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Performance Evaluation of FPU and Subunits for PACS: Stability, Optical Tolerances, and Calibration

This report presents a detailed analysis of the performance and testing of the Focal Plane Unit (FPU) and its subunits for the PACS instrument. It covers critical aspects, including structural integrity, temperature stability, mechanical and optical tolerances, and calibration performance of optical components such as mirrors and gratings. The effectiveness of filters, in-band transmission, and out-of-band suppression is addressed, along with comprehensive testing procedures and results regarding dynamic behavior, pointing accuracy, and jitter performance. Insights into the optical and mechanical performance are also provided.

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Performance Evaluation of FPU and Subunits for PACS: Stability, Optical Tolerances, and Calibration

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  1. FM FPU Subunits A. Poglitsch PACS FPU Subunits

  2. Performance of FPU and Subunits (I) • Structure • Temperature (stability) requirements • Mechanical/optical tolerances, alignment • Mirrors • Optical performance • Chopper • Pointing accuracy • Dynamic behaviour (duty cycle) • Grating • Optical performance of grating • Mechanical performance of grating drive • Precision of position readout/actuator; jitter • Dynamic properties (scanning/dithering/frequency switching) PACS FPU Subunits

  3. Performance of FPU and Subunits (II) • Filters • In-band transmission • Out-of-band suppression • Calibration Sources • Absolute emissivity • Homogeneity PACS FPU Subunits

  4. FPU Structure • Most (but not all) of the FPU structure has been used in CQM • Thermal requirements fulfilled by CQM as far as tested – expect no changes with FM • More detailed tests necessary w.r.t. sensitivity to L0/L1 interface temperature tolerances/variations • Ge:Ga detector thermal interface changed for thermal curing • LHe Optical Alignment Stability Test under way • CQM ILT revealed no anomalies PACS FPU Subunits

  5. Optics: Mirrors • All mirrors tested individually at ambient temperature • Visible light end-to-end test • Documentation with delivery of FPU • No unexplained anomalies observed in QM • But PSF measurements were hampered by telescope simulator defocus PACS FPU Subunits

  6. ± 0.5 arcsec on sky Chopper • Mirror optical performance • Flatness < 150 nm tested; failed but accepted • Roughness < 50 nm tested; passed • Reflectivity > 98% by verified coating process • Actuator performance (static/dynamic) • Range observation ± 4.1°; accuracy ± 1 arcmin tested; ok • Duty cycle observation @ 10 Hz >80% tested; ok • Range calibration ± 9°; accuracy ± 2 arcmin tested; ok • Duty cycle calibration >70% tested; ok PACS FPU Subunits

  7. Grating (I) • Groove period - spec 117.65 (± 0.7) µm • Only measured for CQM: g=117.66 ± 0.11 µm (2 sigma) • Facet angles - spec 34.5° (± 1°) and 62.4° (± 1°) • Average: 34.1846°; standard deviation: 0.3455° • Average: 62.3264°; standard deviation: 0.4233° • Groove regularity - spec 0.3 µm r.m.s. • Not verified  search for grating ghosts in ILT essential! • Positioning precision of machine used for ruling is 0.1 µm w.r.t. reference (reference: Heidenhain glass ruler). Measured ruler error (interferometric) over 100 mm travel:0.15 µm (p-p); no measurement over full length available • Groove depth - spec 56 µm ± 2 µm • Not measured, but 0th order diffraction efficiency indicates no major deviation of grating profile from spec PACS FPU Subunits

  8. Grating (II) • Groove parallelism - spec ± 10” • Each groove to each other: not verified, but hard to miss with machine used for ruling • Grooves to rotation axis: no absolute measurement done, but grating drive wobble measured – uses up error entire budget! Thorough test of position shifts and spectral shear/tilt as function of grating angle during ILT required! • Overall surface flatness - spec 3 (goal 1.5) µm r.m.s. • Verification by WFE measurement with 10 µm interferometer in three grating orders partly achieved (only partial coverage of grating area by beam at a time); machining error <0.5 µm (but internal stress in grating body could lead to deformation)  Confirmation by measurement of PSF / spectral profile during ILT PACS FPU Subunits

  9. Grating Drive • Position precision within the useful angular throw: 8 arcsec (goal: 4 arcsec) • Calibration certificate: ± 3 arcsec; ± 4 arcsec (nom.; red.) • Grating position “jitter” shall be < 0.1 arcsec/√Hz • verified by test • Wavelength switching: amplitude of < 3 arcmin with frequencies of < 3 Hz and a duty cycle of 80% • verified by test • Stabilization time for a grating step of 18 arcsec:< 23 ms • verified by test PACS FPU Subunits

  10. Filters: “Red” Photometer PACS FPU Subunits

  11. Filters: “Green” Photometer PACS FPU Subunits

  12. Filters: “Blue” Photometer PACS FPU Subunits

  13. Filters: “Red” Spectrometer PACS FPU Subunits

  14. Filters: “Green” Spectrometer PACS FPU Subunits

  15. Filters: “Blue” Spectrometer PACS FPU Subunits

  16. Calibration Sources • No measurements of FM CSs possible before FM ILT • Cause of inhomogeneity in CS1 illumination identified in QM FPU; avoided in FM FPU • Emissivity adjusted • Stability expected in spec (similarity with QM) QM QM CS2 CS1 PACS FPU Subunits

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