The choice between a spherical mirror and a parabolic mirror is one of those variables that seems to get blown out of proportion when looking at telescopes. Is it important to understand the difference? Absolutely. Should it be of critical concern to a beginner choosing their first telescope? Let’s talk about it.
This article aims to provide a comparison between these two widely used mirrors in telescope optics and demystify the differences. I also hope it helps beginners understand when, and if, this should be a concern.
Understanding the Basics of Telescope Mirrors
What is a Mirror in a Telescope?
In a reflector telescope, the primary mirror plays a critical role. It collects the incoming light from celestial bodies and focuses it to create a concentrated and magnified image for the observer. The shape of this mirror, whether spherical or parabolic, significantly impacts the quality and clarity of the resulting image.
While magnification in a telescope seems to be well understood by even complete beginners, the concentration of light is often overlooked. One way to understand this fairly easily is to look at the aperture of the telescope (the front that faces the sky) and see how large that opening is. Now look at your eyepiece and measure the size of the glass where you put your eye to see.
The aperture of a beginner reflector telescope may be 114mm, 130mm, or even more, while the glass you look through in your eyepiece is likely to be about 10mm depending on the eyepiece. That difference shows that your telescope is concentrating the light it collects with a 130mm opening into a 10mm circle. This is why you can see things in a telescope you can’t see with the naked eye, and why bigger telescopes allow you to see more.
Both aspects of the telescope mirror: light concentration, and magnification, are critical in the discussion of the types of mirrors in telescopes, and why you might prefer one over the other.
Spherical Mirrors: An Overview
Spherical mirrors, as the name suggests, have a spherical shape. They are a portion of a sphere and can be either convex (bulging outward) or concave (curving inward). In the context of telescopes, we primarily deal with concave spherical mirrors. These mirrors magnify the image, but because of their spherical nature, the incoming parallel light rays do not converge at a single focal point, resulting in a less sharp and slightly blurred image.
Parabolic Mirrors: An Overview
On the other hand, parabolic mirrors have a parabolic shape, enabling incoming parallel light rays to converge at a single point. This unique property provides a sharper, more focused image compared to spherical mirrors. However, they do present their own set of challenges, including a higher manufacturing cost and the occurrence of an optical aberration known as coma.
Delving Deeper: How Spherical Mirrors Work
Concave Spherical Mirrors
In reflector telescopes, concave spherical mirrors are used to magnify the image. However, due to the spherical nature of these mirrors, incoming parallel light rays do not focus at a single point, resulting in a less focused and slightly blurred image. This is particularly noticeable when the mirror’s circumference is large, and can be all but impossible to detect in smaller mirrors without specialized tests and/or equipment.
This lack of perfect convergence of light rays is called Spherical Aberration and gets worse the larger the mirror is.
Convex Spherical Mirrors
Convex spherical mirrors, on the other hand, reflect light rays outward in all directions or toward a single direction. These mirrors are commonly used in car mirrors or security mirrors to make objects appear closer than they are, often resulting in a fisheye-like image.
Analyzing the Advantages and Disadvantages of Spherical Mirrors
Spherical mirrors bring both advantages and disadvantages to the table. On the positive side:
- They are relatively easy to manufacture.
- They are less expensive due to simpler manufacturing processes.
- They are less prone to manufacturing errors.
- More are produced, so they tend to have more consistent quality.
On the downside:
- They lack a single, fixed focal point when the mirror’s circumference is too large.
- The brightness of a celestial object can be diminished.
- Images can be distorted due to spherical aberration.
- They can have issues with diffraction.
Understanding the Functioning of Parabolic Mirrors
Unlike spherical mirrors, parabolic mirrors have a distinct design that allows incoming parallel light rays to converge at a single point. This design results in a more focused and sharper image. However, parabolic mirrors also suffer from an optical aberration called coma (comatic aberration).
Coma is caused because light hits the mirror off-axis, or at an off angle, which reflects light to slightly different points. This creates a comet-like flare instead of a sharp point. This is most commonly visible at the edges of the field of view, and gets worse the faster the telescope is, but can be mostly corrected with a coma-corrector. In fact, astrophotographers almost always have a coma-corrector on their fast Newtonian imaging scopes.
Weighing the Pros and Cons of Parabolic Mirrors
Parabolic mirrors offer several advantages:
- They produce a more focused image.
- They do not suffer from spherical aberration.
- They are ideal for a wider viewing experience.
- They produce more light than a spherical mirror.
However, they also have their own set of disadvantages:
- They suffer from coma.
- They are more expensive to produce.
- Cheap versions often suffer from astigmatism.
- Fewer are produced in small sizes, which can lead to quality control issues.
Spotting the Differences Between Parabolic and Spherical Mirrors
Without removing the mirror and running specific tests with expensive test equipment, the star test is probably the best way to answer that question. If nothing else, performing a star test will make sure that your collimation is as good as it can get, which is even more important than your mirror type.
A general rule of thumb is that the smaller and less expensive a telescope is, the more likely it is to have a spherical main mirror. While there are certainly exceptions to this, costs have to be cut somewhere, and this is usually where it happens. That isn’t necessarily a bad thing, read on.
The Final Verdict: Which Telescope Mirror is Better and Does It Matter?
I can’t tell you how many times I have been asked this question about a telescope mirror. Most people who ask this question are people buying budget telescopes and are “spec hunting”, trying to get the best specifications for their dollar. They also read on some forum about how superior the parabolic mirror is to the spherical mirror (and technically that is completely correct). The problem with this is that a person with a good spherical mirror, in a nice telescope, with excellent collimation and very good eyepieces will get a much better view than someone with a parabolic mirror in a cheap telescope who doesn’t really know how to collimate their telescope and uses cheap eyepieces.
You also have to consider that if you have two $300 telescopes, one has a spherical and one has a parabolic, it is entirely possible that the parabolic model cut costs somewhere else which actually makes it worse, such as in the eyepieces, focuser, or worse still, in the mount. The entire package needs to be considered, not just the mirror.
Your choice of locations is also far more important than your mirror. Someone with a spherical mirror out in a designated dark sky site will have infinitely better views than someone within a hundred miles of a large city who has a parabolic mirror. Read about that in this interesting article, Do dark skies really matter.
In other words, the type of mirror in your telescope is just one factor among many, and probably one of the factors you can not really do much about.
My suggestion is to quit chasing rabbits, buy a reasonable telescope from a name brand like Celestron, Sky-Watcher, etc, learn to collimate it well, allow it to acclimate completely before using it, and buy reasonably good eyepieces. Doing this will have far more impact on the quality of your viewing than holding out for a parabolic mirror that you don’t even have the equipment to verify it actually is parabolic.