Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Calibration of full-disk He I 10830 \AA\ filtergrams of the
  Chromospheric Telescope   The Chromospheric Telescope (ChroTel) is a small 10-cm robotic telescope at
Observatorio del Teide on Tenerife (Spain), which observes the entire Sun in
H$\alpha$, CaII K, and HeI 10830\AA. We present a new calibration method that
includes limb-darkening correction, removal of non-uniform filter transmission,
and determination of HeI Doppler velocities.
  Chromospheric full-disk filtergrams are often obtained with Lyot filters,
which may display non-uniform transmission causing large-scale intensity
variations across the solar disk. Removal of a 2D symmetric limb-darkening
function from full-disk images results in a flat background. However,
transmission artifacts remain and are even more distinct in these
contrast-enhanced images. Zernike polynomials are uniquely appropriate to fit
these large-scale intensity variations of the background. The Zernike
coefficients show a distinct temporal evolution for ChroTel data, which is
likely related to the telescope's alt-azimuth mount that introduces image
rotation. In addition, applying this calibration to sets of seven filtergrams
that cover the HeI triplet facilitates determining chromospheric Doppler
velocities. To validate the method, we use three data sets with varying levels
of solar activity. The Doppler velocities are benchmarked with respect to
co-temporal high-resolution spectroscopic data of the GREGOR Infrared
Spectrograph (GRIS). Furthermore, this technique can be applied to ChroTel
H$\alpha$ and CaII K data. The calibration method for ChroTel filtergrams can
be easily adapted to other full-disk data exhibiting unwanted large-scale
variations. The spectral region of the HeI triplet is a primary choice for
high-resolution near-infrared spectropolarimetry. Here, the improved
calibration of ChroTel data will provide valuable context data.

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The categories of this paper are: math
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The categories of this paper are: alg-geom
22
The categories of this paper are: chao-dyn
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The categories of this paper are: solv-int
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The categories of this paper are: physics
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The categories of this paper are: adap-org
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The categories of this paper are: funct-an
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The categories of this paper are: supr-con
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The categories of this paper are: stat
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The categories of this paper are: hep-lat
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The categories of this paper are: q-alg
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-th
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The categories of this paper are: comp-gas
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The categories of this paper are: eess
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The categories of this paper are: quant-ph