My main telescope is a Newtonian Skywatcher with a diameter of 250 mm and a primary focal length of 1,200 mm (thus, F/4.8). This is a relatively fast optical system well suited for deep sky and rather wide fields of view. For planetary imaging, you will normally want much larger basic focal lengths to achieve a narrow field of view thus allowing you to ‘zoom into’ the smaller sky objects, like the planets.
As a reference, Jupiter, the largest planet in the sky (and in our solar system), is just around 40 arc-seconds. One arc-second is only a tiny 1/3600 of a degree. The Andromeda Galaxy, at about 3 degrees in its main axis, is about 270 times the size of Jupiter, and the Moon, with 0.5 degrees or 32 arc-minutes, about 45 times the size of Jupiter. By the way, it sounds kind of shocking, but yes, the Andromeda Galaxy (M31) is about 6 times bigger in the sky than the moon. Mars is about half the size of Jupiter (not the real size, but the apparent size in the sky)
In order to get a decent image-size of, say, Jupiter, you will need to work at relatively high F numbers, like F/20, F/25 or F/30 (F = Equivalent Focal Length / Diameter). For a given optical tube (where the diameter is already fixed), the only way to increase the equivalent focal length is to add optical elements that effectively ‘stretch’ the original focal length by ‘bending’ the light rays coming from the primary mirror (or primary lens-system in a refractor). The standard method to do this is to add a “Barlow” lens system. Typical Barlows are 2x, 2.5x, 3x, 4x and even 5x. Meaning, they do the equivalent of stretching the original focal length 2 times, or 2.5 times, … The Primary (mirror or lens) + Barlow setup is the most common method when using a camera.
Another way to get magnification is to use an eyepiece (basically, a small magnification lens). The magnification achieved by the system composed by a primary mirror (or lens) and the eyepiece is simply the focal length of the primary divided by the focal length of the eyepiece. This is the standard method when looking with your eye through a telescope. But is can also be used for camera imaging.
And, of course, you can combine both, Barlow + eyepiece. Maybe not the best arrangement, as you should always try to minimize the number of elements in your optical system, specially of lenses, which inevitably introduce imperfections (even if small) in the imaging process (reduce light by reflection or absorption, color aberrations, geometric distortions …).
In any case, I have been playing with combinations of the elements to see how they compare and ‘how far’ (magnification versus image quality) I can take each setup.
