See also Wikiepeda on Newtonians
and scroll down to Jones-Bird.
This Wikipedia image shows that a parabola focuses all incoming light to one point:
(The other way to think of this is that your automobile headlights are parabolic mirrors with the lamp at the focus. The beam is a column of parallel rays.)
A spherical reflector must of necessity create a fuzzy image where the "focus" should be.
(From the University of Texas physics department. Prof. Richard FitzPatrick.)
http://farside.ph.utexas.edu/teaching/3 ... de136.html
Of all the inexpensive beginner telescopes on the market right now (2019), the Celestron 127
"A good way to tell if a telescope is a Bird-Jones is to compare the focal length to the physical length of the telescope tube. The above-mentioned [Celestron 127 EQ] PowerSeeker has a focal length of 1,000 mm, but the physical tube size is 508 mm. This means that there must be something to double the effective focal length, which would be the correcting Barlow lens." -- http://astrowiki.jmhastronomy.com/index ... _Telescope