Focal length reducer Skywatcher ED120 0.85× for ED120 refractor

The Skywatcher ED120 0.85× focal length reducer is an optical accessory engineered for the ED120 refractor. It reduces the telescope's effective focal length by 15%, producing a wider field of view and a faster focal ratio that are useful for both astrophotography and visual observation. The reducer is intended to work with the ED120 optical tube to improve framing of extended deep-sky objects and to shorten exposure times when imaging.

Advantages

The focal length reducer provides a consistent 0.85× reduction, delivering a noticeably wider image circle while preserving the ED-corrected optical performance of the refractor. By lowering the effective focal ratio, it allows shorter exposure times for imaging and easier acquisition of larger targets without changing the main optical tube. The design keeps optical alignment simple and maintains good edge-to-edge performance when used within the compatible backfocus range.

Suitable for

• Wide-field astrophotography of deep-sky objects (nebulae, star clusters, galaxies)
• Visual observing when a broader field of view is required
• Reducing overall focal ratio for faster imaging with smaller sensors
• Improving framing and composition of extended targets

Key specifications

• Reduction factor: 0.85× (15% shorter focal length)
• Compatible optical type: designed for ED120 refractor tubes
• Effect on focal ratio: yields a faster focal ratio proportional to 0.85×
• Primary use: astrophotography and visual wide-field observation
• Optical performance: maintains ED-corrected imaging characteristics when used at specified backfocus

How to use

Attach the reducer between the ED120 telescope's focuser and the camera or visual diagonal following the manufacturer's recommended mechanical spacing (backfocus). Confirm the correct spacing to ensure the optical train reaches the specified focus and to preserve edge performance. Once installed, re-collimate if necessary, then focus and frame the target. For imaging, check focus across the field and make small adjustments to the camera spacing if field correction requires it.

Recommendations

Use the focal reducer together with the ED120 refractor at the recommended backfocus distance and pair it with suitably sized imaging sensors to take full advantage of the widened field. For best results in astrophotography, verify focus and field flatness with test exposures, and consider subframe calibration (dark, flat, bias) appropriate to your setup.