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What is an OTA in A Telescope?

You might have come across the term OTA or Optical Tube Assembly. But what exactly is an OTA, and what does it do? In simple terms, an OTA is the main part of a telescope that collects and focuses light from celestial objects, allowing you to see them more clearly.

OTAs come in different shapes, sizes, and configurations, but they all have the same basic function. They contain the primary mirror or lens, the secondary mirror, and other optical components that work together to produce a clear and magnified image of the night sky. Depending on the type of OTA, you may need to attach it to a mount or tripod to stabilize it and adjust its position.

OTAs are standalone components that you can buy separately or as part of a complete telescope system. They are popular among amateur and professional astronomers who want to customize their telescopes or upgrade their existing equipment. With the right OTA, you can observe a wide range of celestial objects, from planets and stars to galaxies and nebulae, and explore the mysteries of the universe from the comfort of your backyard or a remote observation site.

What is an OTA?

If you’re interested in astronomy or stargazing, you’ve probably heard the term “OTA” before. OTA stands for Optical Tube Assembly, and it’s an essential component of a telescope. Essentially, the OTA is the part of the telescope that holds the optics, including the lenses or mirrors that gather and focus light.

While the term OTA can apply to any type of telescope, it’s most commonly associated with refracting and reflecting telescopes, which use lenses and mirrors, respectively, to gather and focus light. In these telescopes, the OTA is the long, cylindrical tube that houses the optics and protects them from dust, debris, and other environmental factors that could interfere with the telescope’s performance.

One of the benefits of an OTA is that it can be customized and configured to meet your specific needs. For example, you can choose an OTA with a larger or smaller aperture, depending on the amount of light you want to gather. Additionally, you can choose an OTA with a longer or shorter focal length, depending on whether you want to observe distant objects or closer ones.

Components of an OTA

Objective Lens

The objective lens is the primary lens of a refractor telescope. It is responsible for gathering and focusing the light that enters the telescope. The size and quality of the objective lens are important factors in determining the image quality of the telescope. A larger objective lens will allow more light to enter the telescope, resulting in a brighter image. Higher quality objective lenses will produce sharper, clearer images.


The focuser is the mechanism that allows you to adjust the position of the eyepiece. It is an essential component of any telescope, as it enables you to achieve a sharp focus on the object you are observing. There are two main types of focusers: rack-and-pinion and Crayford. Rack-and-pinion focusers are the most common type and are generally less expensive. Crayford focusers are more precise and smoother, but they are also more expensive.


The eyepiece is the lens that you look through to observe the image produced by the telescope. It is responsible for magnifying the image produced by the objective lens. Eyepieces come in a variety of focal lengths, and the focal length of the eyepiece determines the magnification of the image. Shorter focal length eyepieces provide higher magnification, but they also have a narrower field of view. Longer focal length eyepieces provide lower magnification, but they have a wider field of view. Overall, the components of an OTA work together to produce a clear, detailed image of the object you are observing. The quality of the components will determine the quality of the image, so it is important to choose high-quality components when selecting a telescope.

Types of OTAs

Refracting OTAs

Refracting telescopes use lenses to gather and focus light. They are typically more compact and portable than reflecting telescopes. Refracting OTAs are often used for planetary observation because they produce sharp, high-contrast images. They are also great for terrestrial viewing. One of the main advantages of refracting OTAs is their low maintenance requirements. They don’t require collimation like reflecting telescopes do. However, they can suffer from chromatic aberration, which is caused by the lens bending different colors of light at different angles. This can be mitigated by using special coatings on the lenses or by using an apochromatic refractor, which uses multiple lenses to correct for chromatic aberration.

Reflecting OTAs

Reflecting telescopes use mirrors to gather and focus light. They are typically larger and heavier than refracting telescopes, but they can gather more light and produce brighter images. Reflecting OTAs are often used for deep-sky observation because they can gather more light than refracting telescopes. One of the main advantages of reflecting OTAs is their ability to produce images free of chromatic aberration. However, they require occasional collimation to ensure that the mirrors are properly aligned. Reflecting OTAs can also suffer from coma, which is an aberration that causes stars to appear distorted near the edges of the field of view. This can be mitigated by using a coma corrector.

Catadioptric OTAs

Catadioptric telescopes use a combination of mirrors and lenses to gather and focus light. They are typically more expensive than refracting or reflecting telescopes, but they are also more versatile. Catadioptric OTAs are often used for astrophotography because they provide a wide field of view and produce sharp, high-contrast images. One of the main advantages of catadioptric OTAs is their compact size. They can be much shorter than reflecting telescopes of the same aperture, which makes them more portable. Catadioptric OTAs can suffer from some of the same aberrations as refracting and reflecting telescopes, but these can be corrected with additional lenses or mirrors.

OTA Sizes

When choosing an OTA for your telescope, one of the most important considerations is size. The size of the OTA determines the aperture and focal length of the telescope, which in turn affects its light-gathering ability, magnification power, and field of view. Here are some things to keep in mind when selecting an OTA size:


The aperture of an OTA is the diameter of its primary lens or mirror. The larger the aperture, the more light the telescope can gather, resulting in brighter and sharper images. However, larger apertures also mean larger and heavier OTAs, which can be more difficult to transport and set up. Here are some common OTA aperture sizes:

Aperture SizeBenefitsConsiderations
4-6 inchesLightweight and portableLower light-gathering ability
8-10 inchesGood balance of portability and light-gathering abilityMay require a larger mount and tripod
12-16 inchesExcellent light-gathering ability and image qualityHeavy and difficult to transport

Focal Length

The focal length of an OTA is the distance from its primary lens or mirror to the point where the light converges to create an image. Longer focal lengths result in higher magnification power, but a narrower field of view. Shorter focal lengths provide a wider field of view, but lower magnification power. Here are some common OTA focal length sizes:

  • Short focal length (less than 500mm): Ideal for wide-field viewing of large objects like galaxies and nebulae.
  • Medium focal length (500-1000mm): Good for observing planets, star clusters, and other smaller objects.
  • Long focal length (more than 1000mm): Best for high-magnification views of planets and other small objects.

When choosing an OTA size, it’s important to consider your observing goals, budget, and physical limitations. A larger OTA may provide better image quality, but it may not be practical for your needs. Consider your options carefully before making a decision.

OTA Mounting

When it comes to mounting your OTA, there are a few things to keep in mind to ensure a successful setup. Here are some tips to help you:

  • Make sure the mount and tripod you are using are compatible with the OTA you have.
  • Double-check that the OTA is securely attached to the mount or tripod before use.
  • Be sure to balance the OTA properly to avoid any issues with tracking or stability.

In addition to these general tips, there may be specific instructions for mounting your particular OTA. Be sure to consult the manufacturer’s manual for any specific guidelines.

When mounting your OTA, it is important to take your time and be careful. Rushing or being careless can result in damage to your equipment or even injury to yourself.


Now that you understand what an OTA is and how it works, you can make an informed decision when purchasing a telescope. Remember that OTAs come in different types and sizes, and you should choose one that best fits your needs and preferences.

If you are a beginner, a refractor OTA might be a good choice for you because they are easy to use and require little maintenance. However, if you are interested in observing faint objects or want to take astrophotography, a reflector OTA might be a better option because they have larger apertures and are more affordable.

When choosing an OTA, consider factors such as aperture, focal length, and mount compatibility. Also, keep in mind that the OTA is just one part of a telescope system, and you might need to purchase additional accessories such as eyepieces, filters, and a mount.

Overall, an OTA is an essential component of a telescope, and it determines the quality of the images you see. With the right OTA and accessories, you can explore the wonders of the universe and discover new things every night.