# Calibration of full-disk He I 10830 \AA\ filtergrams of the Chromospheric Telescope

## TL;DR

This paper introduces a new calibration method for full-disk He I 10830 Å filtergrams from the Chromospheric Telescope, improving image quality and enabling accurate Doppler velocity measurements for solar studies.

## Contribution

The authors develop a calibration technique using limb-darkening correction and Zernike polynomials to remove large-scale intensity variations and artifacts in ChroTel data, including Doppler velocity determination.

## Key findings

- Effective removal of limb-darkening and transmission artifacts.
- Calibration method validated with solar activity data sets.
- Doppler velocities benchmarked against high-resolution spectroscopic data.

## Abstract

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.