How close can MH be to acrylic top?

[Mariner]

Just got the lighting for my new (used) acrylic tank. I've got a 400w MH bulb about 4 1/2" to 5" above the acrylic center brace of the tank. The acrylic is 3/4" thick. Anyway, the MH has been running for two or three hours now. The acrylic feels warm to the touch, but not melting hot. What do you guys think? Will it be ok long term? I do have a fan in the canopy.
Mariner

[lvreefer]

When I use to service aquariums every now and then a center brace would crack. I have a feeling this is due to temperature swings. Acrylic tends to warp under heat, so keep an eye on it. Keeping the abient temperature down in the canopy or between the brace and the halide will be key.

[kdblove_99]

I have mine 12" and it still cracked the top. I hate Acrylic:)