From a fellow grower: When using my cooltubes to grow your auto flowering pot do I still use both my 600watt mh and hrs or can I just use one or the other for 18-6
Use your MH for veg on 18/6.
HPS for flowering on 12/12.
Leave the mh in for the first couple weeks after switch to help keep the nodes tighter together. 
Since the cultivar in question is an autoflower, I’d keep it lit for 16-20 hours out of 24 during flowering.
Sorry. Missed the auto part.
12/12 works fine for autoflowers. Saves a little on your bill too.
So use the mh for the first few weeks then change over hps?
Just getting back into indoor growing after 30 years. I’ve just started off with some old seeds just to get set up properly things have changed a fare bit since I last grew indoors oh and I’m using home made soil
When I ran my last autos I switched to the hps at 6 weeks I think. 
But they stretched pretty good. Up to 2 in a day. But that was after only 1 week from pistils.
Next time I’d go another week with mh.
The difference (that matters to us) between MH and HPS is spectrum of light. The MH has more blue light, watt for watt, than the HPS. Conversely, the HPS has more red light.
This matters because the blue spectrum seems to be responsible for internode distance; a light with more blue will result in shorter internode distances. Less fibrous stem growth might mean more leaf growth, which in turn means more growth overall.
The reddish spectrum of HPS seems beneficial for flowering. I’ll throw out there that while HPS is the standard, there are top notch breeder-cultivators that use MH exclusively and love the results.
TL&DR: Use the MH as long as you want to vegetate you’re plants. Use the HPS once you change to a 12/12 light cycle.
Edit: now I’m the one forgetting that the plants are autos.
P.s Now that you (hopefully) understand the benefits of MH, you can try swapping earlier or later and see what works best for your space and for your genetics.
Thanks your a champion have a good New Years
And you as well @Doggie.
