The flattener is a lens that evens out the field, as this is slightly curved by the primary optics. This curvature causes stars that are imaged at the edge of the field of view to be less sharp. The flattener, also called a field flattener, corrects this effect. Astrophotographers can then enjoy images where the stars remain sharp right out to the edge of the exposure. The flattener is installed between the telescope and the camera.
This field flattener is ideal for refractors, such as PrimaLuceLab Apochromatic refractor AP 120/900 ED Airy OTA or comparable optics from other manufacturers.
Ideally, the sensor should sit about 55 millimetres behind the flattener. This distance is achieved quite well with an SLR camera (DSLR) using a T2 ring for the camera bayonet (see illustration on the right).
When using an astro camera, it must be taken into account that the sensor is closer to the flattener than with a DSLR. The ideal distance of 55mm can be restored using extension tubes with a T2 thread. Example calculation: The very popular camera Atik 428EX Mono has a flange focal distance of 13mm. This is the distance between the flattener and the sensor. To get to 55mm, an extension tube with an optical path of about 42mm is needed.
