Definition
An optical device that aligns light rays into parallel paths, producing an image that appears to be at infinity. In aviation instruments, a collimator is used so that a projected reference (such as a sighting reticle, gyro horizon line, or heads-up display symbol) can be viewed without the pilot having to refocus the eye between the instrument and the outside world.
Plain English
A small optical part that projects an image so it looks far away, letting the pilot's eyes stay focused on the distance while still seeing the instrument's reference marks.
Context Anchor
Seen mainly in aircraft maintenance, instrument testing, and alignment of aircraft optical systems.
Derivation
From the Latin collimare, a misreading of collineare, meaning 'to align' or 'to bring into a straight line.' The aviation use keeps that idea: the device lines up light rays so they travel parallel to one another.
Why Pilots Care
Accurate instrument alignment prevents false attitude or heading indications that could lead to loss of control or spatial disorientation in instrument flight.
Analogy
It is like setting up a faraway target without needing a real target miles away. The collimator creates the same visual effect in a small test setup.
Intuition Check
A collimator is not a cockpit control or a normal flight instrument. It is usually a testing and alignment tool used to make an image appear far away and properly lined up.
Example Sentence 1
The heads-up display uses a collimator so the airspeed and altitude symbols appear to float at the same distance as the runway ahead.
Example Sentence 2
After overhaul the heading indicator was checked with a collimator to confirm it would display accurate magnetic headings once installed in the panel.