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High-resolution Observations of H$\alpha$ Spectra with a Subtractive Double Pass
Authors:C Beck  R Rezaei  D P Choudhary  S Gosain  A Tritschler  R E Louis
Institution:1.National Solar Observatory (NSO),Boulder,USA;2.Instituto de Astrofísica de Canarias (IAC),La Laguna,Spain;3.Departamento de Astrofísica,Universidad de La Laguna (ULL),La Laguna,Spain;4.Department of Physics & Astronomy,California State University,Northridge,USA;5.Center of Excellence in Space Sciences India (CESSI),Indian Institute of Science Education and Research Kolkata,Mohanpur,India
Abstract:High-resolution imaging spectroscopy in solar physics has relied on Fabry–Pérot interferometers (FPIs) in recent years. FPI systems, however, become technically challenging and expensive for telescopes larger than the 1 m class. A conventional slit spectrograph with a diffraction-limited performance over a large field of view (FOV) can be built at much lower cost and effort. It can be converted into an imaging spectro(polari)meter using the concept of a subtractive double pass (SDP). We demonstrate that an SDP system can reach a similar performance as FPI-based systems with a high spatial and moderate spectral resolution across a FOV of \(100^{\prime\prime} \times100^{\prime \prime}\) with a spectral coverage of 1 nm. We use H\(\alpha\) spectra taken with an SDP system at the Dunn Solar Telescope and complementary full-disc data to infer the properties of small-scale superpenumbral filaments. We find that the majority of all filaments end in patches of opposite-polarity fields. The internal fine-structure in the line-core intensity of H\(\alpha\) at spatial scales of about 0\(.\!\!^{\prime \prime }\)5 exceeds that in other parameters such as the line width, indicating small-scale opacity effects in a larger-scale structure with common properties. We conclude that SDP systems in combination with (multi-conjugate) adaptive optics are a valid alternative to FPI systems when high spatial resolution and a large FOV are required. They can also reach a cadence that is comparable to that of FPI systems, while providing a much larger spectral range and a simultaneous multi-line capability.
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