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.

For the spectrograph body, the JamesR version [3] was used as a starting point. The design was slightly modified, to accommodate the custom optics: same diameter and focal lengths, but different heights and AR coating chosen for the infrared wavelength range. As with previous instruments, a drawback of not using a heliostat, but mounting the entire setup onto the focuser tube of an off-the-shelf refractor, a support structure had to be constructed around the instrument, from V-slot rails and other mechanical elements.

The image below shows my third spectroheliograph:

• 62/400 ED doublet refractor (a twin of the #1 instrument)
• TS optics filter drawers for in-cone ERF filters
  • awaiting the slider for Edmund Optics 1075/50 nm bandpass filter
• ASI 678MM camera, with added peltier cooling and fan (stopped while scanning)
• the JamesR body printed and modified to accommodate:
  • Thorlabs IR optimized 25.4/80 doublet (collimator lens)
  • Thorlabs IR optimized 25.4/150 doublet (objective lens, differs from the solex kit’s size)
  • Thorlabs 1200 ln/mm reflective grating
• 30/125 guide scope, with ESP32 development board[8]

Navigating the spectrum is also challenging. On the one hand, the Liege[4] stops at 9300Å, JSolEx[5] is also limited at these wavelengths, and the BASS2000’s diagram[6] is hard to compare to a live image of the spectrum. Tellurics have also proven to be problematic while navigating the spectrum.

A close-enough narrow band pass filter was chosen as an ERF. CWL 1075nm, FWHM 50nm, OD 4.

We decided, from the very first instrument[7], to use in-cone or full aperture ERF filters, in order to keep most of the light at the wavelength of interest. This is a deviation from the default advice of putting a Herschel prism into the light cone, or using a full aperture neutral density filter. A custom filter drawer was designed to be CNCd from aluminum, to bridge the amateur form factors,  filter drawers and sliders, with the professional form factor of the filter, 50mm diam, 5mm high cell.

A blunder: for the first light, this drawer did not arrive yet, thus only long pass filters were used to limit the thermal load on the focal plane. Still, after the first light, minor thermal damage was observed on the plastic parts. One of the reasons could be the extensive time dedicated to a prominence, which kept the Sun off the slit plate’s center. The sustained damage consists of localized melting of the 3D printed plastic, no boiling, thus no precipitation onto the slit. At this point, the instrument requires no servicing, it is just a reminder that observing the Sun is inherently risky.

First light, He I 10830 in the spectrum

At around UT 2025-12-13 T 12:40,  there was a bright prominence at just the right position to place the slit onto it, without changing the instrument’s (slit’s) orientation. A large field of view allowed for the infrared hydrogen series member, Paschen gamma at 10938.1Å to also show up. In the spectra below, top is the shorter wavelengths.

Flash spectrum: He I 10830 and H Paschen gamma 10938

Prominences in He I 10830 and Paschen gamma

He I 10830 prominence

Paschen gamma, left: in emission when off disk, right: absorption on the disk

Full disk images

We have previously shown, through continuum subtraction, the imprint of coronal holes in He I D3 (confidently) [9] and in He I 7065 (fairly convincing match) [10]. In He I 10830, even the reconstructed disk of the wavelength shows hints of the coronal features.

In the table below, the stacked disks got some manual editing to mitigate slit unevenness and reconstruction artefacts (line/pixel blending because of the curving of the spectral line). Single scans’ reconstructions are presented as the processing software provided the output, with global stretching.

2025-12-13, He I 10830 disk. Direct wavelength reconstruction, stack of 50 scans. No continuum was subtracted. The signal is this strong.

Curves applied

Signal enhanced by continuum subtraction. Coronal holes become obvious

2025-12-13-1249 – a single scan’s delta=(line-continuum) image

2025-12-13-1249 a single scan’s continuum wavelength (+9 pixels away from the guide line)

2025-12-13-1249 a single scan’s helium wavelength (-17 pixels away from the guide line)

A side note: the Paschen-gamma Sun

Similar to the Balmer series (with the classical Hα), in the infrared, there is the Paschen series of hydrogen lines: at around 1875nm the α, 1282 nm β and 1094 nm the γ imaged here. Five scans were successful. The signal is weak, the line itself is blurred and generally hazy, but the prominence was obvious, off the disk, in the flash spectrum.

The disk at the guiding spectral line’s wavelength (the most prominent line, used for de-smiling the curved spectral line)

one of the continuum wavelengths

the Paschen wavelength’s core, notice the subtle darkening, and the somewhat less prominent sunspot, compared to the continuum

stretched delta = paschen-continuum

stacked concept, curved

stacked concept (obtained from the disk and delta), most artefacts manually edited out

Our subjective impression of the Paschen-gamma:

• similar to the higher hydrogens (Balmer epsilon and bluer), but in the UV the continuum is metal dominated, here the continuum is blunt (could also be the CMOS pushed too far)
• somewhat similar to helium lines, both its aspect as a chromosphere and the way processing is to be done
• somewhat similar to a rather anemic hydrogen delta and ionized metals, in that the otherwise brighter plages get darker, but filaments are mostly or totally missing
• the nebulous dip of Paschen lines, noted by professional observers using the bolometer[11], is barely, if at all, noticeable in the spectrum live view. The line is nothing like Balmer α/β/γ or the Calcium II lines, K, H and IR triplet

Links, references

[1] https://solex.astrosurf.com/sol-ex-presentation-en.html

[2] https://groups.io/g/Solex-English/message/1212 and https://groups.io/g/Solex-English/topic/sun_chromosphere_from/100025119

[3] https://cults3d.com/en/3d-model/gadget/sol-ex-by-james-r

[4] https://fermi.jhuapl.edu/liege/s00_0000.html

[5] https://github.com/melix/astro4j/tree/main/jsolex

[6] https://bass2000.obspm.fr/solar_spect.php

[7] https://iopscience.iop.org/article/10.3847/2515-5172/adef50

[8] //csillagtura.ro/solar-autoguider-for-my-shg/

[9] among others, //csillagtura.ro/the-helium-sun-on-2025-02-09/

[10] among others, //csillagtura.ro/the-sun-in-he-i-at-7065a/

[11] https://babel.hathitrust.org/cgi/pt?id=uc1.32106002409180&seq=5