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Refractor Basics

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Refractor Telescope Basics



This site introduces you to the design and characteristics of the refractor. From the simplistic telescope of Galileo to the giants of the astronomical refractor period, refractors seem to have a place in the hearts of most astronomers. At star parties, people will still line up to look through modest sized refractors, even though much larger reflecting type telescopes are available. The venerable refractor just seems to look like what most people think a telescope should look like. Hopefully after you read this, you'll have some idea if a refractor telescope is the right type for your amateur astronomy interests.

If you want to learn more about refractors, read on.


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The Refractor Telescope Design

The oldest and easiest to understand telescope type is the refractor. Used since the early 1600's, the refractor consists of an objective (the imaging forming element) at the front of the telescope, and an eyepiece at the rear.

The bottom of the two images above telescope images shows a ray trace diagram of a refractor. The objective lens (right side of picture) converges incoming light into a focused image.

It's common for a modern refractor to also have a diagonal mirror (left side of image) that reflects the converging rays at a 45 or 90 degree angle into an eyepiece for more convenient viewing. Refractors with such a diagonal (called a star diagonal) see an image that is right side up, but reversed left to right.



Refractor Telescope Characteristics

The original refractor telescopes used a single piece of glass for the objective. That caused horrible chromatic aberration (color dispersion), making color fringes around bright objects, and seriously softening detail. In an effort to reduce this distortion, some of the old refractors were made exceedingly long, over 100 feet.

Increasing the focal length of the telescope caused the converging rays to come together at a much shallower angle, making the critical focus region wider. This reduced the chromatic aberration caused by the different colors coming to focus at different focal points. But it left the user with a very small field of view, and the exceedingly difficult task of wielding a 100 foot long telescope.

Most modern telescopes, like the Orion 9024 AstroView 90mm Equatorial Refractor Telescope, use an objective made of two tailor made pieces of glass. Such telescopes are called achromats.

The different components of the objective are chosen to have different refractive indices. It is thus possible to choose the right curves on each piece of glass to greatly reduce the color problems. The multi-element objectives are designed to bring the red and blue ends of the visible spectrum to the same focal point. These classic two-element designs work quite well at long focal ratios, say f/15 or better.



A number of imported refractors are made at about f/10 or f/11, which make them much more portable in sizes over 3 inches in diameter, but allows a bit more color distortion. Most observers find the amount of color distortion in these popular telescope acceptable.

By either creating the objective out of three elements or using two elements made from modern, more exotic glass that has lower color dispersion characteristics, the apochromat can be constructed. The Orion 9895 ED80 80mm f/7.5 Apochromatic Refractor Telescope is an example of such a telescope. The apochromat greatly reduces chromatic (color) distortion, and also allows quality refractors to be made in focal ratios down to about f/6. For the first time, quality rich-field views can be had with a refractor -- for a price.

The modern refractor telescope has a number of features making it desirable. One is that the refractor requires minimal maintenance. Keeping the lenses covered when not in use, and very occasionally cleaning the objective surface is all that's required. Unlike reflectors, refractors never need to be re-aligned.

Refractors also give the highest contrast images, making them very good for planetary observing. This is because refractors have a clear light path from objective to eyepiece. Most reflector designs necessarily have a secondary mirror in the light path to deliver the image to the eyepiece. Because of the clear light path available in refractors, they give star images uncluttered by the spikes prevalent in many types of reflector telescopes.

Refractors tend to give steadier images. This is for a number of reasons, including that fact that the telescope tube is closed at both ends, preventing air moving in the light path. Also, there's an averaging effect because the light travels through multiple elements.

So why doesn't everyone use refractors? For the amateur astronomer, the biggest reason is cost. Sizes of 4 inches diameter or bigger begin to cost in the thousands of dollars. Because the achromat works best at long focal ratios, the larger refractors are also very heavy and unwieldy.

However, if the low maintenance and steady images, especially for planetary viewing, is what you crave, a refractor is an excellent telescope. If you are just starting out, the long time standard 60mm refractor is a great choice. You'll be able to see more with a bigger scope of another variety, but the simplicity of using a refractor will increase the likelihood that your first telescope won't end up quickly in a garage sale.

If you're pretty sure a refractor is right for you, I suggest you examine the following table to see what kind of telescopes work best for different kinds of observing..

Telescope/Observing Preference Table

(Small Instruments At Table Bottom)

Wide FieldGeneral PurposeNarrow Field
15" f/4.5 DOB10" f/10 DOB12" SCT
12" f/4.5 DOB10" f/6 DOB6" Refractor
6" f/5 Newt8" f/10 SCT6" Maksutov
6" f/5 DOB6" f/8 Newt6" f/10 Newt
3.5" f/8 Refractor4" f/11 Refractor4" f/15 Refractor
4.5" f/4.5 DOB5" f/10 SCT5" f/15 Maksutov
2.4" Refractor4.5" f/10 Newt3.5" f/15 Refractor
Binoculars3" f/10 Refractor3.5" Maksutov

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Some Personal Notes

A few years ago I was going through a transition in telescopes, having finally given away an 8 inch Dob I'd had for 20 years. While shopping for a replacement, I spent a season with only a 50mm refractor to use for my astronomy hobby. It was really a fun season. I had a small equatorial mount from another scope, and it had a clock drive to which I'd added a fast/slow control. The 50mm and tripod could be easily carried around, so I could leave the unit assembled.

In ten minutes I could carry it out, set it in place, and be observing. Within 15 minutes, the optics would cool to the point of providing good images. I was able to observe some Mars features on a 17 arc-second sized planet -- I was amazed. I also got some good Saturn observing that year. The rings were easily visible, but I could not make out the Cassini division with this instrument. That kind of convenience is what's available with a modest sized refractor.

My own experience is that images are at their best through a quality refractor. I once bought a Bushnell 60mm refractor that was on sale. Bushnell telescopes have many critics. I found, as one writer had commented, that the optics on the inexpensive instrument were actually quite good. It gave fine star images. As with most inexpensive telescopes, it used the small 0.975 inch eyepieces, which are usually a form of Kellner. They aren't bad, the main objection is that there's a much smaller selection of these eyepieces available.

The issue I had with the bargain telescope was the inadequate mount. At higher magnifications, the slightest touch or breeze would start vibrations that seemed to take forever to die out. That's something to keep in mind if you are bargain hunting -- don't get a telescope with too flimsy a mount. Or, if you're handy with tools, replace a flimsy mount with a sturdy pipe fitting mount as shown in the 50mm image (see how to build a sturdy tripod).