I bought all the necessary equipment. Flew to a pristine ice ring. Scanned and fired a prospector limpet at 50 yellow asteroids. Zero were deep core. Do I need to fire the prospector limpet at a specific spot on the asteroid?
Here's a post I made on another forum a while back, might help, skip to no. 3

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I average 100 mil an hour with my Imperial Cutter, but I've gotten a lot of practice and am very efficient with opal mining, let me share you some tips.
1. I recommend starting in Solo mode.
2. Because your having problems with pirates, I recommend out fitting a fast ship that can out run them if you encounter them, for example an Orca. You can still get 64 tons of cargo space.
3. It doesn't matter where it is, but you need to find an ice ring close to your current location that is a pristine ice ring (Use eddb or inara). Whats more important is to find a ice ring that is in darkness, and not lit up with sun light. This is important, because a dark ring will make life a lot easier when trying to spot glowing cores (My ideal location is in system V886) .
4. Once you have found the ideal ice ring, and a void opal hot spot, drop out of super cruise near the edge of the hot spot, (you want to be heading to the center in a straight line as you harvest opals).
5. Turn on the night vision for your ship.
6. This will be a bit difficult to explain, do not go into the ice ring directly but position your ship in a way so that your ship cockpit is horizontal to the plane of asteroids in the ring (where you can see the rocks in the upper top of your cockpit).
7. Put all power to engines, and while holding down your pulse wave scanner, go full speed in a straight line towards hotspot centre waypoint while looking up for core rocks. Move your hud and look left to right (don't forget to keep boosting when you can).
8. Now this is important. To find the core rocks we only need understand two things. Small and bright. We will ignore any glowing rock that is not small and glowing. 99% of the time the smallest rock is the core rock.
9. Once you have found a small glowing core rock, go to it and launch a prospector. As you see the prospector moving towards the rock, make sure to select it to see what the actual core is once the prospector hits the rock. If the core is not an Opal go back to step number 6 and keep looking.
10. Once you have found a Opal Core, you will need to become efficient at busting it open, and this can only be done with practice. You should try to only ever use 2-4 charges at most when breaking these rocks open.
11. Before we start its important that you know how to move around the rock and maneuver your ship for efficient fast placement of charges. In terms of movement, we will mainly use the ships downward directional thrusters. Its hard to explain but I mean your ship will be moving around the asteroid rock vertically downwards with your directional thruster as you circle around the rock looking for fissures.
12. Go to your targeting panel/contacts. We always want to put chargers into asteroid fissures with low strength first. Select a asteroid fissure with low strength, maneuver your ship using the directional thrusters until your in front of the fissure. For the first low strength fissure, hold the charge till max and release. Select your second low strength fissure and hold the charge until at least two bars are highlighted before release. Sometimes two low strengh fissures will be good enough to get optimum yeld, if not you now need to select a high strength fissure and put a low yield charge into it (usually hold the charge until first bar is filled up).
13. Detonate the optimum yield and collect the goods.
Hope this helps.