How to probe mission runners
This is a scanning guide that I came across on EVE-O forums. It was posted by Damien Smith. Since the guy is a pirate, it describes how to probe for mission runners in low-sec but it applies just as well for safespot busting.
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To use a probe launcher you need astrometrics 1, which will allow you to use the launcher and 3au probes. This isn't much use though. What you need ideally is a race of your choice's covert ops frig (racial frig 5, elecs upgrades 5, electronics 5, cloaking 1, covert ops 1) and then train the covert ops skill up a bit to reduce the cpu need of the covert cloak, otherwise you'll not be able to online the probe launcher without cpu's.
To bust the average mission in a well laid out system you can get away with using 48au probes (astrometrics 4) and then drop them in a triangle around the target, going for maximum coverage of the system while keeping each probe in range of the target. The range is half the listed distance of the target (debatable) so you have to drop each 3au probe within 1.5au, each 12au within 6au, and each 48au probe within 24au of the target. This is because the listed distance is the probes scanning diameter, not it's radius.
The most common way to probe someone is to start with a wide area scan with 48's, then at the first result drop a 12. You then scan with that 12 and two of the remaining 48's, whichever are in the best position to keep the triangulation around the target. You then drop a second 12 at the second result, and scan with 2 12's and 1 48, and so on. Once you get to 3x 12au probes you can scan with those, and the result they give you will be within 1.5au, which will allow you to use 3au probes.
With 3au probes at the last result, you can try dropping all 3 in the same spot. This often works. If it doesn't then rescan with the 12au's and get a second result, then drop a 3au there, then scan with that one and two of your first 3au's. If that doesn't work then make a final scan with the 12's and drop a 3rd 3au probe. This should work.
A good tip is to bookmark the location of every probe you drop. This makes it easy to redrop expired probes, and gives you warp points that you can use to make bookmarks that are slightly above or below the targets plane for when you're right next to the guy according to the scanner, but your 3au probes just won't pick him up. Just make a few warps around him and try to get some better positions to drop your probes.
I've left out the usage of 192au and observator probes as they're for busting extreme safespots and most people can't bust an average mission before it expires if you have to start from that far away.
Basically though 192au probes are bigger versions of the 48's, and observators are probes that only need one of to start a scan, and they scan the entire system. The downfall of these are that they're very inaccurate. Great for making supersafespots though.