1. What is it?
Before answering this question, the first thing you need to do is to figure out what’s focal reducer and flattener.
2. Focal reducer
The focal length of the astronomical telescope is fixed, and the focal ratio is also fixed. In astrophotography, sometimes the original focal length of the telescope will be too big or the focal ratio will be too small, which is not conducive to the photography of nebula and star clusters (nebula and star cluster photography require a large field of view). Using a group of lenses shortens and reduces the focal length of the telescope to make the focal ratio bigger, expands the image range, and corrects the image field and aberrations. This kind of optical accessory is called Reducer. The focus reducer can shorten the focal length of the telescope to approximately 0.6x-0.8x of the original focal length, but the focus reduction should not be too large, otherwise, the image quality around the field of view will be affected.
The flattener is to add a thick lens to the lens system of the eyepiece to correct the defects of curvature of field and improve image quality at the edge of the field of view. It is a set of lenses used to correct the unevenness of the telescope's field of view. The image plane of the telescope is not flat, it’s curved. The star points around the edges of the photo are radial, which is the curvature of the field. With a flattener, the image plane can be corrected to be flat.
4. The ideas
Generally speaking, the main mirror will form a curved image field at its focal plane as shown in the figure. Adding a specifically designed lens group can effectively improve the curvature of the image field to adapt to the CCD/CMOS sensor of cameras. The focal plane position moves forward.