Always use the focal-length of the telescope to determine which eyepieces and accessories to complement, in this case a barlow. The focal-length of your telescope is 650mm. The planets, and other objects, require higher powers, 150x for instance...
650mm ÷ 150x = a 4.3mm eyepiece
You can get a 4mm eyepiece with a larger eye-lens and greater eye-relief...
Per the 50x-per-inch maxim, your 5"(130mm) telescope can reach up to 250x, but the atmosphere and the collimation of the telescope can limit a power that high. Almost 200x would be possible regularly, if the atmosphere allows, and if the telescope is collimated well. The higher the power, the more precise the collimation will need to be. The quality of the eyepieces and accessories also plays a part in that success, but that doesn't mean that you have to spend a lot of money for each eyepiece, or barlow.
Plossls are comfortable eyepieces in the longer focal-lengths, 9mm and longer...
A 12mm Plossl can be combined with a 3x barlow, and for a simulated 4mm(163x), yet still with the larger eye-lens and greater eye-relief of the 12mm.
A 9mm Plossl can be combined with a 2x-barlow, and for an effective 4.5mm(144x).
That 2x-barlow also offers a 1.5x division, and for even more possibilities. With two eyepieces and a barlow, the equivalent of four eyepieces, magnifications, are at your disposal.
There are these barlows as well. The use of a Newtonian benefits from short barlows...
Welcome to the world of the Newtonian reflector. In that one was chosen, for the larger aperture, there are certain responsibilities that come with one; namely, collimation...
A telescope has to work harder as you go up in power, therefore the alignment of the two mirrors inside the telescope, in tandem with the focusser, must all have their centers lined up together just right, and for sharp, pleasing images.
Edited by Sky Muse, 24 November 2020 - 05:39 PM.