Error analysis and optimal design of polarization calibration unit for solar telescope

Polarization calibration unit（PCU） has become an indispensable element for solar telescopes to remove the instrumental polarization; the polarimetric accuracy of calibration depends strongly on the properties of PCU. In the paper, we analyze the measurement errors induced by PCU based on polarized light theory and find that the imperfections of the waveplate generate the main calibration errors. An optimized calibration method is proposed to avoid the effects from waveplate imperfections, and a numerical simulation is given to evaluate the polarization accuracy by analyzing the relation between calibration error and intensity instability. The work is very important for solar telescopes with high polarization precision up to 10^（-4） I_c.

Methodology for in-flight flat-field calibration of the Lyman-alpha solar telescope(LST)

Flat-field reflects the non-uniformity of the photometric response at the focal plane of an instrument,which uses digital image sensors,such as Charge Coupled Device(CCD)and Complementary Metal-Oxide-Semiconductor(CMOS).This non-uniformity must be corrected before being used for scientific research.In this paper,we assess various candidate methods via simulation using available data so as to figure the in-flight flat-field calibration methodology for the Lyman-alpha Solar Telescope(LST).LST is one of the payloads for the Advanced Space-based Solar Observatory(ASO-S)mission and consists of three instruments:a White-light Solar Telescope(WST),a Solar Disk Imager(SDI)and a dual-waveband Solar Corona Imager(SCI).In our simulations,data fromthe Helioseismic andMagnetic Imager(HMI)and Atmospheric Imaging Assembly(AIA)onboard the Solar Dynamics Observatory(SDO)mission are used.Our results show that the normal KLL method is appropriate for in-flight flat-field calibration of WST and implementing a transmissive diffuser is applicable for SCI.For the in-flight flat-field calibration of SDI,we recommend the KLL method with off-pointing images with defocused resolution of around 18′′,and use the local correlation tracking(LCT)algorithm instead of limb-fitting to determine the relative displacements between different images.

Presentation on the Electronic Control System of Space Solar Telescope

This paper gives the brief view of the electronic control system of SPACE SOLAR TELESCOPE (SST), especially the On Board Data Handling unit (OBDH) on the SST which control the operation of the instrument, acquire data and make data analysis and storage. In OBDH, the Scientific Data Unit (SDU) is a special unit that requires high speed computer. In this paper gives a brief comparison of two possible choices and discuss selection of electronic parts in the space environment.

Solar image reconstruction method under atmospheric turbulence at Fuxian Lake Solar Observatory

Strong atmospheric turbulence reduces astronomical seeing,causing speckle images acquired by ground-based solar telescopes to become blurred and distorted.Severe distortion in speckle images impedes image phase deviation in the speckle masking reconstruction method,leading to the appearance of spurious imaging artifacts.Relying only on linear image degradation principles to reconstruct solar images is insufficient.To solve this problem,we propose the multiframe blind deconvolution combined with non-rigid alignment(MFBD-CNRA)method for solar image reconstruction.We consider image distortion caused by atmospheric turbulence and use non-rigid alignment to correct pixel-level distortion,thereby achieving nonlinear constraints to complement image intensity changes.After creating the corrected speckle image,we use the linear method to solve the wavefront phase,obtaining the target image.We verify the effectiveness of our method results,compared with others,using solar observation data from the 1 m new vacuum solar telescope(NVST).This new method successfully reconstructs high-resolution images of solar observations with a Fried parameter r0 of approximately 10 cm,and enhances images at high frequency.When r0 is approximately 5 cm,the new method is even more effective.It reconstructs the edges of solar graining and sunspots,and is greatly enhanced at mid and high frequency compared with other methods.Comparisons confirm the effectiveness of this method,with respect to both nonlinear and linear constraints in solar image reconstruction.This provides a suitable solution for image reconstruction in ground-based solar observations under strong atmospheric turbulence.

Prototype of solar ground layer adaptive optics at the 1 m New Vacuum Solar Telescope

A prototype of a solar ground-layer adaptive optics （GLAO） system, which consists of a multi-direction corre- lating Shack-Hartmann wavefront sensor with 30 effective subapertures and about a 1 arcmin field of view （FoV） in each subaperture, a deformable mirror with 151 actuators conjugated to the telescope entrance pupil, and a custom-built real-time controller based on field-programmable gate array and multi-core digital signal processor （DSP）, is implemented at the 1 m New Vacuum Solar Telescope at Fuxian Solar Observatory and saw its first light on January 12th, 2016. The on-sky observational results show that the solar image is apparently improved in the whole FoV over 1 arcmin with the GLAO correction.

Vector Magnetograph Observations by the Solar Flare Telescope at Boao

We report that vector magnetograph (VMG) observations of the solar photosphere are being carride out by the Solar Flare Telescope (SOFT) at BOAO. The VMG uses a narrow band Lyot filter (FWHM=0 125A) for Stokes parameter (I, Q, U, V) observations to obtain longitudinal and transverse fields. We have obtained a filter convolved line profile of Fe I 6302 5 for VMG by changing the central wavelength of the Lyot filter, which is consistent with the Sacremento Peak spectral atlas data. Using the line profile, we have determined calibration coefficients of longitudinal and transverse fields by the line slope method. Then we have compared vector fields of AR8422 observed at BOAO with those at Mitaka. The comparison shows that longitudinal fields are very similar to each other, but transverse fields are a little different. Finally, we present Hα and magnetic observations of AR8419 during its flaring activity (M3 1/1B).

Algorithm Design and Test of the Solar Guide Telescope

The Solar Guide Telescope (SGT), an important solar attitude sensor of theSST (Space Solar Telescope, a space solar observing instrument being developed in China), canaccurately produce pointing error signals of the SST for attitude control at high speed. We analyzein detail the error algorithm of the heliocentric coordinates and the edge judging of solar images.The measuring accuracy of +- 0.5 arcsec of the SGT is verified by experiments on the tracking of theSun and by testing a sun simulator. Some factors causing the pointing errors are examined.

Full-disk synoptic observations of the chromosphere using Hα telescope at the Kodaikanal Observatory

This paper reports on the installation and observations of a new solar telescope installed on 2014 October 7 at the Kodaikanal Observatory. The telescope is a refractive type equipped with a tunable Lyot Hα filter. A CCD camera with size 2k ×2k acquires images of the Sun and has a pixel size of 1.21″ pixel^-1 and a full field-of-view of 41＇. The telescope is equipped with a guiding system which keeps the image of the Sun within a few pixels throughout the observations. The FWHM of the Lyot filter is 0.4A and the filter is motorized, capable of scanning the Hα line profile at a smaller step size of 0.01 A. Partial-disk imaging covering about 10＇ is also possible with the help of a relay lens kept in front of the CCD camera. In this paper, we report the detailed specifications of the telescope, filter unit, the installation, observations and the procedures we have followed to calibrate and align the data. We also present preliminary results with this new full-disk telescope.

Irradiation study of liquid crystal variable retarder for Full-disk Magneto-Graph payload onboard ASO-S mission

The Advanced Space-based Solar Observatory(ASO-S)is a mission proposed by the Chinese Solar Physics Community.As one of the three payloads of ASO-S,the Full-disc Magneto-Graph(FMG)will measure the photospheric magnetic fields of the entire solar disk with high spatial and temporal resolution,and high magnetic sensitivity,where liquid crystal variable retarder(LCVR)is the key to whether FMG can achieve its scientific goal.So far,there is no space flight experience for LCVR.Therefore,irradiation study for LCVRs becomes more important and urgent in order to make sure their safety and reliability in space application.In this paper,γirradiation,proton irradiation,and ultra-violet(UV)irradiation are tested for LCVRs respectively.The optical and chemical properties during irradiation tests are measured and analyzed.For optical properties,there is no significant change in those parameters FMG payload concerned except the retardation.Although there is no drastic degradation in the retardation versus voltage during irradiations,the amount of retardation variation is much higher than the instrument requirements.Thus,an in-flight retardation versus voltage should be added in FMG payload,reducing or even avoiding the impact of retardation change.For chemical properties,the clearing point and birefringence of the LC materials almost have no change;the ion density dose not change below 60 krad[Si],but begin to increase dramatically above 60 krad[Si].

Design and application of an autonomous Master Control System for a multi-layer magnetic and helioseismic telescope

With the growing significance of space weather forecasting,multi-layer magnetic and helioseismic telescopes are emerging as a key area of research.However,owing to the diverse operational processes and sophisticated hardware configurations of these devices,there is an urgent need for efficient autonomous observation capabilities.An autonomous Master Control System(MCS)can ensure efficient performance,data consistency,and stability,and the prototype presented here adopts a microservices architecture,breaking down the hardware into multiple subsystems and converting their functions into individual services.A central decision-making system leads the operations,supported by three auxiliary systems and three device control systems.Through inter-subsystem service calls,it achieves rapid imaging and spectroscopic monitoring.To verify system stability and observation efficiency,the system was tested on the Solar Full-disk Multi-layer Magnetograph.Experimental results verify this system can operate automatically for 4 consecutive months,acquire photospheric vector magnetic and Doppler velocity fields within a 15-minute interval,and measure chromospheric longitudinal magnetic and Doppler velocity fields in under 180 s.This ensures consistent and stable solar monitoring and serves as a practical methodological benchmark for the development of similar devices.