You will need to get armor with a max of 42 Light. 4 pieces of armor with 42 Light is necessary to reach the current level cap of 34. Armor that Varik's sells for Prison of Elder armor cores, and the armor that Brother Vance sells or that drops in the 8 win Trials of Osiris package has 42 Light. Prison of Elders armor can also come from Varik's Judgement's Chance packages. You can also ascend any legendary armor piece that has 36 light or less with Etheric Light, and it will then have 42 max Light. Etheric Light primarily comes from Prison of Elders level 34 and 35 challenges, Trials of Osiris (6 win and 8 win packages, and from the chest at the Lighthouse for a flawless card), Iron Banner (one can be purchased at rank 3 and rank 5 iron banner reputation), and there is a chance to get one Etheric Light as a reward from Nightfall.