Can you catch legendary Pokemon without a master ball?

Of course you can. An easy way to do this is get a Pokemon that knows the moves Hypnosis or Spore, and put the desired Pokemon to sleep. Next, whittle away at its HP with not-very-effective moves. Take a long time, so the ever-handy Timer Ball can come into effect. Then use any of the following Pokeballs. I have listed them in preferable order: Ultra Dusk, (if in cave or night time) Timer ( in obvious circumstances) Heal Net (if the type is desirable) Great Poke