Flats or Secondary Mirrors The Secondary Mirror, or Diagonal, often referred to as the Flat, has two critical properties. Its quality and size. The quality of most systems is equal to the quality of their weakest part. Not so with telescopes.  Since Wavefront Errors are cumulative, the optical quality of a telescope is less than the optical quality of its weakest optical component.
For example: My main mirror with a P-V Wavefront error of 1/24 at 550 nm, used with my flat with a P-V Wavefront Error of 1/48 at 550 nm, produced a Combined Wavefront Error of 1/24 + 1/48 or 2/48 + 1/48 = 3/48 or 1/16. Thus my telescope's Combined P-V Wavefront Error is 1/16 at 550 nm.
While Flats require every bit as much attention as the Main Mirror to produce, they can be made with greater precision than curved mirrors, as they can be tested for precisions greater than the maximum precision of the Focault Test.