It's an old question, and there are plenty of guides on the internet that provide better tutorials than this Q&A, so I am only adding this in case someone comes across this SE post:
There is no need to 'waste' prospector limpets on asteroids that have no deep cores.
Asteroids that have deep cores:
- Appear bright orange/yellow during a scan as you get close
- Will be round-ish in shape but quite lumpy
- Will have visible fissures (THIS IS THE IMPORTANT ONE)
I think void opals specifically can only be found in icy rings.
I also often find the deep core asteroids are near the edge of the belt, but YMMV on that.
If you use look-around while scanning then be aware that an asteroid to your left or right might not be as bright in color until you swing around to point at it.
Meaning; if it is the right shape but not as bright as you like, it may still be worth changing course and investigating it.
The best tip for spotting fissures on the asteroid is to turn on your ship's night vision.
I find that fissures stand out a lot better with night vision turned on.
The only time that night vision doesn't work so well is when the belt is lit up by a nearby star, but this rarely happens.
Don't shoot a prospector into an asteroid unless it has fissures.
No fissure = no deep core.
Other than that, hit YT to find videos on deep core mining.