I've read at several places that long focal length telescopes due to their longer focal length are not suitable for watching nebulaes, but I don't understand where is this statement originated
So what is case to take long focal length scopes for planetary viewing and short ones for large deep sky objects?
First of all, let me make it clear that I do not agree with the statement that long-focal-length telescopes are unsuitable for viewing nebulae, and I disagree even more with the statement that short-focal-length telescopes are unsuitable for viewing planets.
However, on the whole, short focal ratios are preferable for deep-sky viewing, because they allow a wider range of choices in magnification. That particular factor is irrelevant for the planets, which are always viewed at high magnifications. So the fact that you have additional choices on the low-power end is irrelevant for planets.
Here's why short focal ratios offer a wider choice of magnifications. High magnifications are achieved by making the focal length of an eyepiece shorter. Rather than using a 20-mm eyepiece, you can use a 10-mm eyepiece to achieve twice the magnification. There is no limit to how small you can make an eyepiece's focal length, so you can always achieve as high a magnification as you want. There are other factors that limit magnification on the upper end, including the telescope's aperture, its optical quality, and the thermal stability of the telescope and the atmosphere. But there are no physical limitations.
To make the magnification lower you need to make the eyepiece's focal length longer. And beyond a certain point, that requires making the eyepiece's physical size bigger as well. A conventional 32-mm Plossl fits into the 1.25-inch focusers common in most inexpensive telescopes; a conventional 55-mm Plossl does not. And since the largest common focuser size even in expensive amateur telescopes is 2 inches, there is a limit to how low a magnifcation -- and hence wide a field -- you can achieve with a local-focal-length telescope.
So, in your example, if you wanted to achieve a true field of view of 1.36 degrees, you could use that 20-mm eyepiece in your 1000-mm focal-length telescope to achieve 50X. But to achieve the same in your 2000-mm focal-length telescope, you would need a 40-mm eyepiece. And a 40-mm eyepiece with 68-degree AFOV doesn't fit into a 1.25-inch barrel. It does (just barely) fit into a 2-inch barrel, so you could indeed get 1.36 degrees out of your 2000-mm focal-length telescope if it has a 2-inch focuser. But that's the limit; you cannot get more. With the 1000-mm scope, by contrast, a 2-inch focuser would let you go all the way up to a 2.72-degree field of view.
I am, by the way, saying exactly the same thing as kathyastro above, but explaining it in a different way. Her explanation is more precise; mine may (or may not) make more sense to some people.
The fact remains that most nebulae are much smaller than 1.32 degrees. So the inability to achieve an ultrawide true field of view in that 2000-mm focal-length scope, while perhaps annoying, is certainly not the end of the world.
Edited by Tony Flanders, 02 July 2020 - 08:02 AM.