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Optical Design

This is the optical design of the telescope, i.e. APO Triplet Refractor. You can reference almost all popular consumer optical designs and their pros and cons above.


Aperture is the diameter of the front opening of the telescope. It’s the most important indicator of a telescope’s performance. The larger the aperture, the more details a telescope will be able to resolve. Aperture is usually measured in either inches or millimeters (mm).

Focal Length

Like in a camera lens, focal length is the measurement of how far the light travels once it enters the main optic of a telescope until it reaches the point of focus. This measurement is half of what determines how zoomed in the view or image will be, with the eyepiece or the camera being the other half. Focal length is usually measured in millimeters (mm). Generally speaking, a telescope with a focal length under 500mm is considered short, 500-1000mm is considered medium, 1000-2000mm is considered long, and 2000mm and up is considered very long. For beginner astrophotographers, a short to medium focal length telescope is recommended to begin.

Focal Ratio (F/Ratio)

Focal ratio is a measurement used to determine how fast a telescope gathers light. It’s a simple formula that takes the focal length divided by the aperture. For example, if a telescope has a focal length of 480mm and an aperture of 80mm, it would have a focal ratio of f/6. Fast scopes (usually f/7 or lower) gather light quicker, but usually lack the long focal lengths used to get zoomed in views. Fast, shorter focal length scopes are best suited for deep sky object viewing and imaging. Slow scopes (usually f/8 or higher) gather light slower and are generally more zoomed in, but usually lack the light-gathering power to reveal faint details for deep sky astrophotography. Slower, longer focal length scopes are best suited for planetary/lunar viewing and imaging.


OTA stands for Optical Tube Assembly. In today's terms, this phrase is simply a fancy way of referring to a telescope on its own. When you see a telescope with "OTA" or "OTA only" in its description, that means that the product only includes the telescope and not a mount or other components.

Optical Tube/OTA Weight

This is the total weight of the telescope only, usually not including accessories like a finder scope or something else unless otherwise specified. This is an important number to pay attention to if you’re buying a telescope and mount separately, as many mounts have a maximum payload (weight) capacity that they can handle. For visual use, the OTA Weight can be a few pounds/kg below the payload capacity of a mount. For deep sky astrophotography, though, a highly recommended rule of thumb is that you keep the OTA Weight around half of the total payload capacity for the mount. This is because deep sky astrophotography requires a much higher degree of accuracy than visual observing, and mounts perform at their best with lighter payloads.

Finder Scope

This is a small telescope or red-dot finder that rides on top of the main telescope. When looking for an object through a longer focal length telescope, it can be easy to get disoriented in space quickly. A finder scope’s purpose is to help you find objects using a much wider field of view. Once you’ve centered the object in your finder scope, the object should be readily visible in the main telescope.


A diagonal, or star diagonal, refers to the right angle part found in most refractor and catadioptric telescopes that reflects light out the back of the telescope and up into the eyepiece.

Highest Useful Magnification

This figure describes the most magnification you’ll want to use visually with the telescope in reference and any given eyepiece. Since the Earth’s atmosphere blurs and distorts the view for all ground-based telescopes, you won’t want to expect better than this magnification rate. A general rule of thumb is that the highest useful magnification is 50x its aperture in inches, or 2x its aperture in millimeters.