The magnification is directly multiplied by the focal length. When using a 4x Powermate youll have 4x the focal length and 4x the magnification. 16x the exposure time to get the same brightness. So a bigger magnification does not necessarily mean youll have better planetary images. It depends a lot on seeing and atmospheric conditions.

Generally stacking smaller but crisper images will give you better results. Therefore my suggestion is to try first with the 2x barlow, and based on those results to try with a bigger magnification later. 5x Powermate is a good piece of equipment, it gives you a lot more magnification.

But keep in mind that bigger magnification doesnt necessarily mean youll get better planetary images. On a good night it will, on a bad night youll only get less images to stack. Cloudynights, IMO its the best forum for these things. Ill give you some criteria used in choosing a lens, and then suggest a few that I would use for this purpose.

Zoom or prime zoom lenses are lenses that cover multiple focal lengths. Prime lenses cover a single focal length. Zooms give you greater flexibility, but primes offer superior image quality at a given price point.

This means that your sensor is significantly smaller than 35mm film, and only uses the center of the image circle created by a standard lens. Larger apertures give you a few capabilities. This is VITAL for sports photography, or any situation where the subject is apt to be moving. AF sensors on your camera. IS Some Canon lenses offer IS. IS is a gyroscopic device that lives in the back of the lens and tries to correct for lens movement. IS is great for shooting STATIC subjects in low light, as it will allow you to use a slower shutter speed than you would otherwise be able to. IS is almost useless for most sports, as the key issue is not camera movement, but getting a shutter fast enough to stop the subjects motion. In reality, this designation has more to do with being designed for professionals than for use as a luxury item. AND build quality than their consumer grade cousins. In my experience, some third party lenses offer an excellent value for the money. That said, ALL lenses have a certain variation in quality between copies of identical lenses. 3rd party lens from one of the three manufacturers I listed, and only if you can test it in store, or if the store has a fair return policy.

There ARE other concerns when buying a lens, but this should be more than enough to get you started. It refers to how ny times longer the longest focal length of the zoom range is compared to the shortest.