, or blurred images due to one of the three primary light colors not in focus (this isn’t a problem with mirrors that reflect light, but more on that next ish). Opticians quickly discovered that a two-element lens configuration (one the crown glass, and another the flint element) resulted in compensating for a great deal of minimizing chromatic aberration. Images were much more acceptable.

So, with the aforementioned handicap a major consideration, why would someone want to invest in the older design of a refractor? The answer is apparent, when gazing at a bright object such as a planet or our moon: greater contrast. The background of space is inky black, compared to a reflector’s dark-gray image (yes, the reflector design has inherent drawbacks too). A lens is a clear aperture, with no secondary obstruction such as the second mirror essential to the reflector design. Given that, a lens will, pound for pound, outperform a mirror of the same diameter. An eight-inch refractor would simply blow the drawers off an eight-inch reflector, especially when it comes to resolving planetary detail.

A refractor, too, is a simpler design. Taken care of, it is a virtually maintenance-free instrument. Although the lens can require occasional recollimation (alignment), it is very rare, usually necessitated by the lens cell having been removed form the optical tube assembly (never, never do this!).

Refractor telescopes are intuitively easier to learn to use, although the curve for using a reflector isn’t as formidable as it may initially seem (there; I’ve said it…and this author is indeed a confirmed refractor fan!).

A major drawback to a large refractor? Cost and size. Large refractors are HUGE animals, and cannot be easily moved. Hardcore refractor aficionados house these instruments behind their homes, in permanent shelters with either a rollaway roof, or a dome (and, I suspect, are not married).

Some folks by now may be suspecting I have stock in this mail order company, but I don’t…they’re just one of many online, but these guys carry the largest stock of so many brands:

A ten or twelve-inch lens needs a tube anywhere from ten to fifteen feet long. That’s very cumbersome, to say the least. Then you have to consider just what you’re going to mount it on. One “pier” mount goes for over fifteen thousand dollars (just the mount, folks).

These big telescopes are also very expensive (at this level, just the tube assembly itself, or OTA). The first time I saw a few prices, I thought a couple of the numbers had an extra zero as a typo…they didn’t. Observatory-sized ones begin at forty thousand dollars and go through the stratosphere from there. Lens casting and grinding is much more demanding than grinding a mirror.

D & G Optical, a company that specializes in making refractor telescope tube assemblies, will give you an idea of what constitutes a “large” refractor:

So, again, why would someone spend more money than some double-wide shelters cost? Image, image, image. I’ve been told not to make the mistake of gazing through a premium refractor unless I’m prepared to buy one. SO far I’ve been able to resist, as one probably doesn’t exist within a two-hundred mile radius of where I live.

Well…back to Earth, huh? My humble, Celestron C150mm f/8 refractor telescope is about the largest one of this type that yoU can realistically move out to a dark site. It’s still a chore, especiallY the older I get!

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We’ll examine that other type, the reflector, along with its pros and cons, next time.

Clear Skies,

Jesse

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