HERE is a web page that really goes into detail about the impact (actually the LACK of impact) of long versus short focal ratio.
If you don't want to read the complete article, below is the conclusion from that page…
“Telescopes of equal aperture are affected the same by atmospheric turbulence, regardless of focal ratio. The error in the hypothesis is that it was assumed that the same atmospheric distortion will cause the same shift in the best focus position in the two telescopes, and this is not true. While the high f-number telescope does enjoy a greater depth of focus, unfortunately the shift in best focus caused by turbulence is also greater. In fact, the two are locked together; the instrument with four times greater depth of focus also has a four times greater linear shift of the best focus position.
There is a long list of valid reasons why high f-number telescopes often perform better than faster ones. Some important reasons are:
a) Slower (i.e., high f-number) optics are exponentially easier to fabricate to the same accuracy as faster optics.
b) As already mentioned, the greater depth of focus of the high f-number telescopes makes them easier to precisely focus.
c) High f-number telescopes have a larger region of the focal plane that is diffraction limited, so off-axis performance is better. This is especially true with Newtonians, where coma and eyepiece astigmatism (mostly the latter) can be noticeable off-axis problems in fast instruments.
d) Slower optics are easier to collimate accurately, and there are less detrimental optical implications to slight misalignments.
e) Many eyepieces perform better with a higher f-number.
f) When comparing two Newtonian reflectors, slower scopes usually have smaller secondary mirrors. While the difference in image quality between, say, a 15% and 20% obstructed telescope is hard to detect, it would be a contributing factor.
If you happen to be observing through two telescopes of the same aperture on the same night, and the longer focus telescope is performing better, some of the reasons stated above are likely to be the explanation. Also, are the two scopes you're comparing of the same design type (i.e., reflector, refractor, SCT)? If not, they most likely have different thermal behaviors.”
I have never looked through a long focus achro (including 4” F15 Edmunds, 6” F15 A Jaegers, and a few others) that bested a “top quality” apo on the moon or planets. And the quality and precision of today’s high-end apos has eliminated the “slower scopes are easier to manufacture” argument. And with today’s 2-speed focusers, achieving exact focus in a faster apo is easy.
Sided-by-side the differences are more readily seen. That does not mean the achros put up poor images. Of course they did not. But the color purity, high quality glass, and high optical figure of modern top-end Apos is unrivaled."
Edited by bobhen, 14 September 2020 - 05:30 PM.