What is the difference between refractive and reflective telescopes? Let’s get an in-depth look at what makes a telescope to be refractive or reflective, what are the pros and cons of both designs, and answer the question: is a reflector or refractor telescope better for beginners?
It’s important to note that the refractive vs reflective terminology is the same as the refractor vs reflector. The words refractive and reflective indicates the physical phenomenon while the words refractor and reflector applies only for telescopes but have the same physics meaning.
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Refractive vs reflective introduction
Refractive vs reflective terminology
It’s important to note that the refractive vs reflective terminology is the same as the refractor vs reflector. The words refractive and reflective indicates the physical phenomenon while the words refractor and reflector applies only for telescopes but have the same physics meaning as refractive vs reflective.
Refraction and reflection of light
Refraction is the bending of light as it passes through a medium with a different refractive index. Reflection is the bouncing of light off a surface, such as a mirror.
The refraction is what happens when light passes through through a medium with a different refractive index, like a lens. This bending causes objects to appear differently than their actual position, and is responsible for phenomena such as mirages and the distorted appearance of objects when viewed through a medium such as water. It is also the reason why a stick appears bent when partially submerged in water.
Reflection is what happens when light meets a reflective surface like a mirror. When a ray of light strikes a smooth, flat surface, like a mirror, it reflects off that surface at the same angle as it hits the surface. This is known as specular reflection. When light strikes a rough surface, it is scattered in many directions, this is known as diffuse reflection.
The main two types of telescopes are based on these two properties, refractive vs reflective or refractors vs reflectors.
Refractive telescopes, also known as refractors, have a long history dating back to the 17th century. They typically have a long, narrow tube with a lens at the front, called the objective lens, which collects and focuses light. The eyepiece, located at the back of the telescope, magnifies the focused light and allows the observer to see the image.
Refractors are known for their sharp and high-contrast images, making them ideal for observing planets and double stars. However, they are also more expensive and can be more sensitive to misalignment or imperfections in the lenses.
Chromatic aberration in telescopes is an optical phenomenon in which light of different colors is bent by different amounts as it passes through the lens or mirror of a telescope. This results in a distorted or blurry image, with colors appearing fringes or halos around objects.
There are mainly two types of Chromatic aberration. Longitudinal Chromatic Aberration occurs when different colors of light are focused at different distances, and is more common in refracting telescopes. Lateral Chromatic Aberration occurs when different colors of light are focused at different positions on the same plane, and is more common in reflecting telescopes. So, comparing refractive vs reflective is important in terms of chromatic aberrations.
To fix this problem, some telescopes use special types of lenses or mirrors that can reduce or eliminate chromatic aberration, such as achromatic lenses, apochromatic lenses, or special coatings.
Different types of refractors
There are several types of refractor telescopes, each with its own set of characteristics and uses. The most common types of refractor telescopes include:
- Achromatic refractor: This type of telescope uses two lenses, made of different types of glass, to correct for chromatic aberration (color distortion) to some degree. They are relatively inexpensive and are a good option for beginners.
- Apochromatic refractor: This type of telescope uses three or more lenses, made of different types of glass, to correct for chromatic aberration to a greater degree than an achromatic refractor. They are more expensive and are preferred by experienced astronomers and professionals.
- ED (extra-low dispersion) refractor: This type of telescope uses special types of glass that has very low dispersion of light, which helps to reduce color distortion even further. They are more expensive than apochromatic refractors, and are preferred by experienced astronomers for high-resolution observations.
- Doublet refractor: This type of telescope uses two lenses to correct for chromatic aberration. It is similar to the achromatic refractor, but the lenses are made from different types of glass which give better color correction.
- Triplet refractor: This type of telescope uses three lenses to correct for chromatic aberration. It is similar to the apochromatic refractor, but the lenses are made from different types of glass which give better color correction.
- Petzval refractor: This type of telescope uses a special type of lens design that was invented in the 19th century, which allows for a very short focal length and a wide field of view. Petzval refractors are typically used for wide-field photography.
Pros and cons of refractors
- They are easy to use, as they have relatively few moving parts.
- They offer excellent image quality and contrast, making them great for viewing planets and other deep-sky objects.
- They have long focal lengths, which allows for high magnification and detailed observation of small objects.
- They are generally more lightweight and portable than reflecting telescopes.
- They are relatively easy to maintain and clean.
- Their tube is always closed so the optics are protected from humidity and dust. It’s an important point when comparing refractive vs reflective.
- They are relatively expensive compared to reflective telescopes of the same aperture size.
- They have relatively smaller diameter than reflectors so they collect less light and more difficult to observe faint objects.
- The lenses can be chromatic, except if they are design to be achromatic.
- They can be affected by thermal expansion and contraction, which can cause image distortion.
- Need to be careful when cleaning external lenses to protect the coating and prevent chromatic aberrations.
- They are heavy and bulky.
On the other hand, reflective telescopes, also known as reflectors, use mirrors to collect and focus light. The most common type of reflector is the Newtonian telescope, named after Sir Isaac Newton, which uses a concave primary mirror at the front of the telescope to collect and focus light onto a flat secondary mirror, which in turn reflects the light through an opening in the primary mirror to the eyepiece.
Reflectors are typically less expensive and more forgiving of misalignment and imperfections than refractors, but they can produce images that are less sharp.
Telescope mirrors are coated with a thin layer of metal in order to enhance their reflectivity and protect them from damage. The most common types of coatings used on telescope mirrors are aluminum, silver, and gold.
Aluminum coatings are the most widely used type of coating for telescope mirrors. They are relatively inexpensive to produce and provide a high level of reflectivity (around 88%) for visible light. However, aluminum coatings are not as durable as other types of coatings and can be easily scratched or damaged.
Silver coatings are also commonly used for telescope mirrors. They provide a slightly higher level of reflectivity (around 92%) than aluminum coatings, but are also more expensive to produce. Silver coatings are also more durable and resistant to damage than aluminum coatings.
Gold coatings are the most expensive type of coating used for telescope mirrors. They provide the highest level of reflectivity (around 96%) of all the coatings, but they are also the most delicate and susceptible to damage. Gold coatings are used in high-end telescopes and are typically reserved for specialized applications where the highest level of reflectivity is required. They are also more performant in infrared.
In addition to these traditional coatings, there are also newer coatings being developed such as dielectric coatings which are made of multiple thin layers of dielectric material, which reflects certain wavelengths of light better than others. They are used in high-end telescopes and are typically reserved for specialized applications where the highest level of reflectivity is required.
When choosing a telescope mirror coating, it’s important to consider the intended use of the telescope and the level of reflectivity and durability that is required. For general observing and casual use, an aluminum coating may be sufficient. However, for more specialized applications or for those who want the highest level of performance, a silver or gold coating may be a better choice.
It’s also important to note that refractive telescope lenses are also coating, but their coating is anti-reflective to maximize the light transmitted through the telescope. So, refractive vs reflective telescopes have both coatings but different ones.
Different types of reflectors
There following list gives the main types of reflecting telescopes:
- Newtonian Reflector: This type of telescope uses a concave primary mirror to reflect light onto a flat secondary mirror, which then reflects the light out the side of the telescope tube to the eyepiece. They are relatively inexpensive, and are a good option for beginners and amateur astronomers.
- Cassegrain Reflector: This type of telescope uses a concave primary mirror to reflect light onto a convex secondary mirror, which then reflects the light back through a hole in the center of the primary mirror to the eyepiece. They are more compact than Newtonian reflectors and are preferred by some for their portability.
- Dobsonian Reflector: This type of telescope uses a Newtonian reflector design, but with a simplified and more cost-effective mounting system. The telescope is mounted on a simple alt-azimuth mount, which allows it to be easily moved and pointed in any direction. They are relatively inexpensive and are a good option for beginners and amateur astronomers.
- Ritchey-Chrétien Reflector: This type of telescope uses a specialized type of Cassegrain reflector design that uses a concave hyperbolic primary mirror and a convex hyperbolic secondary mirror. This design corrects for some of the aberrations found in traditional Cassegrain and Newtonian reflectors. They are more expensive and are typically used by professional astronomers.
- Maksutov-Cassegrain Reflector: This type of telescope uses a combination of a spherical primary mirror and a thick meniscus corrector lens. This design corrects for some of the aberrations found in traditional Cassegrain and Newtonian reflectors. They are more expensive and are preferred by some for their compactness and portability.
Pros and cons of reflectors
- They can have large mirrors so they can collect more light than refractors and detect faint objects.
- They have no chromatic aberration, which means that all colors of light are brought to the same focus point.
- They are relatively inexpensive compared to refractive telescopes of the same aperture size.
- They can be lightweight when tube is open and they can be compact for some designs.
- They are less affected by thermal expansion and contraction, which can cause image distortion.
- They have a problem called spherical aberration, which means that light is not brought to a single focal point, creating blurrier image than refractors.
- They need to be collimated, which means that the mirrors need to be aligned correctly, otherwise the image will be distorted.
- They are more difficult to maintain and clean, especially the primary mirror, which is located at the bottom of the telescope tube.
- They can be bulky and heavy when having larger primary mirror.
Both refractive and reflective telescopes have their own advantages and disadvantages, and the choice between them often comes down to personal preference and intended use. For example, professional astronomers may prefer reflectors for their large aperture and wide field of view because it’s more easy and les expensive to manufacture large mirrors than large lenses.
Amateur astronomers may prefer refractors for their portability and ease of use, but they are also smaller and more expensive than reflectors. Ultimately, refractive vs reflective comparison is a good starting point to look for the different types of telescopes, but both types can provide amazing views of the night sky, and it’s all about finding the best fit for you.
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