IR Cryogenic optomechanics E. Bendek, Dec 2009 V30 Outline Introduction Example Instrument Description Optical overview Functions overview Vessel, vacuum and cryogenics. Introduction IR instruments are the workhorse at modern observatories
Wide range of wavelength coverage. Several modes available: Imaging Spectroscopy IFU
Polarimetry Example: ISAAC ISAAC is located at Nasmyth B, UT1. It is fed by a f/15 beam. Instrument Description Infrared Spectrometer And Array Camera Cryogenic instrument capable of: Imaging, spectrometry and polarimetry for infrared wavelengths. Short Wave Arm: 1 2.5 um,
1k x 1k Hg:Cd:Te Hawaii array. Set point at 65 K. Long Wave Arm: 1 5 um, 1k x 1k InSb Aladdin array. Set point at 30 K. Aladdin LW detector Instrument Description Instrument capabilities: Imaging: Obtain direct images of a field.
Polarimetry: Allows to identify polarization components of polarized light. Spectroscopy: Obtain the spectral distribution of a target. In total 10 optical modes or configurations are possible! Optical Overview Optics: Common path:
Calibration mirror (F12): Select the light source. Slit/Mask wheel (F1): Define the field with slits or mask. Mode selector (F3): Select Imaging or spectroscopy mode. Collimator (F5): Adjust the Collimation of the beam. Detector Selector (F4): Select the SW or LW camera. 2 Cameras, SW and LW each one has: 2 filters wheels
1 objective wheel Optical Overview LW LW Spectroscopy Imaging Optics are cryogenically cooled to avoid thermal emission. The Cold Structure holds the optics and keeps it between 70 and 80 K The motors are
inside the structure. Remove M1 3 Mirror Collimator Rotate M7 SW Imaging FW 1 FW 2 Objective 1 SW Spectroscopy Optical Overview Sample of Flat field images. Normal Flat Slit mask wheel offset M1 out of position
Filter offset M7 out of position Functions Overview Electromechanical devices that move an optical component. Rotating mass mounted on the outer ring of a pre-loaded ball bearing. A pre-loaded worm wheel system drives the function. 5 - phase stepper motor from Berger-Lahr move drive the worm.
A set of two magneto resistors is used for initialization. Toothed wheel mounted on the motor axis gives a move signal. Typical ISAAC drive Functions Overview Cryogenic motors Specially prepared Berger Lahr 545/100 motors. 5 phases, 1000 half steps/rev. No feed back on function position. (no encoders) Berger Lahr 545 / 100, already prepared for cryogenic operation Functions Overview
Cryogenic motors preparation More than a factor 10 cheaper than a UHV qualified motor like a Phytron VSS! Preparation is done in house. On the left the motor is dismounted, on the right new parts for cryogenic operation are shown Functions Overview No position feedback exists, then every lost step causes a positioning error. Common problem: The motor or the amplifier burns a phase Then, the motor torque is reduced. A speed reduction is needed
to keep the torque high enough to drive the function precisely. Torque(N*mm) 120 Serie1 Serie2 100 80 60 40 0 2000 4000 6000
Speed (Steps/sec) 8000 Functions Overview F1 Mode selector Counterweight Aluminum mirror, not coated (1032gr) Bellows coupling Worm wheel Stepper motor Functions Overview
Initialization system Voltage Consist of 2 sensors, one in the worm and one in the tooth wheel. The zero or init position occurs if: V worm = V Tooth wheel Worm sensor A backup sensor par exists in case of failure. Worm signal Init Position Tooth wheel signal
Movement feedback Tooth wheel sensor Position Detail of the init system Functions Overview Cold functions of the instruments Function Description Init Resolution (enc/rev)
Mode F1 Slit & mask wheel Normal 402.000/rev Circular optimized F2 Grating Backup 840.000/rev Circular F3
M1 (mode selector) Normal 64.227 LL-UL Linear F4 M7 (Detector selector) Normal 400.000/rev Linear F5 Collimator Normal
50.000 LL-UL Linear F6 Filter Wheel 1 (SW) Normal 126.800/rev Circular F7 Filter Wheel 2 (SW) Normal 268.000/rev Circular
804.000/rev Circular F11 Objective Wheel 2 (LW) Fallbac k Fallback 804.000/rev Circular Functions Overview SW Camera inside the Cold Structure SW Camera removed Functions Overview
M7 and collimator holder Slit mask wheel Vacuum and cryogenics Vacuum and cryogenics Vessel volume: ~ 3m3 Weight: ~ 2.500 Kg Vacuum:
Turbomolecular: Operational vacuum: ~ 5x10-7mbar Cold Structure temp: 70 K Cooling: Compressor: 1 Leybold RW6000 Scroll Pump Seiko Seiki Maglev. 2 x Leybold Cold head. Vacuum and cryogenics
Inside ISAAC Cryo-cooler cold head Detector Cold Structure (optical Layout) Vaccum vessel (electropolished stainless steel) The interventions ISAAC retrofited super insulation and radiation shield Fixing the Cold Structure IR Cryogenic optomechanics E. Bendek, Dec 2009 V30
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