An accurate metrology system is required to stabilize the differential path lengths in the Nulling Interferometry Cryogenic Experiment (nice) to within 0.45nm peak-to-peak to achieve broadband mid-infrared nulls with long exposure times, which are required for potential future space missions, such as the Large Interferometer for Exoplanets (life) mission, that aim to directly image and characterize temperate terrestrial exoplanets. For this purpose, a differential heterodyne laser distance metrology is developed to enable differential path length measurements that are stable over multiple days with sub-nanometer accuracy at a bandwidth of 1 kHz. The system aims to solve several challenges that arise in the context of NICE, such as the need for long-term stability, the high intensity attenuation through the NICE beam path, and the requirement that the metrology be able to deliver low-latency feedback for closed-loop operation to compensate vibrations and drifts of the nulling testbed. The metrology uses a 633nm HeNe laser and operates at ambient temperature and pressure with a beat frequency of 10 kHz, which is generated by acousto-optic modulators. To improve long-term stability, the compact optical layout is optimized for low susceptibility to temperature variations. Over periods of 2 s, the intrinsic instability of the metrology is ≈ 80pm RMS when sampling at 10 kHz, and it is stable to within ≈ 0.5nm peak-to-peak for 2 hours. When correcting the distance measurements for the temperature of the metrology board, it is stable to within ≈ 1nm peak-to-peak for 14 hours. The metrology fulfils the stability and bandwidth requirements for nice for a duration of at least 2 hours. To achieve stability over even longer time periods, the metrology will later be placed in a temperature-controlled vacuum environment.
The Large Interferometer For Exoplanets (LIFE) is an envisioned nulling interferometry space mission to characterize the atmospheres of terrestrial exoplanets in the mid-infrared (MIR) wavelength range (∼4-18.5 μm.) The star-to-planet flux contrast for an Earth-twin exoplanet is ≈ 107 at these wavelengths. Previous studies have shown that a “raw” null-depth of 105 provided by the interferometer is sufficient as long as the residual starlight can be removed through signal modulation, phase-chopping and data post processing. Two main technological challenges for a nulling interferometer are instrument stability and sensitivity. Several test-benches were built for LIFE’s ancestral mission concepts DARWIN and TPF-I. Operating at ambient conditions, they demonstrated excellent stability and suppressed the artificial starlight by up to 106 (depending on the spectral bandpass). However, instrument sensitivity/throughput for astronomical sources can not be characterized at background dominated ambient conditions. Cooling the instruments to cryogenic conditions reduces the thermal background and enables sensitivity driven instrument characterization. The Nulling Interferometer Cryogenic Experiment (NICE) is a single Bracewell nulling interferometer test-bench for LIFE. The ultimate aim of this test-bench is to attain a sensitivity level that demonstrates the feasibility of detecting an Earth-twin around a Sun-like star at 10 pc with a spectral bandwidth of 10% at 10 μm. The development of NICE is divided into two phases, the warm and cold phase. The warm phase focuses on the alignment of the optical components and maintaining their position and angular stability to achieve a null depth of 10−5 − 10−6 at 4 μm over several hours. In the cold phase, NICE will be cooled to 15 K to suppress the thermal background, and the throughput and sensitivity of the instrument will be characterized. This paper describes the development plan of NICE and presents the optical layout of the NICE warm phase. It also presents the preliminary null-depth reached by the NICE warm phase and the residual alignment errors in the system.
The multislit spectro-polarimeter (MSSP) is a grating-based littrow spectrograph with five slits at the entrance aperture. The polarimeter consists of a nematic liquid crystal variable retarder as the modulator and a Savart plate as an analyzer. It is one of the facility instruments on the Multiapplication Solar Telescope at the Udaipur Solar Observatory, developed to measure the magnetic fields of the Sun in the photosphere and chromosphere. MSSP currently operates only at 630.2 nm (FeI), but will be upgraded to cover CaII at 854.2 nm, HeI at 1083.2 nm, and FeI at 1565.3 nm. The spectrograph has a spectral dispersion of 15.8 mÅ ± 1.2 mÅ at 630.2 nm. The polarimeter has a sensitivity of the order of 10 − 2 and the root mean squared noise in the Stokes spectrum (continuum wavelength points of Stokes Q, U, and V) is 0.015I. To obtain an estimate of the instrument induced polarization, an analytical model is developed to determine the polarization introduced by the telescope. A polarimetric calibration (PolCal) unit is used to calibrate the downstream optical path from the telescope exit pupil up to the detector in MSSP. A residual polarization cross talk of 10% is measured in the data after applying PolCal corrections. The polarimetric data obtained from the engineering run (first-light) are inverted using NICOLE, to extract the magnetic field parameters. The field strength derived from MSSP observations is compared with the data obtained from helioseismic and magnetic imager and is found to lie within ±70 G in the umbral region and ±200 G in the penumbral region.
Stokes V, the circular polarization of light, from the solar corona is weak of the order of 10 − 4 times the intensity (Stokes I). Measuring weak source polarization, for a faint source such as corona, is difficult and requires long integration time. To obtain long uninterrupted measurements, a space-based polarimeter would be preferable over a ground-based observatory. A full-Stokes polarimeter is designed such that the modulation matrix provides high efficiency in measuring the weak Stokes V signal, while minimizing the cross talk from Stokes Q and Stokes U. The prototype polarimeter consists of a single crystal retarder as the modulator and a Wollaston prism as a dual beam analyzer. The modulation is performed by rotating the retarder continuously. An optimum modulation matrix is derived taking into account the systematics and polarization cross talk due to satellite jitter. We present the results of cross talk simulation and the steps taken to obtain the modulation matrix. A polarimeter designed to observe the solar corona at Fe XIII 1074.6-nm emission line is presented. Jitter and drift from a low earth orbit satellite are taken to simulate and experimentally verify the cross talk and polarimetric efficiency of the polarimeter. A plane parallel crystal retarder produces polarized fringes. F-ratio of the beam incident on the retarder is one of the factors that effects the contrast of the polarized fringes. Results from a simulation performed to compare the polarizance produced by the retarder, when placed in a converging beam and a collimated beam of light, is presented.
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