Refractor telescopes are unique among the various telescope designs available on the hobby market because they do not depend on mirrors to gather light. The designs based on mirrors have an obstruction in the light path; the secondary mirror that directs the light to the eyepiece or camera.
The secondary mirror and the structure to support the secondary mirror reduces the effective aperture of a mirror-based telescope. Some people also feel that this obstruction can lead to a reduction in image quality. Refractors avoid this obstruction and so offer a clear light path to the eyepiece or sensor. As a result a 100 mm refractor telescope effectively has more light gathering ability than a 100 mm reflector telescope.
The Refractor Design
Refractors have an objective lens upfront and an eyepiece or sensor in the rear. The purpose of the objective is to gather light and concentrate it down to the eyepiece where an image is formed. A simple diagram is shown here.
The classic “spyglass” or binoculars have this straight-through design. The image is inverted, but a correcting prism can be inserted in the path to correct the image for terrestrial use which is what is done with binoculars.
For astronomy, we usually add a diagonal that turns the image 90 degrees so the eyepiece is at a more comfortable viewing position. This inverts the image again so that it appears right-side up. However, it comes up reversed left and right. For astronomy purposes, we don’t generally care about this left and right reversal and tend to avoid adding a prism to correct this as it will reduce some of the light that reaches the eye or camera sensor.
The original single-lens objective refractors, as popularized by Galileo in the 1600s, had a serious problem with color distortion. The objective lens tends to split the light into colors, like a prism. The colors don’t arrive at the eyepiece precisely together and this distorts the color called chromatic aberration or CA. To address CA three objective lens designs have emerged, the achromatic, ED refractor, and the apochromatic.
Think of them as the low, medium, and higher-priced models in the market. If you are a visual observer the achromats or ED achromats may be just what you are looking for.
If you are an astrophotographer, consider the ED achromats your entry-level models with the APOs as the premium models for AP.
Entry-level and lower-cost refractor telescopes are of the achromatic design. You may also see these called as achromats or achro refractors.
Their objective lens is based on two lens elements made of different types of glass. The result is that the colors arrive at the eyepiece more closely merged than a single lens objective, but the effect is not perfect. Because the objective lens is made up of two lenses they may also be referred to as doublets.
Image quality can be very good, but when viewing bright objects, such as the Moon and planets, there can be a color fringing that appears in the eyepiece but is not actually a characteristic of the object being observed. This CA tends to be more pronounced in achromatic refractors of lower focal ratios, typically F5 to around F8. Longer focal ratio achromats, greater than F8, show less CA, but it is still there.
Many people are very tolerant of CA for visual astronomy because they know what it is, so they ignore it. Others are more sensitive to it and find it quite annoying.
The benefit of the achromatic refractor is its low cost and lightweight. That is why it tends to be the prevalent design in the entry-level market. Achromatic refractors are available in a variety of aperture sizes and are very popular, even among experienced observers, for their low cost and lightweight. The lightweight also allows them to be put on lighter weight mounts which further reduces cost.
Recent improvements in the achromatic refractor design have been made based on the introduction of ED or extra-low dispersion glass that further reduces CA. These ED refractors, based on a 2 element objective, have created a category between the APO and the achromat. You will see them referred to as ED achromats, ED APO, or ED doublets.
From a pricing point of view, they fall between the achromat and the APO refractors. For visual use, they are very good with CA reduced by 90% or more, compared to the same focal ratio achromat. But they are not quite as free of CA as the APO designs.
ED achromats make an excellent upgrade from the entry-level achromats for visual use. And some entry-level astrophotographers have found them good enough for AP as well. In some cases, they will use computer tools to process the images to eliminate the remaining CA effects.
To better address the CA issue the apochromatic refractor was developed, often referred to as an APO for short. The apochromatic design is based on a third objective lens element and more exotic glass so it is also referred to as a triplet. The apochromatic refractor virtually eliminates chromatic aberration.
The APO refractor is heavily favored for astrophotography. The CA of the achromats shows prominently in photographs. The apochromatic refractor fixes this issue. Apochromatic refractors are also favored by higher-end visual observers who want all CA removed from the image.
APO refractors are much more expensive than achromatic refractors. The three lens objective and the glass used adds greatly to the price. They are also significantly heavier than the achromats which means that the mounts that support them must be stronger, more robust, and more expensive.
When purchased as part of a package, typical of the entry-level models, a refractor can include a mount and tripod, a finder of some kind, one to three eyepieces, a focuser system, and perhaps other accessories.
Higher-level refractors are often sold separately and will require the purchaser to add such things as a mount, mounting rings, dovetail, diagonal, finder, and eyepieces. When evaluating cost and value, take a look at what is included in the package.
You may prefer to buy a refractor that does not include accessories so that you can pick your own or leverage what you already have. Just take this into account as you do cost comparisons.
Focusers can vary greatly in quality and flexibility. Some of the entry-level designs are made of plastic which may be adequate for lighter 1.25” eyepieces but which may struggle to handle larger wide view eyepieces or cameras.
Focusers are typically of two sizes, 1.25 inch or 2 inch. The 2 inch focuses usually include a 1.25 inch adapter so both size eyepieces can be used. The key benefit of the 2 inch is that they can accept the larger 2 inch eyepieces that can offer wider views. There may also be benefits for astrophotography based on the type of camera and sensor size being used.
Focusers can be single speed or dual speed. The dual-speed focusers have a course adjustment, similar to the single-speed focusers, which allows for rapid focusing. But they will also include a fine adjustment knob that provides a 10:1 or 11:1 reduction to make very precise fine focus adjustments easier. A dual-speed focuser can be very beneficial to visual and AP users alike.
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