He I 10830, 2025-12-27, powered by duct tape
Duct tape turned out to be one of the darkest materials I could build into the spectroheliograph, to limit parasite light and scatter on the chromium-bright (visually black) focuser for example.
He I 10830
Even though there is a band pass filter, 1075/50 in front of the instrument, those 50 nanometers still cause trouble. Duct tape to the rescue. The slit also got a light trap in front of it, partly to limit the thermal load on the plastic, and mainly to trap light reflected off the slit. The trap is an aluminum radiator from an old circuit board (some might have noticed, I have quite a few circuit board related components in my setup) that I inspected and confirmed it is indeed black. I cut it in half and use the wings as light traps, to prevent that light from getting onto surfaces I have less control over. And duct tape, duct tape went into the focuser tube. And. It. Helps. A. Lot. Also, a second bandpass filter has been ordered, 1100/50, to further limit the parasite light. These are laser and fluorescence filters, so no helium wavelength available off the shelf. I may one day go for the not that cheap custom option. Or just say that He I D3 is much more feasible.
To the best of my knowledge, just like with the Fe II 5018 dark plages and visible prominences, this image is among the first in this wavelength (I know only about the proof of concept level precedent by Christian Buil), and the best quality to date, as far as amateur astronomy is concerned.
The disk I present here is the reconstruction of the wavelength, there is no local contrast enhancement, and there is no continuum subtraction. Direct wavelength image. A stack of 136 scans.
The Oxygen Sun — O I 7772
Heavily amplified oxygen signal, projected back onto the 7772Å disk
With this new spectroheliograph, still in the tuning, I observed the infrared oxygen triplet at 7772, 7774 and 7775 Å, on 2025-12-13. As an ERF, I used a Nantong Foric bandpass filter, OD2-ish with the CWL close enough.
To the best of my knowledge, this is the first full solar disk with (amplified) oxygen features and prominences.
A subjective impression is that this oxygen line is somewhat similar to a weak helium. The line is in clear absorption on the disk, but turns into emission as going off the limb. Prominences are readily visible if bright, or become visible after applying math. Continuum subtraction shows that spots seem to be less dark, and the plages are also a bit shaded, less overall contrast.
He I 10830 — Challenger Deep
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.
