Optimisation of the PACS Chopper Markus Nielbock Ulrich Klaas Jeroen Bouwman Helmut Dannerbauer Jürgen Schreiber Ulrich Grözinger

Markus Nielbock –PACS Chopper Herschel Space Observatory Science with Herschel: formation and evolution of galaxies and stars ISM physics and properties solar system bodies Strengths of Herschel: cover SED peaks of ISM wide area continuum mapping spectral mapping ([O I], water) thermal IR to submm wavelengths (55 – 672 μm)

Markus Nielbock –PACS Chopper Herschel Space Observatory main mirror: 3.5 m (effective aperture: 3.3 m) cryostat containing three instruments: HIFI: submm hi-res spectrometer SPIRE: submm bolometer and spectrometer PACS: MIR to FIR imaging photometer and IF spectrometer photometer: 60 – 85 μm or 85 – 130 μm and 130 – 210 μm simultaneously, same FOV (3.5’ x 1.75’) spectrometer: 55 – 73 μm or 73 – 98 μm and 102 – 210 μm FWHM = 5” – 13” simultaneously at R = 1000 – 5000 FOV = 47” x 47” orbit around L2 (1.5 x 10 6 km away from earth)

Markus Nielbock –PACS Chopper The PACS Chopper mirror baffle (32 mm x 26 mm) drive coils sky window calibration source onoff focal plane geometry FOV Is used as … 1) classical chopping (flipping) mirror 2) beam deflection mirror compensates for bright telescope background (~ 85 K) feeds in signal of internal calibration sources  critical for sensitivity  critical for flux calibration

Markus Nielbock –PACS Chopper Angular Calibration Conversion of digital electronic position sensor read-out (  32767) into rotation angle non-linear origin at mechanical rest position not on optical axis laser reflection measured vs. sensor signal Req. 1: duty cycle of ≥ 80% for science observations (± 4.1°   3’ on sky) and chopping at 10 Hz Req. 2: duty cycle of ≥ 70% for calibration (± 8°) and chopping at 5 Hz  10 ms transition time  30 ms transition time Req. 3: deviation < 1’ (0.7” on sky) Required Properties

Markus Nielbock –PACS Chopper Chopper Control current supply of drive coils magnetic induction moves chopper discrete current steps cause damped oscillations  controlled current variations needed (acceleration and braking) instead of step function electronic control loop (PID): 7 control loop parameters 5 parameters for resonance oscillation suppression filter

Markus Nielbock –PACS Chopper PID Optimisation initial set of parameters chopping measurement adjust parameter(s) final set of parameters input record movement analyse bad good input MATLAB Model generate

Markus Nielbock –PACS Chopper The Best Result Chopper angle (digital units) Supply current [mA] stability corridor movement acceleration braking stimulus transition within 19 ms  not in spec, but efficient enough Example: photometer operated with 1.25 Hz chopping  duty cycle: 95% Further improvement is probably not needed, but will be tried.

Markus Nielbock –PACS Chopper Wish us luck!