In context of wavefront control for high-contrast coronagraphy, we present the spectral characterization of speckles generated by deformable mirrors (DMs) at various distances downstream of the pupil plane. We have measured the focal-plane electric field at 600 to 690nm of speckles generated with an in- and out-of-pupil DM on a highcontrast testbed using a self-coherent camera. These measurements are analyzed analytically and are compared to numerical end-to-end simulations of the setup. This will aid in the design of wavefront correction algorithms with multiple DMs.
In the context of the SAFARI instrument (SpicA FAR-infrared Instrument) SRON is developing a test environment to
verify the SAFARI performance. The characterization of the detector focal plane will be performed with a backilluminated
pinhole over a reimaged SAFARI focal plane by an XYZ scanning mechanism that consists of three linear
stages stacked together. In order to reduce background radiation that can couple into the high sensitivity cryogenic
detectors (goal NEP of 2•10<sup>-19</sup> W/√Hz and saturation power of few femtoWatts) the scanner is mounted inside the
cryostat in the 4K environment. The required readout accuracy is 3 μm and reproducibility of 1 μm along the total travel
of 32 mm. The stage will be operated in “on the fly” mode to prevent vibrations of the scanner mechanism and will
move with a constant speed varying from 60 μm/s to 400 μm/s. In order to meet the requirements of large stroke, low
dissipation (low friction) and high accuracy a DC motor plus spindle stage solution has been chosen.
In this paper we will present the stage design and stage characterization, describing also the measurements setup. The
room temperature performance has been measured with a 3D measuring machine cross calibrated with a laser
interferometer and a 2-axis tilt sensor. The low temperature verification has been performed in a wet 4K cryostat using a
laser interferometer for measuring the linear displacements and a theodolite for measuring the angular displacements.
The angular displacements can be calibrated with a precision of 4 arcsec and the position could be determined with high
accuracy. The presence of friction caused higher values of torque than predicted and consequently higher dissipation.
The thermal model of the stage has also been verified at 4K.
A cryogenic iris mechanism is under development as part of the ground calibration source for the SAFARI instrument.
The iris mechanism is a variable aperture used as an optical shutter to fine-tune and modulate the absolute power output
of the calibration source. It has 4 stainless steel blades that create a near-circular aperture in every position. The
operating temperature is 4.5 Kelvin to provide a negligible background to the SAFARI detectors, and ‘hot spots’ above
9K should be prevented. Cryogenic testing proved that the iris works at 4K. It can be used in a broad range of cryogenic
optical instruments where optical throughput needs to be controlled.
Challenges in the design include the low cooling power available (5mW) and low friction at cryogenic temperatures. The
actuator is an ‘arc-type’ rotary voice-coil motor. The use of flexural pivots creates a mono-stable mechanism with a
resonance frequency at 26Hz. Accurate and fast position control with disturbance rejection is managed by a PID servo
loop using a hall-sensor as input. At 4 Kelvin, the frequency is limited to 4Hz to avoid excess dissipation and heating.
In this paper, the design and performance of the iris are discussed. The design was optimized using a thermal, magnetic
and mechanical model made with COMSOL Finite Element Analysis software. The dynamical and state-space modeling
of the mechanism and the concept of the electrical control are presented. The performance of the iris show good
agreement to the analytical and COMSOL modeling.
For the on-ground calibration setup of the SAFARI instrument cryogenic mechanisms are being developed at SRON
Netherlands Institute for Space Research, including a filter wheel, XYZ-scanner and a flipmirror mechanism. Due to the
extremely low background radiation requirement of the SAFARI instrument, all of these mechanisms will have to
perform their work at 4.5 Kelvin and low-dissipative cryogenic actuators are required to drive these mechanisms.
In this paper, the performance of stepper motors, piezoelectric actuators and brushless DC-motors as cryogenic actuators
are compared. We tested stepper motor mechanical performance and electrical dissipation at 4K. The actuator
requirements, test setup and test results are presented. Furthermore, design considerations and early performance tests of
the flipmirror mechanism are discussed. This flipmirror features a 102 x 72 mm aluminum mirror that can be rotated 45°.
A Phytron stepper motor with reduction gearbox has been chosen to drive the flipmirror. Testing showed that this motor
has a dissipation of 49mW at 4K with a torque of 60Nmm at 100rpm.
Thermal modeling of the flipmirror mechanism predicts that with proper thermal strapping the peak temperature of the
flipmirror after a single action will be within the background level requirements of the SAFARI instrument. Early tests
confirm this result. For low-duty cycle operations commercial stepper motors appear suitable as actuators for test
equipment in the SAFARI on ground calibration setup.