Why are telescope mirrors nearly flat? My understanding of telescope mirrors is that they are (generally speaking) a slice out of an imaginary sphere. The center of this sphere is the focal point of the telescope, where the detector is placed. The bigger the sphere, the further away the focal point will be.
With this in mind, why are telescope mirrors nearly flat? Would it not make sense to instead use the same diameter mirror, but with a smaller imaginary sphere so the detector doesn't have to be so far away?
I have made a crude image to demonstrate my point. The blue arc and point represent a more traditional &quot;nearly flat&quot; mirror, whereas the red arc and point represent a mirror that has the same diameter but with a smaller sphere and therefore closer detector.
The main reason is that the red mirror produces a smaller image in its image plane, which requires a smaller pixel spacing in the detector (or a stronger eyepiece) to get the same resolution.
In a Newtonian telescope the focal length cannot be too small:

(Image Krishnavedala)
Also, a more ideal shape is a paraboloid (not a sphere). Spherical mirrors are relatively easy to manufacture, and &quot;nearly flat&quot; ones are very close to paraboloids.