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OTIS has recently undergone cryogenic vacuum testing in Chamber A at Johnson Space Center in Houston, TX. A key outcome of this test was to verify there is adequate range of motion in PMSA and SMA actuators to align them to AOS/ISIM under flight-like conditions. The alignment state of the PMSAs and SMA was measured using photogrammetry and cross-checked optically using a variation of a classical Hartmann test. In the “Pass-and-a-Half” (PAAH) configuration, fiber sources near the Cassegrain focus propagate light through the full optical train and small tilts on the PMSAs create an array of spots on the science instrument detectors, mimicking the effect of a Hartmann mask. Comparison of measured and modeled spot arrays provides the alignment state of the SMA and the global tilt of the primary mirror. This paper will discuss the methodology, testing, and analysis performed to measure the alignment state of OTIS using the Hartmann method and verify the primary and secondary mirrors can be successfully aligned on orbit to meet performance requirements.
Calibration results using highly aberrated images for aligning the JWST instruments to the telescope
In support of this cryogenic alignment of the AOS to ISIM, an array of fiber optic sources, known as the AOS Source Plate Assembly (ASPA), are placed near the intermediate image location of JWST (between the secondary and tertiary mirrors) during thermal vacuum ground-test operations. The AOS produces images of the ASPA fiber optic sources at the JWST focal surface location, where they are captured by the various science instruments. In this manner, the AOS provides an optical yardstick by which the instruments within ISIM can evaluate their relative positions to and the alignment of the AOS to ISIM can be quantified. However, since the ASPA is located at the intermediate image location of the JWST three-mirror anastigmat design, the images of these fiber optic sources produced by the AOS are highly aberrated with approximately 2-3μm RMS wavefront error consisting mostly of 3rd-order astigmatism and coma. This is because the elliptical tertiary mirror of the AOS is used off of its ideal foci locations without the compensating wavefront effects of the JWST primary and secondary mirrors. Therefore, the PSFs created are highly asymmetric with relatively complex structure and the centroid and encircled energy analyses traditionally used to locate images are not sufficient for ensuring the AOS to ISIM alignment.
A novel approach combining phase retrieval and spatial metrology was developed to both locate the images with respect to the AOS and provide calibration information for eventual AOS to ISIM alignment verification. During final JWST OTE and ISIM (OTIS) testing, only a single thru-focus image will be collected by the instruments. Therefore, tools and processes were developed to perform single-image phase retrieval on these highly aberrated images such that any single image of the ASPA source can provide calibrated knowledge of the instruments’ position relative to the AOS. This paper discusses the results of the methodology, hardware, and calibration performed to ensure that the AOS and ISIM are aligned within their respective tolerances at JWST OTIS testing.
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