I rescued a dying sun coral from my lfs some time ago and it didn't want to open up for about 3 months, but when it did, it became one of my most prized corals! I believe that you really need to be persistent and patient with them. I feed mine once a week (should feed more, but I'm lazy

) by taking it out of the tank and putting it in a bowl of tank water to feed. This way I can just spray food all over it without worrying about polluting my water or having my shrimp and hermits crawl all over the coral and steal the food right out of their mouths! I wait until the polyps have completely eaten their fill before I put it back in the tank.
My method is as follows:
Put coral in bowl of tank water.
Put food in feeding syringe and spray over each polyp.
Feed again once polyps have "sucked up" food.
Repeat several times.
Return coral to tank by quickly shaking off leftover food in bowl and then quickly lifting coral out of bowl and back into tank.
I feed mine a combination of Hikari frozen brine shrimp and mysis shrimp.
As stated, they are not photosynthetic so light means nothing to them. In fact, too much light can cause nuisance algae to grow on any exposed skeleton and can irritate the polyps. Mine lives in a 10gal nano lit with only a single 15w NO actinic strip. I've put them in bright lighting before and found that there was no preference between bright or subdued lighting.
Good luck with your new sun coral! Once they get going, they are very nice
