The 5" Konus is an excellent entry-level telescope. If you are in the UK you're going to have problems with Jupiter--for the next few years its orbit keeps it low to the horizon, so details will be hard to get. Saturn is moving into a rings-edge-on phase so we're going to lose a few years there too, unfortunately. With Mars you'll see polar caps and some dark markings on good night when Mars is fairly close (as right now). The person who said you'd see Olympus Mons was regrettably a tad over-enthusiastic. This is a 5" Konus not a 5" Takahashi, and even so, Olympus Mons--a big volcanic mountain on Mars--is an extremely difficult visual target.The more general answer to your question is that you can do a lifetime of observing with a 5" aperture, once you learn to appreciate its limits. If you like using a GermanEquatorial Mount, which is what came with your mount, then you could at some future point get a more stable set up (and less frustration hunting for things) by looking into a Vixen Great Polaris or Super Polaris. Otherwise at some point you might consider a Dobson configuration, but it won't track.
130mm is 130/25.4 = 5.1 inches.
1 mm=0.0393 inches
130mm = 130*0.0393 =5.1
So 130 mm is 5.1 inches wide
Direct Conversion Formula
130 mm * ( 1 in / 25.4 mm ) = 5.118110236 in
mm to inches
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