Growing tomatoes under lights

I'm growing some tomato plants under a 400 watt metal halide lamp. They are about three weeks old and several are a few days from blooming. Right now I have them under 18 hours of light and 6 hours of darkness.
Should I continue to keep them under the MH lamp or should I switch over to my high pressure sodium lamp? Also, should I change the light schedule at all?
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Tomatoes = long day, since they bear fruit during summer. I'd switch to HPS anyway, since it costs less to run per usable lumen.
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My tomatoes first flower around the summer solstice. After that they set fruit and flower some more as the days begin to shorten.
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Billy

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Which would be close to 18-6.

Up until the equinox, with declining production until killing frost?

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it?
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Billy

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