The use of this corrective lens is very useful when the solar telescopes of the "Allround" series are used for astrophotography at night. It is not necessary for using the Apos as a solar telescope.
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.
The reducer shortens the focal length of the telescope and is therefore also called a focal reducer. This makes the optics "faster" for photography and the image brighter. It also increases the field of view.
Since a stronger signal is captured in the same exposure time with a reducer, you can expect a higher signal-to-noise ratio and thus better detail in your images.
