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How Reducer Affects the Light Path

Aug 18, 2025 by WO Team

A reducer is an optical accessory placed at the back of a telescope. As the name suggests, its primary function is to reduce the telescope’s focal length. Most reducers are also designed as flatteners, correcting field curvature while modifying the light path.

Why Use a Reducer?

Astrophotographers employ reducers for multiple reasons:
  • Shorten focal length → Capture a wider field of view.
  • Lower focal ratio (f/number) → Achieve brighter, faster exposures.
But why not simply use a telescope with a shorter native focal length?
  • Larger aperture advantage – A long focal length telescope typically has a larger objective lens. When paired with a reducer, it delivers both higher resolution and greater light-gathering power than a smaller short-focus scope.
  • Versatility – By changing reducer magnification (e.g., 0.8×, 0.7×), imagers can adapt the same telescope to match different targets, from wide nebulae to smaller galaxies.

Optical Effect of a Reducer

Parameters with the reducer are denoted by a prime (′), and the reducer factor is R.

Example with a 0.8× Reducer:

Parameter
Native System
With 0.8× Reducer
Effect
Focal Length
600 mm
480 mm
Shorter focal length widens field
Focal Ratio (f/number)
f/6
f/4.8
Faster, brighter imaging
Image Circle
44 mm
35 mm*
Smaller corrected circle*
Field of View (APS-C 22 mm sensor)
~2.1° × 1.4°
~2.6° × 1.7°
Wider FOV
*Practical Note:
In real setups, the effective image circle is often limited by mechanical components such as the rear adapter or thread diameter. Even though the optical design may provide a larger circle, the usable FOV may be clipped by these restrictions. Therefore, formulas provide theoretical values, but actual results depend on the telescope’s mechanical design and reducer integration.

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