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How Refractor Telescopes Work

Refractor telescopes use glass lenses to bring an image into focus. Refractors are the original telescope design, and are popular with beginners and advanced users alike. High-quality refractors are prized for their high contrast, aberration-free views and images. Entry-level refractors are preferred for their ease of use and affordability. No matter what skill level you are or what your budget is, the refractor design is a tried and true telescope choice for almost everyone.

Doublet vs. Triplet vs. Quadruplet/Petzval

You'll often hear terms like doublet, triplet, and quadruplet (or Petzval) used to describe refractor telescopes. Each of these is a type of optical design, which is a refractor's most important trait. Doublet, triplet, and quadruplet simply refers to the number of glass lens elements inside the telescope that bring the light into focus, and each one of these types has their own pros and cons. We dive deeper into the differences between doublets, triplets, and quadruplets further below, but here's a quick overview:

 Doublet Refractors Triplet Refractors Quadruplet Refractors
Least Expensive More Expensive Most Expensive
2 lens elements 3 lens elements 4 lens elements
Good for visual & imaging
Good for visual & imaging
Some quadruplets are designed for imaging only
Usually have heavy chromatic aberration, unless made with good glass Have little to no chromatic aberration, depending on the quality of the glass used
Have little to no chromatic aberration, depending on the quality of the glass used
Requires use of a field flattener or flattener/reducer for most imaging purposes
Requires use of a field flattener or flattener/reducer for most imaging purposes
Does not require a field flattener

For a beginner, a doublet refractor is more than adequate, especially if you're only planning to be visually observing. Click here to shop our wide selection of doublet refractor telescopes under $500. More experienced observers and imagers may prefer the optical qualities of a triplet APO refractor telescope. Finally, those looking for an out-of-the-box solution for astrophotography may prefer the built-in field flattener (which we cover under the Quadruplet Refractor section below) in a quadruplet refractor telescope.

It's worth noting that the refractor optical designs of doublet, triplet, and quadruplet aren't the only aspect to consider, particularly when buying a high-end refractor, especially for imaging. Other factors, such as price, aperture, focal length, focal ratio, glass used, focuser size and design, included accessories, and whether it's an achromatic or apochromatic (APO) design should play into your decision as well. With that said, read on below to help you narrow down your decision even further!

Doublet Refractor Telescopes

Celestron StarSense Explorer Doublet Refractor

Best for beginners and observers on a limited budget

Doublet refractors are the least expensive refractor design, using only two lens elements, making for an affordable entry-level telescope. These telescopes are perfect for kids and beginners as they're intuitive to use and require very little maintenance. More experienced observers may notice that doublets suffer from chromatic aberration, also known as false color or color fringing. Chromatic aberration is an optical imperfection where only two (red & green) of the three main colors (red, green, & blue) come to focus at the same focal plane, which we cover in more detail under the FAQ section below. This results in blue/violet-colored halos around bright objects, such as the moon, planets, and stars. Beginners just getting started, however, shouldn't worry about chromatic aberration too much, as even a doublet can provide great views if you can look past the color fringing. Overall, doublet refractors are a fantastic choice for any beginning or intermediate observer.

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Triplet Refractor Telescopes

Radian Raptor 61 Triplet Refractor

Best for experienced observers & imagers who prefer little to no false color

Triplet refractors use three lens elements to correct for chromatic aberration, and the triplet design is widely considered the gold standard in terms of refractors. Since the three lens elements can allow all three main colors (red, green & blue) to come to focus at the same point, you can usually expect the view to be free of color fringing through a triplet refractor. There are some cases, however, when triplets may not give completely chromatic-aberration-free views, depending on what glass is used. We cover this more in the FAQ section below, but the important takeaway here is that triplet refractors are some of the best performing telescopes money can buy. We recommend triplet refractors for any intermediate to advanced amateur astronomer who want a premium visual performance scope that can take stunning images, too.

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Quadruplet Refractor Telescopes

William Optics RedCat 51 Petzval Refractor

Best for astrophotographers looking for a turnkey imaging telescope

Quadruplet (also known as Petzval) refractors use four lens elements in their design. You might be wondering: why would I need a quadruplet if a triplet already corrects for all three colors? That's a great question, and the answer has to do with another common aberration in refractors: spherical aberration. Spherical aberration is when not all light beams come to focus on the same focal plane. A field flattener can be used to correct this — field flatteners flatten the incoming light to be parallel with the focal plane, allowing you to retain sharp, round stars even towards the edges of your image. Nearly all doublet and triplet refractor designs will require the purchase of a field flattener or flattener/reducer combo for deep sky astrophotography. Without a field flattener, stars may appear sharp in the middle of the frame, but will appear out of focus towards the edges. The quadruplet design features the field flattener built in, so you don't have to purchase an extra accessory for imaging. The downside to this design, however, is that most quadruplets cannot be used visually. For this reason, quadruplets are primarily used only for imaging, unlike doublets or triplets.

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Still have questions? We have answers.

What is Chromatic Aberration, and Do I Need To Worry About It?

Chromatic aberration, also known as false color or color fringing, is a common trait among optical telescopes, particularly in refractors. It occurs when one or more colors of light focus at different locations, causing bright objects to appear to have colored halos around them, including stars, planets, and the moon. Though it can be eliminated in higher-end refractors, chromatic aberration is commonly found in entry-level and some intermediate refractor telescopes. Find an example of what chromatic aberration looks like here:

Chromatic Aberration or Color Fringing in Refractors

If you're a beginner, you don't need to worry about chromatic aberration! Nearly all beginner refractors will feature it to some extent, and it doesn't affect the view too drastically. As amateur astronomers get more experienced, most generally prefer to make the jump from a doublet refractor to a triplet, as triplets will usually have much less of this color fringing. Using a quality refractor will also make dramatic improvements for astrophotography, as chromatic aberration is more present when imaging.

How Do I Know Which Refractor Telescope To Buy?

Generally speaking, we recommend that all beginners interested in the refractor design go with an entry-level doublet. There are many fantastic options for beginner doublet refractors out there, such as the smartphone-assisted StarSense Explorer series from Celestron.

As you grow with the hobby, you may want to consider upgrading to a higher-end doublet refractor or a triplet. If you plan to do serious deep sky imaging, you'll likely want to consider a triplet refractor like the Radian Raptor 61 or a quadruplet design as they can usually give better overall performance.

Are Refractor Telescopes Good For Astrophotography?

Yes, absolutely! Refractor telescopes can be some of the best-performing telescopes for capturing the night sky. The refractor is a renowned telescope design for imaging. So much so, in fact, that we recommend all beginner deep sky astrophotographers start with a small refractor due to their ease-of-use, high contrast images, and maintenance-free operation. All refractor telescopes are capable of astrophotography to some degree. Of course, this is highly dependent on the quality of the refractor, though. For entry-level and beginner refractors, you may only be able to use a smartphone adapter to take images through the eyepiece with your phone. This can give good beginner images of the moon and sometimes the planets. Higher-end refractors like APO doublets, triplets, and quadruplets, on the other hand, usually feature a larger 2" focuser that allows you to connect a DSLR or a dedicated astronomy camera for capturing even more detailed images of the night sky.

What is the Difference Between Achromatic vs. Apochromatic Refractors?

Achromatic and Apochromatic are terms that describe how much chromatic aberration a refractor telescope or lens suffers from. Achromatic, or achro, is a dated term to describe telescopes free of color when doublets first evolved from singlet refractors. In truth, achromats only correct for two of the three main colors. Apochromatic, also written as APO, correct for all three colors, among some other optical requirements. However, not all APO refractors are created equal. Some manufacturers will use the term APO to describe a higher-end doublet refractor, even though doublets usually suffer from chromatic aberration. Why is this? The answer lies with the type of glass used, which you can read more about below.

What Types of Glass Are Used in Refractors, and What Glass Qualities Should I Look For?

The type of glass used in a refractor has a significant impact on the quality of the view and image. For example, a doublet refractor with an ED (extra-low dispersion) lens element can often come close to the optical performance of a triplet with standard glass. For this reason, you may see some doublet refractors referred to as APO doublets if they use ED lens elements or other premium glass, even though the term APO is usually reserved for triplets and quadruplets. These higher-end doublet refractors usually have a price tag that compares them with lower-end triplet telescopes. Glass type is often obsessed over in the amateur astronomy community, but in truth, it is just one of many important aspects to consider when purchasing a high-end refractor telescope. Perhaps a more well-rounded specification for optical performance is comparing spot diagrams for different telescopes, as spot diagrams take into account the rest of the telescope design aside from glass like baffling. Regardless, below is a chart of some of the more common glass types and their general rankings as preferred by the amateur community.

 Standard Good Better Best
BK7 (Schott) FPL-51 (Ohara) FPL-52 (Ohara) Fluorite/FL
FK61 (CDGM) OK4 (LZOS) FPL-53 (Ohara)
FCD1 (Hoya) FPL-55 (Ohara)
FK51 (Schott) FCD100 (Hoya)