Here are a couple of sketches of Jupiter I did last month. I thought they might demonstrate the kind of views you can get of planets with a large Dob. The first was made on Saturday 1st April at 2am, and the second was on Saturday 8th April at 11.30pm. The scope was a 16 inch f 5 Dobsonian with a 1/10th wave primary mirror and a good quality unspecified secondary. If I remember correctly, I worked out that the secondary mirror covers 21% of the area of the primary. (Anything less than 20% is supposed to be good for planets.) Teeters Telescopes in the US do a 16 inch f 5 Dob Planet Killer, so they must think it’s a good spec scope for planets. I realise you’d get better views with a 6 inch apochromatic refractor. I brought my scope for deep sky objects, but was pleasantly surprised by what I could see on Jupiter.
Half the trouble with observing planets with a Newtonian is tube currents. This isn’t an issue with this scope – because it has no tube. I usually pull up the shroud on the upper side just above the primary, to let out any warm air.
Obviously the main thing when observing planets is to wait until they’re high in the sky. The light is then passing through the atmosphere at more of a right angle, meaning the light travels a shorter distance through the air. This results in less blurring. I also avoid looking over houses close by, as warm air rises from these too.