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.