It is imperative that we have high confidence that the optical performance capability of JWST is well-understood
before launch. With the telescope operating at cryogenic temperatures and sporting a 6.6 meter primary mirror
diameter, the optical metrology equipment required to measure the optical performance can be quite complex. The
JWST Test team undertook an effort to greatly simplify the optical metrology approach, while retaining the key
measurements and verification methodology. The result is a cryogenic optical test configuration and
implementation using Chamber A at NASA's Johnson Space Center that uses the science instruments to help
understand JWST's optical performance.
A large aperture dynamic wavefront sensor (WFS) was tested and qualified for use against its design requirements. The WFS was designed to measure the relative slope of dynamic wavefronts; therefore, the test system created dynamic wavefronts, moving at 35 Hz to 315 Hz, with slopes on the order of 50 nanoradians (nR). The essential test system was an f/2.3 parabolic mirror with a laser source at the focal point, offset laterally by a fold mirror. The reflected light was nominally collimated and incident on the WFS at zero degrees. The source hardware was mounted on two crossed-translation stages that could drive a 540 μm, 1/2 Hz trapezoidal motion, inducing tilt in the collimated beam. This 100 microradians (μR) wavefront modulation calibrated the WFS. The fold mirror was mounted on a PZT, which oscillated the fold mirror from 35 Hz to 315 Hz, at tilt angles near 10 μR. This tilt moved the virtual source point, inducing wavefront tilts in the collimated output beam on the order of 100 nR. These fast, very small wavefront tilts were used to test the WFS performance. The test system, procedure, and calibration procedures are described.
A compact, low-cost wavefront sensor has been demonstrated to measure dynamic disturbances in 1-3 m diameter optical systems. With 448 subapertures and 4 KHz frame rate, it can measure disturbances up to 2 KHz at a level of 1/3800 waves rms at 0.65 μm. It also has a linear dynamic range of 3 X 10<sup>6</sup>:1 for ease of alignment. The principles of operation and test data are presented for the subaperture sensors (called NanoTrackers), which are lateral shearing interferometers capable of measuring tilt to 1.7 nanoradians rms at 8 µW of input power as well as phasing for segmented optical systems to 75 picometers rms (at 4 KHz).
Optical systems with segmented mirrors require precision coalignment of their segments to attain the desired performance. A novel sensor for measuring piston over a ± 10 wave range by analyzing the image of a white light point source is described. The sensor combines a Young's two-slit aperture pair for coarse sensing with another set of apertures, which include a phase retardation portion for fine sensing. The course sensor has an accuracy of ± 0.25 wave over the full range. The fine sensor works within ± 0.5 wave of zero with ± 0.005 wave accuracy, and a repeatability of the order of ± 0.001 wave. Theoretical derivation and experimental results are provided.
Foucault knife-edge testing has been the classic qualitative test method for optical surface and wave-front quality measurement for more than one hundred years. We have developed an upgraded quantitative method, using off-the-shelf digital microprocessors and a solid state camera, to provide white-light evaluation of generalized optical surfaces with the precision and accuracy of computer-aided laser interlerometry. We describe a method that upgrades the knife-edge test from a qualitative test to a precise quantitative evaluation. The hardware is described and initial results for some aberrations are shown comparing the knife-edge test with interferometric measurements.
A novel piston sensing concept is described for measuring piston over the +/- 0.5 wave range. The image of a white light point source is optically processed using a half-wave phase plate to enhance the presence of piston in the incident wavefront. The resultant point spread function is recorded on a CCD camera and analyzed using a simple intensity balance algorithm. Theoretical derivation and experimental results are provided, with emphasis on piston values below 0.25 waves. Piston values are measured to +/- 1/200 wave accuracy or better with a repeatability on the order of +/- 1/1000 wave.
Foucault knife edge testing has been the classic qualitative test method for optical surface and wavefront quality
measurement for over one hundred years. We have developed an innovative method, using off the shelf CIigit.a1
microprocessors and a solid state camera, whereby quantitative evaluation of surfaces has been achieved.
This paper will describe a method that upgrades the knife edge test from a qualitative test to a precise quantitative
evaluation. The hardware is described and results are shown comparing the knife edge test with interferometric