Refractor

Refractor Telescope Basics

This site introduces you to the design and characteristics of the refractor. Refractirs are very popular among amateur astronomers, in fact the survey results of over 300 amateur astronomers (shown above) indicate that refractors are the most used type of telescope among amateur astronomers.

From the simplistic telescope of Galileo to the giants of the astronomical refractor period like the great Yerkes Refractor, 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.

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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.

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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. The long focal length lessened the converging angle of the focused rays, reducing the chromatic dispersion.

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. One of the longest telescopes in the early era was the 150 foot long telescope of Johannes Hevelius.

Most modern telescopes, like the Orion 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.

The focal ratio size necessary for acceptable color dispersion varies by telescope diameter.  A 60mm telescope starts to be acceptable as to color dispersion at only about f/7, or with a focal length of only 420mm. But a 100mm or 4 inch telescope has to be f/12 or better to deliver acceptable dispersion. That's a whopping 1200mm or 4 feet long. Not a bad length for a low setting Dobsonian reflector, but pretty unwieldy for a tall mounted, heavy refractor.

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. The following Chroma Ratio Table illustrates how telescope diameter and focal ratio together are what determine acceptable chromatic dispersion focal ratios.

Chroma Ratio Table

Dia mm	Dia in	6.00	7.00	8.00	9.00	10.00	11.00	12.00	15.00	20.00
50.00	1.97	3.05	3.56	4.06	4.57	5.08	5.59	6.10	7.62	10.16
60.00	2.36	2.54	2.96	3.39	3.81	4.23	4.66	5.08	6.35	8.47
70.00	2.76	2.18	2.54	2.90	3.27	3.63	3.99	4.35	5.44	7.26
80.00	3.15	1.90	2.22	2.54	2.86	3.17	3.49	3.81	4.76	6.35
90.00	3.54	1.69	1.98	2.26	2.54	2.82	3.10	3.39	4.23	5.64
100.00	3.94	1.52	1.78	2.03	2.29	2.54	2.79	3.05	3.81	5.08
125.00	4.92	1.22	1.42	1.63	1.83	2.03	2.24	2.44	3.05	4.06
150.00	5.91	1.02	1.19	1.35	1.52	1.69	1.86	2.03	2.54	3.39

Orange cells indicate Sidgwick (3) criteria reached
Aqua cells indicate Conrady (5) criteria reached

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 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. A good example is my 50mm refractor, shown on a homemade Pipe Fitting Tripod. It stands ready to go at a moments notice. Keeping the lenses covered when not in use, and very occasionally cleaning the objective surface is all that's required. Unlike reflectors, refractors seldom need to be re-aligned unless droppped or jarred enough to affect alignment.

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.

Image Taken with 50mm Refractor

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 on a budget, you can start small. Lest you think there's nothing you can see with a small refractor, check out the images of the solar eclipse of 1994. Click on the image block above for a full-sized version. Those images were taken with my little 50mm refractor.

Lunar Straight Wall Region w/60mm Telescope

If you are just starting out, the long time standard 60mm refractor is a great choice. And again, if you think such a size is too small, just check out the above image of the Straight Wall region of the Moon. It was taken with my DIY 60mm Refractor. 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 simplicity is what you are after, and on a budget, I know a fellow that has a Celestron 60LCM Computerized Telescope. Though the computerized 60mm is advertised for the beginner, the fellow I know isn't a beginner, he's an old guy like me who's long grown tired of lugging big telescopes out. So he often uses the computerized 60mm for the ultimate in low maintenance and ease of use. No aligning, no mirrors to clean, cools down quick, and a computer find the targets for you. Interested in a refractor yet?

Vixen A70lf on Pipe Tripod

Another good choice if you are on a budget is the nice sized 70mm Vixen Optics Mini Porta Mount and A70LF Telescope. I bought a Vixen A70lf awhile back, and have found it to have very good optics. It has an f/12.9 focal ratio, so it delivers images that have no detectable color distortion. Yet, at a 36 inch focal length it still has a repectably large field of view. If you have a tripod already, or are interested in making the pipe-fitting tripod, you can save even more by buying the Vixen 2602 A70Lf Telescope without tripod. I've written a review at Vixen Telescope Review. You'll notice on the image above that the Vixen uses the same pipe tripod shown with the 50mm telescope.

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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 diffraction 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).