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.
Time-domain astronomy is important in the field of modern astronomy, and monitoring observations in the mid-infrared region with 1% photometric accuracy to study the variables and transients is becoming essential. The non-uniformity of the sensitivity caused by the optical characteristics of instruments and differences in the response curves of individual detector pixels degrade photometric accuracy. Therefore, to achieve 1% photometric accuracy, a flat-field correction for the non-uniformity with an accuracy of better than 1% is required. We developed a flat calibration unit (FCU) consisting of a silicon lens, a blackbody source, and two flat folding mirrors. We conducted proof-of-concept tests of the FCU by measuring the accuracy and stability of flat frames obtained using the FCU. The accuracies of the flat frames were 0.23% at 7.7 μm, 0.43% at 9.6 μm, 0.34% at 11.5 μm, and 0.84% at 20.9 μm, which are sufficient to achieve 1% photometric accuracy. The flat frames obtained using the FCU were stable over a period of 29 h within the accuracies of 0.13% at 7.7 μm, 0.12% at 9.6 μm, 0.22% at 11.5 μm, and 0.52% at 20.9 μm, indicating that it is sufficient to obtain flat frames once per night.
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.
MIMIZUKU is a mid-infrared instrument for the TAO 6.5-m telescope under construction in the Atacama Desert, Chile, and will be the world’s first mid-infrared monitoring observation station. We aim to achieve a photometric accuracy of 1%. For this purpose, highly accurate flat fielding with an accuracy of 0.1% is needed. Although flat fielding has been conducted using sky images and dark images conventionally, the correction has uncertainties of several percent. The reason is that the non-linearity of the detector is not considered. To improve this, it is necessary to create flat frames from data in the same count level as during observation. Highly accurate flat frames were derived by taking differential counts against the time variation of the atmospheric radiation. However, this method cannot be used under stable conditions suitable for observations. Therefore, we developed a flat calibration unit which irradiates the detector uniformly and vary the irradiation intensity with time to enable the improved flat fielding under any conditions. We designed the unit that irradiates the detector uniformly by placing a silicon lens and a blackbody source in front of the camera. The blackbody source is put at the pupil position of the optical system. We made some tests to create flat images with the unit. By improving flat fielding, we have successfully corrected for patterns originating from the detector, which appeared in the conventional one. We also clarified that the accuracy of the improved flat fielding was 0.29%, while the accuracy of the conventional one was 1.3%.
MIMIZUKU is the first-generation mid-infrared instrument for the TAO 6.5-m telescope. It has three internal optical channels to cover a wide wavelength range from 2 to 38 µm. Of the three channels, the NIR channel is responsible for observations in the shortest wavelength range, shorter than 5.3 µm. The performance of the NIR channel is evaluated in the laboratory. Through the tests, we confirm the followings: 1) the detector (HAWAII 1RG with 5.3-µm cutoff) likely achieves ∼80% quantum efficiency; 2) imaging performance is sufficient to achieve seeing-limit spatial resolution; 3) system efficiencies in imaging mode are 2.4–31%; and 4) the system efficiencies in spectroscopic modes is 5–18%. These results suggest that the optical performance of the NIR channel is achieved as expected from characteristics of the optical components. However, calculations of the background levels and on-sky sensitivity based on these results suggest that neutral density (ND) filters are needed to avoid saturation in L ′ - and M′ -band observations and that the ND filters and the entrance window, made of chemical-vapor-deposition (CVD) diamond, significantly degrade the sensitivity in these bands. This means that the use of different window materials and improvements of the detector readout speed are required to achieve both near-infrared and long-wavelength mid-infrared (>30 µm) observations.
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