The correct use of sensors

How sight works:
All objects are assigned an obscuration level. A ship with a high level of obscuration is difficult to detect, while a low level of obscuration is easy to detect. Sensors provide varying levels of detection. In order to see an enemy vessel your sensors must provide a higher level of detection than the target's obscuration. Things like storms, nebulas, cloaking devices, and strange warp points can increase a ship's obscuration level, making it harder to see. Meanwhile, you can increase your detection level by improving your sensor technology or switching to better types of sensors.

Basic Sensors:
These provide a large radius of sight. You can see planets, asteroids, warp points, and "normal" ships and such. However, basic sensors provide only a low level of detection. Ships with cloaking devices or ships hiding in some kinds of nebulas will not be detected by basic sensors.

Tachyon Sensors:
These sensors provide a much higher level of detection, making them suitable for detecting cloaked ships and other obscured vessels. Unfortunately, tachyon sensors tend to provide a shorter detection range and are *extremely* expensive. You can't afford to place them on all your ships, so you're going to have some blind spots. Also note that you need high-level tachyon sensors in order to detect a high-level cloaking device; level 1 tachyon sensors probably won't detect a level 3 cloaking device.

Long Range Scanners:
I've never used them myself, but I think scanners allow you to see the cargo and components of an enemy ship. The range is very short, making them somewhat difficult to use. Also, certain kinds of armor prevent a ship from being scanned. Note that these scanners do NOT provide any sort of sight; you need to have sensor coverage in addition to scanners.

At higher levels, Tachyon Scanners can have a good range (range of level + 1).

Long Range Scanners do get better range with each level.
They are helpful for telling if an enemy cargo ship is empty or packed full of stuff like troops, how many supplies/ordnance they have, and what components they have, etc.

The main advantage is when there are several groups of enemies, you can pick out which one has ships that will be the most damaging and deal with them, or which ones might hurt them the most to loose.
Any ship I put Tachyon Scanners on also gets a Long Range Scanner and Basic Sensors.

Knowledge is power.

The Scattering Armor is the one that blocks Long Range Scanners, but it is 50kt. Not practical for smaller ships unless you have something to hide. Also provides a defensive bonus.

There is also a Scanner Jammer, it reduces the effective range of Long Range Scanners, meaning the enemy has to get a lot closer to scan the ship it is on.

Long Range Scanners also has a way of relieving tension and providong humor value when dealing with the AI...

Like finding out an enemy fleet just sitting in a nebula is just sitting their because they ran out of supplies, and not due to preparing to attack a warp point leading to my systems... they left when an almost empty ship showed up, they then limped back to their territory... in the adjacent system...

edit.
No clue on the Temporal Scanners/Psychic Receptors. I'm not sure what scan levels stuff like cloaks or stealth armor prevents with them or if it effects them equally, or if they have any advantage over Tachyon Scanners.
I do know that Tachyon Scanners can get to a higher level.

If you open up the Components.txt Data file, you can see exactly what the differences are... Note that the max level for all 3 devices is sufficient to detect the maximum level cloaking device.

Name                                            := Tachyon Sensors
Description                                     := Modulating tachyon sensor grid which can detect cloaked ships in a system.
Picture Number                                  := 36
Maximum Level                                   := 26
Tonnage Space Taken Formula                     := 40
Tonnage Structure Formula                       := 40 + (([%Level%] - 1) * 10)
Cost Minerals Formula                           := 1500 + (([%Level%] - 1) * 15)
Cost Organics Formula                           := 0
Cost Radioactives Formula                       := 500 + (([%Level%] - 1) * 5)
Supply Amount Used Formula                      := 0
Ordnance Amount Used Formula                    := 0
Can Be Placed On Vehicle Types                  := Ship, Base, Satellite, Drone
Can Be Placed In Ship Sections                  := Inner Hull, Outer Hull
Component Type List                             := Technological
General Group                                   := Sensors
Custom Group                                    := 0
Number Of Requirements                          := 1
Requirements Evaluation Availability            := AND
Requirements Evaluation Allows Placement        := TRUE
Requirements Evaluation Allows Usage            := TRUE
Requirement 1 Description                       := Empire must have at least tech level 4 in Sensors.
Requirement 1 Formula                           := Get_Empire_Tech_Level("Sensors") >= (4 + ([%Level%] - 1))
Number Of Abilities                             := 2
Ability 1 Type                                  := Sight Level
Ability 1 Description                           := Allows scanning at level [%Amount1%].
Ability 1 Scope                                 := Space Object
Ability 1 Range Formula                         := 0
Ability 1 Amount 1 Formula                      := 2 + (([%Level%] - 1) * 1)
Ability 1 Amount 2 Formula                      := 0
Ability 2 Type                                  := Sight Range
Ability 2 Description                           := Allows scanning out to a range of [%Amount1%] sectors.
Ability 2 Scope                                 := Space Object
Ability 2 Range Formula                         := 0
Ability 2 Amount 1 Formula                      := 2 + (([%Level%] - 1) * 1)
Ability 2 Amount 2 Formula                      := 0
Weapon Type                                     := None

Name                                            := Psychic Receptors
Description                                     := Psychic sensors which can detect vessels hidden in a system.
Picture Number                                  := 45
Maximum Level                                   := 20
Tonnage Space Taken Formula                     := 40
Tonnage Structure Formula                       := 40 + (([%Level%] - 1) * 10)
Cost Minerals Formula                           := 1500 + (([%Level%] - 1) * 15)
Cost Organics Formula                           := 0
Cost Radioactives Formula                       := 500 + (([%Level%] - 1) * 5)
Supply Amount Used Formula                      := 0
Ordnance Amount Used Formula                    := 0
Can Be Placed On Vehicle Types                  := Ship, Base, Satellite, Drone
Can Be Placed In Ship Sections                  := Inner Hull, Outer Hull
Component Type List                             := Technological
General Group                                   := Sensors
Custom Group                                    := 0
Number Of Requirements                          := 1
Requirements Evaluation Availability            := AND
Requirements Evaluation Allows Placement        := TRUE
Requirements Evaluation Allows Usage            := TRUE
Requirement 1 Description                       := Empire must have at least tech level 1 in Psychic Technology.
Requirement 1 Formula                           := Get_Empire_Tech_Level("Psychic Technology") >= (1 + ([%Level%] - 1))
Number Of Abilities                             := 2
Ability 1 Type                                  := Sight Level
Ability 1 Description                           := Allows scanning at level [%Amount1%].
Ability 1 Scope                                 := Space Object
Ability 1 Range Formula                         := 0
Ability 1 Amount 1 Formula                      := 2 + (([%Level%] - 1) * 1)
Ability 1 Amount 2 Formula                      := 0
Ability 2 Type                                  := Sight Range
Ability 2 Description                           := Allows scanning out to a range of [%Amount1%] sectors.
Ability 2 Scope                                 := Space Object
Ability 2 Range Formula                         := 0
Ability 2 Amount 1 Formula                      := 2 + (([%Level%] - 1) * 1)
Ability 2 Amount 2 Formula                      := 0
Weapon Type                                     := None

Name                                            := Temporal Sensors
Description                                     := Temporal sensors which can detect vessels hidden in a system by looking for temporal distortions.
Picture Number                                  := 46
Maximum Level                                   := 20
Tonnage Space Taken Formula                     := 40
Tonnage Structure Formula                       := 40 + (([%Level%] - 1) * 10)
Cost Minerals Formula                           := 1500 + (([%Level%] - 1) * 15)
Cost Organics Formula                           := 0
Cost Radioactives Formula                       := 500 + (([%Level%] - 1) * 5)
Supply Amount Used Formula                      := 0
Ordnance Amount Used Formula                    := 0
Can Be Placed On Vehicle Types                  := Ship, Base, Satellite, Drone
Can Be Placed In Ship Sections                  := Inner Hull, Outer Hull
Component Type List                             := Technological
General Group                                   := Sensors
Custom Group                                    := 0
Number Of Requirements                          := 1
Requirements Evaluation Availability            := AND
Requirements Evaluation Allows Placement        := TRUE
Requirements Evaluation Allows Usage            := TRUE
Requirement 1 Description                       := Empire must have at least tech level 1 in Temporal Technology.
Requirement 1 Formula                           := Get_Empire_Tech_Level("Temporal Technology") >= (1 + ([%Level%] - 1))
Number Of Abilities                             := 2
Ability 1 Type                                  := Sight Level
Ability 1 Description                           := Allows scanning at level [%Amount1%].
Ability 1 Scope                                 := Space Object
Ability 1 Range Formula                         := 0
Ability 1 Amount 1 Formula                      := 2 + (([%Level%] - 1) * 1)
Ability 1 Amount 2 Formula                      := 0
Ability 2 Type                                  := Sight Range
Ability 2 Description                           := Allows scanning out to a range of [%Amount1%] sectors.
Ability 2 Scope                                 := Space Object
Ability 2 Range Formula                         := 0
Ability 2 Amount 1 Formula                      := 2 + (([%Level%] - 1) * 1)
Ability 2 Amount 2 Formula                      := 0
Weapon Type                                     := None

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The other nice advantage of long range scanners is that it lets u tell where the AI is sending ships, allowing u to know if they are headed to one of your new colonies. This allows u the time to build up an intercept force, block a warp point better or build some more defenses on that planet. Whats esp fun is if u see a message to several ships that u are at war with head to a sector location with the command to join fleet, take some supply ships there with some mines or sats whichever u prefer and lay them out. Eventually they will stop the ships but i have seen one empire lose 13 ships going to that sector after i put a satellite fleet in that sector.

Did I read that right Dansherril? Long range scanners allow you to see an AI ship's ORDERS? OMG, thats insanely useful... I've not yet bothered to research them (I newb on first game) but they just went to the top of my list.

Will it let you see orders of ships in Multiplayer games?

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Long range scanners do let you see other people's orders in multiplayer. For example, I have a carrier with long range scanners hiding in a storm in my friend's system, and I can see which warp point he's going to. But that will only help if they're using autopilot -- if they're giving each ship new orders every turn, you'll see nothing. Also, you can only see as many orders as will fit into the little text box at the bottom of your screen -- which means, basically, one. Even if you right-click on the ship, the Orders window is always empty. It would be kind of cool if that got fixed.

I used to make Long-Range Scanners a priority so I could tell whether the opponent's ship was point-defense or missiles, shields or armor. But now I usually just trade with everyone for their ship designs and combat reports. Then their designs show up in the combat simulator, so I can look them up when desired.

I get to thinking about SE IV.
My knowledge of Se IV is limited (played the demo a lot, not far in full game), but I know there were 5 things listed for stuff like cloaking devices and mines.

Checking the data files, I see the following.
Active EM Scans
Passive EM Scans
Psychic Scans
Gravitic Scans
Temporal Scans

Perhaps similar needs to be added to SE V, instead of having all sensors types have a general scan type.
It also could mean different items might be easier or harder to detect with different sensors/scanners.
Maybe also a reason to add a few new sensor/scanner and stealth components of some sort.

I really don't see a difference between scanners and sensors, synonyms in my mind. You basically only have two modes to scan/detect in :passive and active (possibly a third or more when we evolve to the next dimension), so I would design detectors based of that mentality along with the anti-detection components like stealth and such.

Max out sensors and scaners - place 1 tackyon and 1 longrange on your ships and enjoy seeing everything . But do note that ships in nebulas/storms can be invisibile even to the lv 27 tachyon sensor .

Can someone give me the figures of obscuration/detection for the different sectors? What level of sensor will detect you in an asteroid field, nebulae, storm, etc? They all simply say "medium" on level of obscuration.

"We are the Vahl, resistance is...welcomed"

I think its level 15'ish

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8 and 16 IIRC. Check out the data files.