Apochromatic refractors are widely touted as the “perfect telescope” – no chromatic aberration, no obstruction, and often with extremely high-quality optics providing sharp images with minimal scatter or other issues with image quality. Meanwhile, achromatic refractors tend to be shafted and largely discounted by many astronomers while also wildly overrated by others. What exactly the terminology of either of these words even means is also wildly unclear depending on who you ask, due to confusion about nomenclature and biased owners of extremely expensive gear wanting to feel confident that their purchase was a cut above the rest.
What is an Apochromat or Achromat anyway?
Let’s get the first part out of the way. If you are doing deep-sky astrophotography and plan on using a refracting telescope, you need an apochromatic refractor. “Apochromat” is an all-encompassing term for anything that is not an achromat. Achromats are simple refractors that use a doublet objective lens with crown and flint glass, and not any kind of ED or fluorite glass in the objective.
All refractors focus different wavelengths of light at different points, and the stronger the curvature of the lenses (i.e., the larger the scope or faster the f-stop) the worse this problem gets. It produces an issue known as chromatic aberration – smeared, out-of-focus halos most easily seen around bright stars, the Moon, and planets. Slow achromats are poor for imaging due to their physical size and focal length, while fast ones put up intolerable chromatic aberration. Controlling chromatic aberration is an uphill battle that refractor optical designs face, and almost any apochromat is going to be better than an achromat for imaging use. The real selection challenge lies in what type of apochromat you want and whether you need one for visual observation. Apochromats use ED, or extra-low dispersion, glass, which is glass that has a better refractive index, meaning it focuses different wavelengths of light closer to the same point to minimize chromatic aberration and other issues, allowing it to achieve a sharp image suitable for photography and/or superior for visual use.
These days, apochromats can refer to a broad range of telescopes, from ED doublets to Petzval sextuplet designs. In reality, however, there are essentially three main types of “apochromat”:
- Triplet apochromats are usually the most universally referred to as apochromats. They use three lenses for the objective, with at least one containing some type of ED glass.
- ED doublets use an objective lens with two lens elements like an achromat, but one lens uses ED glass in place of the crown or flint element.
- Petzval refractors are either triplet or doublet scopes with an integrated field flattener/reducer element. They can use a longer f-ratio objective lens to reduce aberrations, while the built-in reducer/flattener eliminates spacing, vignetting, and other requirements as well as allowing for the addition of an additional reducer/flattener to make the telescope even shorter in focal length for imaging.
- Technically, Petzvals can use a non-ED optical design making them essentially achromats. However, this is only the case with telescopes such as older Tele-Vue refractors which are no longer sold. Most Petzvals are apochromats i.e.. use ED glass.
- Quadruplet refractors are usually an ED doublet with a 2-element Petzval lens.
- Quintuplet, sextuplet etc. refractors are usually triplets with a Petzval lens which may have more than 2 elements.
These three terms are still extremely broad, as optical design and arrangement within each category can vary greatly and influence performance as a result, but in general, these three archetypes all have distinct differences in form factor and performance. All three of these designs (as well as achromats) have their own advantages and disadvantages. This is dictated by a few factors, starting with the type of glass used.
ED glass is rated by its Abbe number, essentially a performance score out of 100. The higher the Abbe number, the lower the chromatic aberration and scatter the glass has. There are a few types of ED glass, all with their own benefits and drawbacks.
“SLD” or “super low dispersion” glass is a term referring to ED glasses with an Abbe number over 90.
Fluorite (Abbe number 95) a naturally occurring mineral, is considered the best type of ED glass. Technically, it’s not glass but crystal.
Pros of fluorite:
- Extremely consistent quality
- Actually has even lower scatter than any kind of glass due to molecular structure
- Takes a very smooth polish and cools down to ambient temperatures a little more rapidly
Cons of fluorite:
- Extremely expensive
- If used as the front lens element and not protected, fluorite is rapidly attacked by chemicals in the air. This is worsened by salt, humidity, and pollution. Older fluorite refractors that use it as the front element in a lens have “foggy” lenses which have been essentially corroded, making them the only type of refractors which cannot last indefinitely.
- Due to environmental regulations and cost, fluorite is not as commonly available as it used to be, especially since synthetic ED glass can come very close in performance.
Ohara FPL-53 (Abbe number 94.94) is Ohara Japan’s best attempt to create a glass copy of fluorite.
Similar glasses to FPL-53 include the Ohara FPL-55 (Abbe number 94.66), a newer version with similar performance to FPL-53 but slightly better properties for larger lenses and easier properties when worked within the optics shop. Hoya FCD-100 is virtually identical in performance and properties to FPL-53.
Pros of FPL-53 and equivalents:
- Best possible Abbe number other than fluorite
- Fairly common and easily adapted to wide variety of optical designs
- Can take a very good optical figure
Cons of FPL-53:
- It’s not fluorite
- Costs more than cheaper ED glasses
- Expands and contracts more with temperature, requiring greater cool-down time and also increasing difficulty in making a good lens in the shop, which means shoddy FPL-53 lenses can occur and good ones cost more.
Ohara FPL-52 (Abbe number 90) and its equivalent, LZOS OK-4, used to be the best ED glasses available until FPL-53 became available. Today they are seldom seen, though performance is only slightly worse and other properties are essentially the same.
Ohara FPL-51 (Abbe number 81.54) is commonly used in cheaper apochromats. It is similar to Hoya FCD-1, which is actually slightly superior on paper with an Abbe number of 81.6.
Pros of FPL-51:
- Consistent quality
- Cheaper than FPL-53 or fluorite
- Thermally more stable, requiring less cooldown time in the field and easier to work with to a fine standard of quality
Cons of FPL-51:
- Worse refractive index means it’s harder to design an aberration-free scope
- Almost inevitably produces some chromatic aberration when used without another ED glass, e.g. Explore Scientific FCD-1 triplets, which have blue chromatic aberration on bright targets but otherwise tack-sharp images
- Often is misrepresented as FPL-53 usually by omission of clearly stating glass type
CDGM FK-61 (Abbe number 81.6) is supposed to be similar to FPL-51, and is a type of ED glass made in China. It is, on paper, essentially a copy. However, FK-61 glass is not as consistent in quality control as its Japanese counterpart. As such, many telescopes using FK-61 glass may have optical issues stemming from the glass itself, affecting the ability to achieve a fine optical surface or inducing additional scatter.
Other glass is used for the secondary lens element(s) of a refractor where a low Abbe number but good optical quality are needed. Lanthanum and BK7 crown glass are the most commonly used. BK7 is a type of glass vaguely similar to Pyrex (though slightly inferior to mirrors in reflecting telescopes). Schott borosilicate N-BK7 is commonly used. Achromats use flint and crown glass, the latter of which is often BK7.
Other glass types exist. Thorium oxide was used in camera lenses and tank telescope eyepieces through World War II as it provided similar performance to lanthanum glass but was cheaper and easier to produce. Unfortunately, thorium is radioactive, which causes lenses with it to brown/yellow over time, requiring UV light (such as from the Sun) to restore clarity. The radioactivity of the thorium glass is not very dangerous on its own but can ruin film, while long-term use of a thorium eyepiece has been linked to an increased risk of cataracts. Optical parts with thorium glass can be fun curiosities to keep on your desk, but shouldn’t actually be used.
A wide variety of lens arrangements and optical designs are possible with apochromatic and achromatic refractors and can have wildly variable performance. It’s important to note that a telescope using any kind of glass can achieve good images, and the correlation of achromats and lower-Abbe glasses with lower performance is purely just that – correlation. If designed right, an optical engineer can make a good refractor with any available material and number of lens elements, but it may be optimized for different things. In addition to the glass type and the actual curvature of lens elements, the spacing of lens elements with an air gap, foil spacers, etc. can produce different results. Some refractors even use oil in between lens elements, which has its own refractive properties and may improve performance (but of course induces complexity and a potential point of failure).
Performance & Optimizations
All refractors can be good or bad depending on the manufacturing quality and design optimization. In terms of optical performance, there are a few things that can be adjusted. Glass type, spacing, focal ratio, and lens arrangement are the tools in the optical designer’s toolkit. While a “perfect” apochromat with a fast f/ratio, sharpness both on- and off-axis, and minimal scatter, etc. is possible, the reality is that due to cost constraints, most refractors have to be optimized for certain performance attributes over others.
You might hear the term “Strehl ratio” used in reference to refractors. This is a score for the efficiency of the telescope, i.e., how much light is properly focused and not scattered, put out of focus due to chromatic aberration or defects in the optical figure, etc. An acceptable telescope has a Strehl ratio of 0.8, while a good one is typically 0.9, and extremely high-end refractors are often Strehl 0.99 or greater. The Strehl ratio can also be used for measuring performance at a certain wavelength of light.
It’s important to note that a telescope with Strehl 0.8 is not usually literally losing 20% of the light entering it but rather might have some performance issues with one extreme wavelength of light or suffer from defects that only show up at high magnifications and blur the image but do not affect images or low-power views. A lot of fast achromats and cheap ED doublets have Strehl ratios that are quite poor but still provide a satisfying experience if you know their limitations.
Focal ratio – Slower focal ratio refractors are simply easier to design to a high standard of optical performance because the curvature of the objective lens is weaker, and the light cone entering the eyepiece/camera is far less steep. However, this of course kills photographic speed and limits the achievable field of view for visual use.
On-axis sharpness – A refractor designed for imaging may sacrifice some sharpness at the center of the field of view in exchange for a sharper, flatter field of view (especially at a faster focal ratio), since slightly “fat” stars may not exceed the resolution of the camera sensor being used.
Off-axis sharpness – Many refractors have field curvature by default and need a flattener to display a sharp image across the field of a camera, but an eyepiece induces its own aberrations that may cancel this out. Ironically, this can mean that refractors designed mainly for imaging (Petzval or not) can have outright nauseating field curvature with an eyepiece.
Chromatic aberration spectrum – Since all refractors suffer from at least minor chromatic aberration (reduced to practically nonexistent with the best telescopes) it is going to show up more at certain wavelengths than others. Depending on the purpose of the telescope and/or other design constraints, what wavelengths this falls under can be adjusted and is extremely important.
Achromats place both of the extreme ends of the visible spectrum – red and blue – out of focus, creating purple halos. This is in an attempt to control other optical aberrations, and also because when the telescope is long enough in focal ratio these wavelengths are brought closer to focus. If the out-of-focus wavelengths were concentrated towards one end of the spectrum, a longer f/ratio achromat would still struggle with either red or blue sharpness. Likewise, at low powers with a faster or larger achromat, the red and blue chromatic aberration blur together more than if it were just one color, allowing for images that still appear decent.
The bulk of early apochromats were designed for sharp images at the eyepiece and thus are optimized for a fairly even Strehl across the visual spectrum, or to have slight chromatic aberration at each end of the spectrum like an achromat. This is not true today; most cheaper apochromats are designed for imaging, and given that their market is composed mostly of people who never even look through their telescope, there is little point in optimizing for such. As a result, many apochromats nowadays have been optimized for performance and higher Strehl towards the bluer end of the spectrum to avoid blue halos around bright stars in astrophotos. However, in order to cut corners and keep costs down, good imaging performance and sharp stars in these cheaper ED scopes come at the price of severe red chromatic aberration. This may not be noticed in most visual observations or images, but it affects planets, namely Mars and Jupiter, along with some double stars.
Owners of many smaller and less expensive apochromats are surprised at just how bad their views of Mars are in particular, with the planet appearing as a blurry ball while a $100 60-80mm achromat or 100mm tabletop Dobsonian walks all over their prized instrument with clear images of the Red Planet’s dark surface markings and polar ice cap. FPL-53 scopes are often, ironically, far worse in this regard than FPL-51 refractors (particularly when the latter are produced at a slower focal ratio).
Quality control – This is the big one. Manufacturers like Astro-Physics, Stellarvue, and Takahashi polish and figure their lenses to an extremely high standard and will also reject entire glass batches that have the slightest imperfection. This is not true when you buy an ED doublet that costs a few hundred dollars. However, this may just mean a lot of per-unit variation. Additionally, as previously mentioned, slight sharpness/scatter issues impact visual observation far more than they impact imaging. Deep-sky astrophotographers often get away with surprisingly shoddy optics. Additionally, slow achromats are easier to manufacture to high standards of quality due to the less exotic properties of the glass used, the minimal number of optical surfaces, and the weak curvatures of the lenses, and thus even cheap mass-manufactured ones can often be surprisingly sharp.
While the above is all true, the current market for telescopes dictates that in general, glass type tends to trend at least somewhat with quality, e.g. there are more bad FPL-51 scopes than FPL-53 ones, and many achromats are simply manufactured to low quality standards in order to cut costs since their market is primarily budget shoppers anyway. However, plenty of fantastic telescopes with “inferior” ED glass or achromatic optics have been produced, and as such, it’s important to keep in mind that the quality of many refractors does just come down to the optical design and quality control standards for that particular product.
Which is Right for Me?
As refractors are of course more often than not a compromise in design to achieve their particular goals, their physical attributes can be just as important as the optical. A Petzval refractor delivers a wide and sharp image, but the huge increase in weight (and cost) is pointless if your camera sensor would be fully illuminated with a cheap doublet and reducer/flattener at the same aperture/focal length specs. Triplets can provide sharp images for visual use, but they take longer to cool down to ambient temperatures and achieve those sharp images – and they weigh a lot more, too.
For imaging, selecting what you want comes down to the camera, your mount, your focal length/speed requirements, and your budget. Petzvals are generally best for wide, flat fields at a fast f/ratio but cost and weigh more, while also being limited by their short focal lengths. Most slower FPL-53 and FPL-51 doublets or triplets can do double duty for visual use in some form but work great for smaller targets and will allow you to shoot at a fairly fast f/ratio with a smaller sensor and reducer. Fluorite doublets are ideal for those looking for a lightweight scope with maximum performance while cheap FPL-53 doublets work great for those getting started with imaging and limited by their budget or mount.
If you’re looking for a no-nonsense visual scope for the Moon, planets, and double stars, a fluorite or FPL-51 doublet – usually f/6.5 or slower – is best for most people, and a small, slow f/ratio achromat of similar speed can also be great. Fluorite doublets cool down the most rapidly, while FPL-51 is fairly close; FPL-53 can take a bit longer. Heavier triplet designs may actually be inferior and take longer to cool down as well as costing more; many FPL-53 doublets can also be notoriously bad for planets, as previously mentioned. A fast ED doublet or achromat will provide a wider field of view for deep-sky viewing, and the former works for imaging; both are terrible for high-resolution viewing. Our OTA rankings page is a good start in your journey.
It’s also important to note that refractors for visual use are not particularly economical at all. They are luxury telescopes. If you want one for visual use, shopping for the best quality achromat or ED doublet possible is wise. You might want to look at the used market. Old achromats from Vixen (often sold as Celestron but clearly marked as Japan-made) are particularly prized as are Swift, Royal Astro, Unitron, and Towa (sold as Meade). The best refractors designed for visual observation are arguably the fluorite and other ED scopes from Takahashi, Vixen, and Astro-Physics, along with almost anything from Stellarvue and many of the TEC/LZOS/APM and TMB telescopes. Some of these are discontinued, while some are available new. Budget achromats with good optics sold today include the Explore Scientific FirstLight and Celestron Omni XLT refractors.
If you’re shopping new for an apochromat primarily to look through an eyepiece, in addition to the aforementioned premium brands, Explore Scientific’s Essentials 127ED with either FCD-1 or FCD-100 glass is a wonderful telescope for visual use and some astrophotography, while FPL-51 ED doublets sold by Astro-Tech and other rebrands are also great for visual at a bargain price, albeit so-so for imaging. Many of the Long Perng refractors and the Sky-Watcher Evostar line are usable for spectacular imaging results as well as great views at the eyepiece.