Abstract
2025-12-13, He I 10830 disk. Direct wavelength reconstruction, stack of 50 scans. No continuum was subtracted. The signal is this strong.
Using a Sol’Ex type spectroheliograph[1], we imaged the full solar disk in the He I 10830 line. We obtained rich helium details on the direct disk reconstruction, without applying continuum subtraction, as it is the practice in He I D3. As a side note, the Paschen-gamma disk was also imaged. The original Sol’Ex design was modified and adapted to the near infrared / short wave infrared range, using off the shelf and DIY elements, as well as some high end custom components.
Building the instrument, design considerations
Reaching the Helium line at 10830Å, with the available instruments, seems challenging:
- CMOS sensors, nominally, approach zero QE this deep into the NIR/SWIR, and InGaAs sensors, while available, are at a prohibitive price range
- materials begin to behave in counter intuitive ways: anodized, black aluminum becomes reflective „white” at these wavelengths
- off the shelf items become sparse, the few professional optics vendors come into play
- amateur astronomy runs low and runs out of the available bandpass filters that could be used as ERFs
In spite of the challenges, there is some precedence. Christian Buil (Sol’Ex) recorded a proof of concept a while back [2]. So we reached out to Mr. Buil for his insights, and he was kind enough to point us into the right direction regarding the grating and lenses.
