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The Fiber Injection Unit (FIU) sits on the Keck I AO bench ahead of AO correction. The unit's primary role is to feed telescope light to the science, sky, and Ca H&K fibers. The unit utilizes a pair of dichroics to send light to the relevant subsystems:
In addition, the FIU contains atmospheric dispersion correction (ADC) systems for both the science and Ca H&K wavelengths. For the science arm, a pair of prisms are rotated to provide dispersion correction based on the telescope position. For the Ca H&K arm, the fiber is on a two axis stage and is translated in such as way as to place the fiber under the correct wavelengths of light. The total wavelength range in this arm is small enough that only the differential position between this arm and the science and guide arms is corrected, there is no correction of the dispersion over the narrow wavelength range of the Ca H&K arm.
| Wavelength Range | Subsystem | Notes |
|---|---|---|
| 382-402 nm | Ca H&K Spectrograph | Includes two fibers for science and sky |
| 445-870 nm | Science Spectrograph | Includes two fibers for science and sky |
| 950-1200 nm | Guide Camera | Used for acquisition and for fast tip tilt correction |
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| The FIU layout when in science mode. Light from the telescope is directed to the FIU off of a tip tilt mirror located on the PCU stage. |
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| The FIU layout when in calibration mode. The calibration fibers are illuminated and a fold mirror is moved in to place to direct their light on to the science fiber. |