Design Of A 1-m Unobscured Infrared Collimator And Laser Tracker Test System

AbstractThe large collimator is designed to test laser tracking systems with apertures as large as 1 m. To reduce the 1 -m aperture of the large collimator to a reasonable working size,two confocal parabolas (Mersenne) reimage the pupil to 0.1 m with a 1.4 -m off -axis paraboladecentered 1 m. Near the reimaged pupil two cylindrical mirrors are used to introducenear -field defocus and astigmatism while a two -axis tilt mirror simulates target motionand atmospheric tilts. A second Mersenne reduces the beam size further with the intermed-iate focus used for far -field atmospheric turbulence and thermal blooming simulation. Abandpass beamsplitter allows the incoming laser beam to proceed to a laser analyzer sec-tion, for directional and spot size information, while allowing the infrared radiation fromthe target to pass to the tracker. A large focal ratio allows for zooming the target by a"trombone" set of movable mirrors and fixed target. Rotation is provided by a K- mirrorimage rotator. The optics are chilled in a cold box and situated on an optical bench in avan for transportation.IntroductionThe large collimator device described in this paper was designed to test a laser tracker.The device must simulate optical targets at varying distances and the atmospheric effectsalong the path length. Simultaneously, the large collimator must be capable of analyzingthe optical beam coming from a tracker and passing through a simulated atmosphere to deter-mine its spot shape parameters. The large collimator must also be portable and computercontrolled.Figure 1 illustrates the contents of the optics van. The major features are a 30 -ft-long optical bench that is lifted up by vibration -isolation supports off of the van whenthe collimator is being used to test a tracker. The major optical components are a largetracking flat at the rear of the van and a 1.4 -m off -axis parabola that is the primarymirror of the first Mersenne 10X beam expander. The other items on the table are two coldboxes containing the remaining optics that are discussed in this paper (see Figure 2). Atthe front of the van is a control room for the computer and the electronics needed to runthe collimator. Not shown is a pallet containing the vacuum pumps and refrigerators forcooling and evacuating the cold box and the air conditioning system for controlling thevan temperature.The placement of the optics in the cold box is complex. A simplified optical schematicof the collimator is shown in Figure 2. The main function of the collimator is to projectan image of a simulated target into the tracker. A low power laser beam is returned fromthe tracker and is analyzed to control a simulated laser target burn.Atmospheric SimulationOne of the most important functions of the large collimator is to simulate the atmo-spheric effects on the tracker beam and the tracker's view of the targets. Much of thepresent configuration of the optics in the collimator is dependent upon the aberration gen-erators needed for this simulation. We have chosen to simulate two general types ofatmospheric effects. First, near -field effects are called isoplanatic because the iso-planatic region covers the whole image plane in the tracker. These effects can be simu-lated near a pupil location in the collimator. Far -field effects are called non -isoplanatic, as perturbations here affect only a small part of the tracker image. Theseeffects need to be simulated with a location near an image in the collimator.The far -field simulation consists of a rotating phase plate.' Space has been reservedin the far -field optics carriage for a thermal blooming simulator of a future design. Thenear -field simulation consists of a two -axis fast tilting mirror to simulate base and tar-get motion and two warpable cylindric mirrors for simulation of defocus and astigmatism.Mersenne Beam ExpandersA Mersenne 10X beam expander is utilized to reduce the 1 -m aperture down to a workablesize for the near -field optics. The beam expander conveniently reimages the tracker