Abstract
There are strong scientific cases and practical reasons for building ground-based solar synoptic telescopes. Some issues, like the study of solar dynamics and the forecasting of solar flares, benefit from the 3D reconstruction of the Sun’s atmosphere and magnetic field. Others, like the monitoring and prediction of space weather, require full disk observations, at the proper sampling rate, combining H-alpha images and Doppler velocity and magnetic field. The synoptic telescopes based on Magneto Optical Filters (MOF) using different lines are capable of measuring the line-of-sight Doppler velocity and magnetic field over the full solar disk at different ranges of height in the Sun’s photosphere and low chromosphere. Instruments like the MOTH (Magneto-Optical filters at Two Heights), using a dual-channel based on MOFs operating at 589.0 nm (Na D2line) and 769.9 nm (K D1line), the VAMOS instrument (Velocity And Magnetic Observations of the Sun), operating at 769.9 nm (K D1 line), and the future TSST (Tor Vergata Synoptic Solar Telescope), using a dual-channel telescope operating at 656.28 nm (H-alpha line) and at 769.9 nm (K D1 line), allow to face both aspects, the scientific and the operative related to Space Weather applications. The MOTH, VAMOS and TSST data enable a wide variety of studies of the Sun, from seismic probing of the solar interior (sound speed, rotation, details of the tachocline, sub-surface structure of active regions), to the dynamics and magnetic evolution of the lower part of the solar atmosphere (heating of the solar atmosphere, identification of the signatures of solar eruptive events, atmospheric gravity waves, etc.), to the 3D reconstruction of the solar atmosphere and flare locations. However, the use of MOF filters requires special care in calibrating the data for scientific or operational use. This work presents a systematic pipeline that derives from the decennial use of MOF’s technology. More in detail, the pipeline is based on data reduction procedures tested and validated on MOTH data acquired at Mees Solar Observatory of the University of Hawaii Haleakala Observatories and at South Pole Solar Observatory (SPSO), at the Amundsen-Scott South Pole Station in Antarctica, during Antarctica Summer Campaign 2016/17.
Highlights
Multi-height observations of the Line-of-Sight (LoS) Doppler velocity and magnetic field signals over the full solar disk facilitate a wide range of studies of the Sun
The data reduction pipeline is designed to accomplish four major calibration functions: (1) standard dark subtraction (SDS) and flat field correction (FFC); (2) corrections for atmospheric extinction and orbital trend (STOC); (3) leakage correction (LC), based on a distinct series of frames, acquired with the Magneto-Optical Filters (MOF) filter turned off, necessary to correct for the parasitic light leaking through the non-ideal crossed polarizers; (4) rotation, resizing, alignment and cropping of the images acquired by the four CMOS cameras into a coherent final dataset
The MOTH has been upgraded in 2016 to a new version, characterized by a new optics design and CMOS camera sensors of 3072 Â 3080 pixels, for a final 1.46 arc-seconds spatial resolution. This instrument has been deployed to the South Pole Solar Observatory (SPSO) for two observation campaigns (November 2016–January 2017 and December 2017–January 2018)
Summary
Multi-height observations of the Line-of-Sight (LoS) Doppler velocity and magnetic field signals over the full solar disk facilitate a wide range of studies of the Sun. A similar observation of solar global oscillations was performed using a MOF-based instrument and sunlight reflected from the Moon, to reduce Earth atmosphere noise (Fussell et al, 1995) Another MOF-based instrument using potassium (K) is the VAMOS (Velocity And Magnetic Observations of the Sun), operating with the K D1 line, developed at Osservatorio Astronomico di Capodimonte, Napoli (Italy), whose results and pipeline are presented in a series of papers by Cacciani et al (1997), Moretti et al (1997), Oliviero et al (1998a, b), Vogt et al (1999), Oliviero et al (2011). DIMMI-2H (Moretti et al, 2010; Stangalini et al, 2011) was proposed as a double channel MOF imager candidate as an optional payload for the ASI space mission ADAHELI (Berrilli et al, 2010) and ESA proposal ADAHELI-PLUS (Greco et al, 2010; Berrilli et al, 2015)
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