The Sunrise Chromospheric Infrared spectroPolarimter (SCIP) is an instrument for the third flight of the SUNRISE balloon-borne solar observatory planned for 2022. To verify the high spatial and spectral resolutions required in the balloon flight, the SCIP optical unit was subjected to a thermal-vacuum test in which the SCIP optical unit was installed in a vacuum chamber and was exposed to the thermal environment expected in the flight. We verified the heater control performance and the temperature distribution in the SCIP optical unit and confirmed the optical performance by injecting the laser and white light through a vacuum window.
SUNRISE III is an international balloon-borne solar observatory with a 1m diameter telescope and is scheduled to fly in 2022. The Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) is being developed as a focal plane instrument for SUNRISE III, which will perform multi-wavelength spectropolarimetric observations with high spatial and spectral resolution (0.21 arcsec and 2 × 105 ). The SCIP is a quasi-Littrow-type spectropolarimeter mainly composed of an echelle grating and two aspheric mirrors. A polarizing beam splitter is used for simultaneous measurement of p- and s-polarization. To ensure the imaging performance, correct the image rotation and shift, and avoid vignetting, we performed the optical alignment. The optical elements were firstly aligned by shimming with mechanical precision of 1-20 arcmin for tilt and 0.1-0.3 mm for shift using a coordinate measuring machine. After the mechanical alignment, we constructed the telecentric feed optical system to mimic the light distribution instrument of SUNRISE III telescope, which is called Image Stabilization and Light Distribution (ISLiD). To reduce the astigmatism, we measured the contrast of the spot in the spatial direction and the width of the spot in the wavelength direction and adjusted the two aspheric mirrors. For the correction of the image rotation, the Z-tilt of grating was adjusted with a wavelength tunable laser by evaluating the tilt of the slit image in the wavelength direction. The wavelength position in cameras was adjusted with tunable lasers and sunlight. We evaluated the modulation transfer function of SCIP using a Ronchi ruling target with a white light source.
The Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) is a near-IR spectro-polarimeter instrument newly designed for Sunrise III, a balloon-borne solar observatory with a 1-m diameter telescope. In order to achieve the strict requirements the SCIP wavefront error, it is necessary to quantify the errors due to environmen- tal effects such as gravity and temperature variation under the observation conditions. We therefore conducted an integrated opto-mechanical analysis incorporating mechanical and thermal disturbances into a finite element model of the entire SCIP structure to acquire the nodal displacements of each optical element, then fed them back to the optical analysis software in the form of rigid body motion and surface deformation fitted by polynomials. This method allowed us to determine the error factors having a significant influence on optical performance. For example, no significant wavefront degradation was associated with the structural mountings because the optical element mounts were well designed based on quasi-kinematic constraints. In contrast, we found that the main factor affecting wavefront degradation was the rigid body motions of the optical elements, which must be mini- mized within the allowable level. Based on these results, we constructed the optical bench using a sandwich panel as the optical bench consisting of an aluminum-honeycomb core and carbon fiber reinforced plastic skins with a high stiffness and low coefficient of thermal expansion. We then confirmed that the new opto-mechanical model achieved the wavefront error requirement. In this paper, we report the details of this integrated opto-mechanical analysis, including the wavefront error budgeting and the design of the opto-mechanics.
Polarization measurements of the solar chromospheric lines at high precision are key to present and future solar telescopes for understanding magnetic field structures in the chromosphere. The Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) for Sunrise III is a spectropolarimeter with a polarimetric precision of 0.03 % (1 σ). The key to high-precision polarization measurements using SCIP is a polarization modulation unit that rotates a waveplate continuously at a constant speed. The rotating mechanism is a DC brushless motor originally developed for a future space mission, and its control logic was originally developed for the sounding rocket experiment CLASP. Because of our requirement on a speed of rotation (0.512 s/rotation) that was 10 times faster than that of CLASP, we optimized the control logic for the required faster rotation. Fast polarization modulation is essential for investigating the fine-scale magnetic field structures related to the dynamical chromospheric phenomena. We have verified that the rotation performance can achieve the polarization precision of 0.03 % (1 σ) required by SCIP and such a significant rotation performance is maintained under thermal vacuum conditions by simulating the environment of the Sunrise III balloon flight. The waveplate was designed as a pair of two birefringent plates made of quartz and sapphire to achieve a constant retardation in a wide wavelength range. We have confirmed that the retardation is almost constant in the 770 nm and 850nm wavelength bands of SCIP under the operational temperature conditions.
The Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) is a near-IR spectro-polarimeter instrument newly designed for Sunrise III, which is a balloon-borne solar observatory equipped with a 1 m optical telescope. To acquire high-quality 3D magnetic and velocity fields, SCIP selects the two wavelength bands centered at 850 nm and 770 nm, which contain many spectrum lines that are highly sensitive to magnetic fields permeating the photosphere and chromosphere. To achieve high spatial and spectral resolution (0.21 arcsec and 2 × 105), SCIP optics adopt a quasi-Littrow configuration based on an echelle grating and two high-order aspheric mirrors. Using different diffraction orders of the echelle grating, dichroic beam splitter, and polarizing beam-splitters, SCIP can obtain s- and p-polarization signals in the two wavelength bands simultaneously within a relatively small space. We established the wavefront error budget based on tolerance analysis, surface figure errors, alignment errors, and environmental changes. In addition, we performed stray light analysis, and designed light traps and baffles needed to suppress unwanted reflections and diffraction by the grating. In this paper, we present the details of this optical system and its performance.
The Sunrise balloon-borne solar observatory carries a 1 m aperture optical telescope and provides us a unique platform to conduct continuous seeing-free observations at UV-visible-IR wavelengths from an altitude of higher than 35 km. For the next flight planned for 2022, the post-focus instrumentation is upgraded with new spectro- polarimeters for the near UV (SUSI) and the near-IR (SCIP), whereas the imaging spectro-polarimeter Tunable Magnetograph (TuMag) is capable of observing multiple spectral lines within the visible wavelength. A new spectro-polarimeter called the Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) is under development for observing near-IR wavelength ranges of around 770 nm and 850 nm. These wavelength ranges contain many spectral lines sensitive to solar magnetic fields and SCIP will be able to obtain magnetic and velocity structures in the solar atmosphere with a sufficient height resolution by combining spectro-polarimetric data of these lines. Polarimetric measurements are conducted using a rotating waveplate as a modulator and polarizing beam splitters in front of the cameras. The spatial and spectral resolutions are 0.2" and 2 105, respectively, and a polarimetric sensitivity of 0.03 % (1σ) is achieved within a 10 s integration time. To detect minute polarization signals with good precision, we carefully designed the opto-mechanical system, polarization optics and modulation, and onboard data processing.
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