KEYWORDS: Tunable filters, Feedback control, Control systems, Digital signal processing, Electronic filtering, Repetition frequency, Signal processing, Mirrors
In ground-based mid-infrared observations the background radiation must be removed. Chopping is a background removal method requiring fast switching of the observation field. For MIMIZUKU, the mid-infrared instrument for the TAO telescope, we have developed a cold chopper which switches the observing field by tilting a movable mirror inside MIMIZUKU, instead of tilting the large secondary mirror.
We require a short transition time, sufficient amplitude, high frequency and steadyness for observation in the chopper movement.
With Repetitive Control we significantly increase performance by iteratively improving a feedforward trajectory and continously adapting to changes in the nonlinear dynamics.
This allows for much shorter transition time (<30 ms) and more freedom in the design of a feedback controller. Furthermore, repetitive disturbances originating from the cryo-cooler can be countered thus improving stability on sky.
Controller design, stabilisation, choice of reference trajectory, real-time computability and performance trade-offs are subjects in this research.
MIMIZUKU, the mid-infrared instrument for the 6.5-m telescope at the University of Tokyo Atacama Observatory (TAO), employs a cold chopper to perform chopping, which tilts a mirror placed on the internal cold optics at about 30 K. The mirror rotates around two orthogonal axes, and its tilt angle is controlled by the balance between the restoring force of the flexural pivots and the magnetic force driven by the coils in the system. In this study, we developed a final-product model of the chopper and tested its onboard performance in MIMIZUKU. This experiment showed that the mirror could be operated with a stability of 3.83×10−4 and 3.29×10−4 degrees, and a transition time of 31.2 and 32.2 milliseconds for each rotation, when both rotations were driven at 5 Hz with an amplitude of 0.59 degrees, satisfying the performance requirements.
Cold choppers are fast beam-switching tip-tilt mirrors installed in the cold optics of mid-infrared instruments. They enable chopping observations, required for ground-based mid-infrared observations to subtract the bright background radiation, without moving telescope mirrors. In the era of next-generation extremely large telescopes, the telescope mirrors cannot be moved due to the size. Therefore, cold choppers are a key technology for groundbased mid-infrared instruments for such large telescopes. In this study, we develop a prototype cold chopper for TAO/MIMIZUKU, the mid-infrared instrument for the TAO 6.5-m telescope, and evaluate the performance in a cryogenic environment at 20 K. It is confirmed that the prototype shows almost the same response as at room temperature and achieves 2-axis square-wave motion with an amplitude of 0.84 deg, a settling time of ∼40 ms, and a frequency of ≥2 Hz. The evaluated power dissipation is ∼5mW. Stability is found to be slightly worse than required (6 × 10−4 deg) due to mechanical vibration caused by the cryocooler used in the experiment. We plan to mount this chopper on MIMIZUKU to check the effects of such vibrations in the on-board environment.
This paper briefly explains the development status of the present segmented mirror control system in Seimei telescope. Modeling and control issues in segmented mirror control are then described to share our experience. The total degrees of freedoms (DOFs) in the ring and the segments are 57. 57 DOFs are separated into 54 non-fixed modes and 3 fixed modes. 3 fixed modes are selected by introducing the concepts of the virtual ideal mirror surface. 54 non-fixed modes are the target of the segmented mirror control system. The centralized control system (CCS) in the outer-segments-fixed method has been developed and has been successfully installed in Seimei telescope, and now scientific results are beginning to be reported.
An internal cold chopper is one of the key elements for the ground-based mid-infrared astronomical instruments of the next generation large telescopes such as TMT, GMT, and ELT. For ground-based mid-infrared observations, time variations of radiation from atmosphere, telescope, and instrument are often many times greater than the radiation from targets. Relatively fast (~5 Hz) beam switching technique (chopping) using the oscillating secondary mirror is often used for 8-m class telescopes to remove these background variations, but this is impractical for the 30-m class telescopes due to their much larger secondary mirror. Instead of it, internal chopping mirror is desired for the next generation groundbased mid-infrared astronomy. However, since the mid-IR instruments are usually cooled as low as ~30 K, such internal chopping mirror should also be cooled to that low temperature and low power dissipation of <1 W is required to maintain the low temperature environment, thus it is not so easy to have such cold chopper. As a testbed for that, we are developing an internal cold chopper for the mid-infrared instrument MIMIZUKU for the University of Tokyo Atacama Observatory (TAO) 6.5-m telescope. We use a voice-coil-like structure as actuators and capacitive gap sensors as position sensors. We construct a feedback control system for the rectangular command signal. We made a prototype cold chopper for TAO/MIMIZUKU and are currently evaluating this system. We present the current design of the cold chopper for MIMIZUKU and results of performance evaluation in the laboratory.
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