In order to avoid any brightness reduction at the corners of the camera sensor (vignetting) for astrophotography, the field of view delivered by the telescope must be sufficiently large. The required diameter of the image field depends on the size of the camera’s sensor.
For DSLR cameras: when using a camera with an APS-C format sensor (approximately 15mm x 23mm), the field of view should have a diameter of 28mm. For a full frame camera with a sensor size of 24mm x 36mm, a minimum diameter of 42mm is recommended.
Optical image defects and image field curvature are ever more noticeable at the edge of the image field. A field of view is described as evenly illuminated (or corrected) if it shows mostly point-shaped stars up to the diameter specified.