3" 0.79x Reducer & Field Flattener for APO Refractors

The 3" 0.79x reducer and field flattener is designed for astrophotography with high-quality APO refractor telescopes. It reduces focal length by 0.79x while correcting field curvature, delivering sharp stars across the entire image – even with large APS-C sensors such as the ASI2600.

This reducer is ideal for refractors with focal lengths around 800–1000mm, making it a perfect match for 127mm class telescopes. It provides a wider field of view, faster imaging, and improved edge-to-edge performance without requiring extreme backfocus distances.

Astrophotography Performance

Image: Crescent Nebula by Johannes Jensen
Setup used:
♦ 102mm f/7 APO refractor
♦ 0.79x reducer/flattener
♦ DSLR camera – 36 x 360 second exposures

Key Features

• 0.79x focal reduction for wider field and faster imaging
• Flattens field curvature for pinpoint stars across the frame
• Large 3" optics – ideal for APS-C sensors (e.g. ASI2600)
• Optimised for refractors around 800–1000mm focal length
• No extreme backfocus required
• Improves illumination and reduces edge distortion

Backfocus & Spacing

Backfocus distance is measured from the M48 thread to the camera sensor and depends on the telescope focal length:

• Up to 350mm – 70mm
• Up to 420mm – 65mm
• 450–500mm – 61mm
• 510–600mm – 58mm
• 610–700mm – 57mm
• 710–790mm – 56mm
• 800mm and above – 55mm

For most 127mm class refractors (~950mm focal length), the correct spacing is 55mm. Spacers and extension rings can be used to achieve precise backfocus.

Compatibility & Connection

The reducer connects directly via an M68 thread or can be inserted into a 3" focuser. Adapters are available for compatibility with a wide range of telescope focusers.

Suitable for:

• APO refractors with 2.5" or larger focusers
• 127mm class refractors (~900–1000mm focal length)
• APS-C cameras such as ASI2600MC Pro
• DSLR and mirrorless astrophotography setups

This is a high-performance solution for astrophotographers looking for a wider field, faster imaging speed, and excellent edge correction.