Hi there...
I'm currently making a Comet map, and I have a question:
I saw on maps like AstroGears comets coming down (the first one, with the wraith explosions!)
Is there any easy way to do without specific locations?
How does a 0.5 grid works? or even more, is it able to make a 0.5 grid?

Generally you let an obs or wraith or whatever fly towards the comets destination and follow it by a 1 or a few locations which are always centered every second or so (depends on how long you'd like the trail). At those location(s) you create and kill wraiths or whatever you like for the explosions that make the trail.

To speed it up either you use a speed ground unit like a vulture or use the grids you mentioned.

Well a 0.5 grid would probably possible using rines or lings but I suppose you want an air grid.
The smallest air unit is an interceptor but I dont know if it can be used for grids.
Next smallest would be scourge but that would create a grid slightly smaller than 1x1.

a 0.5 air grid is a combination of the 1.5x1.5 air grid using devourers and the 1x1 grid using observers and scourges.

I think you still have the map where I made that 0.5 steps into the 8 directions?

How big are you going for the comet? I would use a devourer grid, much easier, although a bit bigger.

The way Tux did it in the map, I believe, is he had it pick 2 random numbers, then the grid would go down and right(or up and left, doesn't really matter) for that number of spaces, and check to see if any units were there, if not, it would go back a few spaces, create the meteor, tail, and do the meteor. If there was a unit, it would just try again until it found a space without one, I'm betting he just had a death count that held any comets that should be fired, so it just kept trying until that death count reached 0.