EDIT: I've come across a new challenge. My colonies with no minerals want protection, which requires ships, which requires maintenance facilities, which requires certain resources on said planet. Is there a way to say "keep 1000 of mineral X on this colony via mass drivers", or is it best to just make a big commercial maintenance ship and have it resupply when needed?
Assuming you're already delivering the minerals in question, open the Mining tab for that colony, double-click on minerals in question, and set the Reserve amount to the desired threshold (1000, in this case). This will retain that amount as long as it can, though as AIStar noted, the reserve won't ensure that more minerals are delivered if the colony runs low. Otherwise, it may be easier to just to drop off maintenance supplies from home every once in a while.
Tell me if my reasoning for how to build sensors is wrong or not:
Active sensors should be built to be your weapon's maximum range plus expected travel time per 5 seconds, for maximized range.
Since I don't have missile weapons yet, and my resolution 1 active sensors for missile defense covers my larger laser's max range. I don't need anything other than that active sensor, and can save long range detection for large thermals and EM sensors.
AKA: long range active sensors aren't worth building.
Corollary: long-range active sensors also provide two additional benefits.
1. Knowledge. Knowing where your enemy is from further out means you can maneuver to either give or avoid battle depending on relative strengths, assuming you also have the speed advantage.
2. Missile tracking. When firing at missiles specifically, your ships receive additional bonuses based on the length of time you've been tracking them. These bonuses are dependent on the research tech series "Max Tracking Time for Bonus vs Missiles" under "Sensors and Control Systems," scaling linearly if I recall properly. As such, it's better to build your anti-missile sensors to cover the necessary distance to maximize this bonus: add extra range to your fire control sensors based on the time before it hits weapons range multiplied by the tracking speed of your anti-missile defenses and/or expected speed of your opponent's missiles (if you have advance intelligence on your enemies due to an unfortunate first contact, these should be related in either case for obvious reasons), and you can try to maximize your accuracy just as your defense systems open fire. Working from my flawed memory again, this tech also shouldn't require redesigns, so it can be helpful to add some additional tolerance to your anti-missile fire control sensors in order to accommodate future software update packages from home.
This of course contrasts with the big disadvantage noted by Madman: when you turn on a searchlight in a dark room, everyone else can see it. In fact, if you are radiating and they are not, they can typically see it before you can see them if they have equivalent passive EM sensors. This may end up becoming another advantage if you're intentionally building a wild weasel. Large active sensors also require more uridium and incur higher maintenance costs both in failure rates and MSP to repair when compared either to smaller sensors, which may or may not be significant depending on mineral and tonnage constraints. This is also true when compared to equivalent-sized passive sensors, but this difference is far more marginal. The new sensor model also means it's nowhere near as critical to build massive active sensors as it used to be. As both actives and passives now scale according to the square root of the signal return, you'll hit diminishing returns as your sensors increase in size.
Worth noting, you don't need search sensors on every ship (contrasting with both types of fire control systems), as long as you have it available somewhere. As such, recommended usage can be linked loosely to your preferred doctrine. It can be helpful to set up large active sensors on your command ships if you choose to operate squadrons with dedicated command elements (potentially with possible smaller backups on the lighter ships for detached duty or if/when those command ships end up drawing fire due to said searchlights). If you go for a generalist option without dedicated command ships, it's a bit more likely that you may want to cut back on your actives to save space. On the flip side, a carrier-focused doctrine takes the idea of command ship squadrons to its furthest limit: actives are typically the first to get cut entirely from parasite craft due to strict tonnage limitations, since they can rely on their mothership to light up their targets for them.