lighting

i have a 600 w hps about 60 cm above and the leaves are starting to curl at the sides are my lights to close?

Are the top leafs turning a lighter shade of green? I would think that they are above plant plenty, I would think that it is heat alone and not the light. Unless you have seedlings or veging that are under 25 cm tall.
What you want to do is blow heat away from bulb and out of room, then see about lowering the lights to 48 cm. Must get heat lowered first
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sounds like heat stress but your lights sound like there high enuff 60 cms is heaps , whats your ambient temp in your room ?if u have a spare fan focus it on the light to push the heat away