Reduce focal length and correct the image field
A reducer-flattener combines two functions: it shortens the effective focal length and corrects the image field for astrophotography. Your telescope captures a wider area of the sky, while stars can remain sharp across more of the sensor.
The optical design matters: many models are made for specific telescopes, focal ratios and camera spacing. Check compatibility and the supported image circle in the individual product specifications.
Two functions in one optical element
The reduction factor shortens the effective focal length and changes the focal ratio accordingly. For example, a 0.8× model turns a telescope with a 560 mm focal length into approximately 448 mm; with the same sensor, the captured area of the sky becomes wider. The flattener also corrects field curvature so stars can remain sharp toward the edges. The actual image circle and image quality depend on the correct combination of telescope, corrector, spacing and sensor.
Which reducer-flattener is right?
Products in this category are not universal adapters. Options include specific versions for Schmidt-Cassegrain telescopes, reducer-flatteners for selected ED and APO refractors, and models for RC or other flat-field telescopes. Reduction factors vary by product, for example 0.63× for certain SC systems or 0.8×, 0.79× and 0.9× for selected refractors.
What to check before choosing
- Telescope type and model: Check the telescope series, aperture and focal length explicitly listed. A matching thread alone does not mean the optics are compatible.
- Focal ratio and reduction factor: The factor shortens focal length and makes the system faster. Make sure your telescope is within the supported focal-ratio range.
- Back focus and spacing: The distance from the reducer to the sensor must match the optical design. It is critical in some systems; account for the camera, filters, adapters and other accessories.
- Image circle and sensor: Check that the corrected area covers your sensor size. Not every reducer-flattener fully illuminates APS-C or full-frame sensors.
- Connections: Check SC threads, barrel size and camera connections such as T2, M48, M63, M68 or M92. Additional adapters may be required.
- Focusing and accessories: Make sure the corrector fits your imaging train and that your telescope can reach focus.
Examples in this range
The selection includes a 0.63× reducer-corrector for Schmidt-Cassegrain telescopes, 0.8× models for selected ED refractors and 0.67× telecompressors for RC and flat-field telescopes. Depending on the model, larger 0.79× correctors support sensors up to full frame. Back focus, threads and image circles vary considerably, so always use the specifications on the relevant product page.
If you only need to reduce focal length, see the focal reducers category. For field correction without a reducer, browse field flatteners and correctors.