Sharp stars across the entire image
Field correctors improve the image away from the center so stars remain as point-like as possible toward the edges of astrophotographs. Depending on the telescope, this may be a field flattener for a refractor or a coma corrector for a Newtonian telescope.
Correctors are designed for particular optical systems, focal ratios and sensor sizes. Check compatibility carefully, especially the required distance between the corrector and the camera sensor.
Field flattener or coma corrector?
Many refractors have a curved focal plane. A matching field flattener flattens the image field for photography, helping stars remain sharp toward the sensor corners. A standard flattener usually does not change focal length; still, check the stated factor and product specifications.
Newtonian telescopes often show coma near the edge of the field, where stars appear stretched like small comets. A coma corrector is designed to address this aberration. The right model depends in part on the Newtonian’s focal ratio. Some correctors are designed for a particular range, such as approximately f/3.5 to f/6, or optimized especially for f/4.
Check compatibility before buying
- Telescope and optical design: Choose a corrector explicitly suited to your refractor or Newtonian. Some flatteners are designed for specific telescope models.
- Focal ratio: Compare your telescope’s f-number with the corrector’s supported range. A different optical system may not be corrected adequately at the edges.
- Sensor size and image circle: Check that the corrected and illuminated image area covers the sensor diagonal. APS-C and full-frame sensors have different requirements.
- Back focus: The specified distance between corrector and sensor is important. Include filters, adapters, the camera body and, where applicable, an off-axis guider.
- Connections: Check barrel size, threads such as T2, M48 or M68, and any required camera adapters. A camera-bayonet adapter or T-ring may need to be purchased separately.
- Focus and accessories: Make sure the corrector fits your imaging train and that your telescope can reach focus with it installed.
Examples in this category
The range includes model-specific flatteners for Skywatcher refractors such as Evoguide and Esprit, flatteners for selected refractors around f/5.5 to f/6, and coma correctors for Newtonian telescopes. Depending on the model, connections may include T2 or M48; supported sensor size and back focus vary. Always use the specifications on the individual product page.
A reducer also shortens focal length and widens the field of view. Some products combine both functions. See the focal reducers and reducer-flattener combinations categories.