And so you are shopping for a telescope for astrophotography and you are considering getting a refractor.
That’s a good choice: they are popular, basically maintenance-free, fairly compact and versatile.
But this type of telescope comes in two (three if you are picky with definitions) flavors, so which one to get for astrophotography? Better to go with an achromatic or with an apochromatic refractor?
What Is A Refractor Telescope And What It Does To My Images?
In a refractor telescope, light passes through one or more lenses in order to be focused and magnified. Photographic lenses are, in this sense, refractors.
A refractor works differently from a reflector (newtonian telescope), in which the light is bounced off mirrors to get focused rather than passing through an optical element (lens).
While refractor lenses and telescopes are great in many ways, they have their Achilles heel in the form of chromatic aberration.
This is simply the unavoidable result of having light passing through a glass.
Starlight, like sunlight, is not monochromatic. Instead, it is composed of many colors.
When light passes through a glass, its different components, i.e., the different colors, are bent differently and you can see them individually.
This is not different from making sunlight pass through a prism to observe colors of the rainbow and the phenomenon is called dispersion.
Having the different colors focusing on a different point or a different plane is what we commonly call chromatic aberration, or CA for short.
We have discussed chromatic aberration in detail in this article, so let me just illustrate the two types of chromatic aberrations and consider what happens to the three fundamental colors (Red, Green, and Blue).
Longitudinal Chromatic Aberration (LoCA) is when the lens cannot focus the three colors on the same plane.
With LoCA, you will have fringing around highly contrasted edges that changes with focus: if your focus is too short, you’ll see fringings that are, say, blue. Overshoot the focus in the other direction and you’ll have fringing of a different color, say, red.
While not easy to correct in post, you can minimize it in-camera if you accurately focus on the stars and can step down your lens (or reduce your telescope aperture with a mask).
The other type of chromatic aberration is Lateral (or Transversal) Chromatic Aberration (TCA). This occurs when the lens cannot focus all three colors on the same point of the focal plane.
Here, the CA increases the more the subject is off-axis: if you photograph stars, those near the edges of the frame will show stronger CA.
TCA is easier to correct in post than LoCA by aligning the R, G, and B channels, but cannot be reduced in-camera.
Because in newtonian telescopes and mirror lenses light does not pass through the lens, they are essentially CA free. But they have their own set of shortcomings, such as coma, collimation issues, big size, long cooling time, etc.
Apochromatic vs Achromatic: What’s the difference?
As you may have guessed from the name, the difference between achromatic and apochromatic has something to do with the way the different colors are affected when sunlight (or starlight) passes through the lenses in the telescope.
In a telescope, when the light passes through the lens, it is bent to focus at a precise point on the instrument’s focal plane. But because of dispersion, we already saw that the different colors starlight is made of are also bent by a slightly different amount.
The result is that a single lens telescope would not be able to focus the three fundamental colors (Red, Green, and Blue) on the same point.
To correct for this, modern telescopes use more complicated optical schemes involving more lenses made of high-quality glass.
Compared to achromatic telescopes, apochromatic ones are:
- Heavier: they have more optical elements in it
- More expensive: they have more optical elements of better quality
- Some high-end apochromatic telescopes incorporate the otherwise extra field flattener
- Almost all apochromatic telescopes will let you reach focus with your DSLR camera, and all of them should allow you to focus with a mirrorless camera
This is because apochromatic refractors are principally built with the goal of delivering high-quality images in astrophotography.
Achromatic Refractor Lens
Modern achromatic telescopes, also known as ACHRO, use two lenses to bring two colors to focus in the same plane, typically red and blue.
They usually pair a negative and a positive lens that are made of different types of glass: crown and flint glass. These glasses have a different refractive index, and therefore, different light dispersion.
In a nutshell, the low dispersion crown glass positive lens is used to compensate for the higher dispersion of the flint glass in the negative lens.
Still, they cannot fully correct dispersion and you will see some residual chromatic aberration, mostly in the form of violet and blue halos around the stars.
This is the same problem you have with using most of the legacy lenses.
This is not a big concern if you use your refractor for visual observations, but it can be annoying if you plan to do astrophotography with it.
Note that not all achromatic telescopes are born equally: the higher the f-ratio of the telescope, the better the reduction of false colors.
That is why LoCA can be reduced in-camera when stepping down your lens aperture.
In telescopes, you don’t have a diaphragm to control the aperture, but you can always create a mask to reduce the obstructed part of the telescope aperture (thus reducing it) for the times you use your telescope for astrophotography.
Apochromatic Refractor Lens
Apochromatic refractors are designed to bring all three colors into focus in the same plane, effectively producing images that are CA-free.
Now, do understand that there is not a single red, green and blue color; there are multiple wavelengths that can be considered red, green, and blue: these are the residual colors and depending on the quality of your telescope, they can creep into the image, although often they go unnoticed.
Also, apochromatic telescopes do a better job than the achromatic ones in correcting for spherical aberrations.
How Do APO And ACHRO Telescopes Differ From ED?
ED stays for “Extra-low Dispersion” and is not a type of telescope but rather a type of glass.
ED glass is a better quality glass than that typically used in achromats: it has lower dispersion and provides better color correction.
If you are into photography, you may have noticed that high-end and pro photographic lenses often include the ED included in the name.
One of the most highly regarded photographic lenses for wide-field astrophotography falls under this category: the Samyang 135 f/2 ED UMC.
Fluorite glass is also another type of high-quality glass that gives an even better color correction. FPL-51 and FPL-53 are another type of fancy glasses used in APO telescopes.
There is a third type of refractor, one we didn’t discuss yet and that is kind of a hybrid beast.
If you take the same two-lenses design of an achromatic telescope but use ED, Fluorite, or FPL-51/FPL-53 glass for at least one of them, you have built what is called a semi-APO refractor.
The Sky-Watcher Evoguide 50ED is a great example of such a semi-APO (often just called APO) refractor, using one FPL-53 element coupled to an ED lens.
Terminology Used
The terminology used is pretty confusing, particularly when manufacturers tend to bend the definitions in favor of their products by labeling a semi-apochromatic refractor as apochromatic.
Anyway, as said before:
- APO is shorthand for apochromatic
- Semi-APO is shorthand for semi-apochromatic
- ACHRO is shorthand for achromatic
- ED is shorthand for Extra-Low Dispersion and is a type of glass, same as FPL-51/FPL-53
Then there is also the distinction between doublet and triplet, which relates to the number of lenses used: a true APO is a triplet.
Conclusion
After this rather long technical article, what practical conclusions can we draw? What to choose?
Consider the main use you will have for it: are you into astrophotography? Go with at least a semi-APO refractor. They offer the best color correction, minimal chromatic and spherical aberrations.
Are you going for visual observations on the move? If the budget allows for it, I would still prefer semiAPO and APO telescopes, as you never know. But if you are on a budget, go for an achromatic one, trying to avoid those with low f-ratio, as they will suffer from stronger aberrations.