How spectroheliographs work

Alright, imagine Jodie Foster in Contact mode (my favorite all-time space movie), staring in awe at the cosmos, but with just a hint of that Silence of the Lambs intensity.

"A spectroheliograph… Dr. Lecter… is an elegant instrument, designed for a very specific purpose. It isolates and records images of the Sun in a single wavelength of light—like hydrogen-alpha, calcium-K, or any other atomic signature. It’s not just seeing the Sun, it’s… understanding it, peeling away its secrets, layer by layer, like an onion… or perhaps a census taker’s liver. With fava beans and a nice chianti—except in this case, the chianti is a carefully tuned diffraction grating, splitting the light into its spectral components."

She takes a breath, eyes wide, the kind of wonder only science can evoke.

"It sweeps across the Sun, like a radio telescope scanning the cosmos, but instead of searching for extraterrestrial life, it’s mapping solar activity, sunspots, prominences—phenomena invisible in ordinary white light. Think of it like a cosmic barcode scanner for our star… except instead of groceries, we’re scanning the thermonuclear rage of a giant plasma ball 93 million miles away."

She leans forward, whispers dramatically: “And that… is how we listen to the whispers of the Sun.”

Cue dramatic music.

Minh Trong, the founder of MLAstro, is a civil engineer with a passion for astronomy who has dedicated years to perfecting solar imaging technology - enter the MLAstro Spectroheliograph. Operating a remote observatory in Hanoi, Vietnam, Trong’s journey into solar observation led him to discover the Spectroheliograph (SHG), a powerful yet often complex tool for capturing detailed images of the Sun at specific wavelengths. Frustrated by the limitations of existing designs, he set out to refine and improve upon them, culminating in the creation of the SHG 700.

How spectroheliographs work

In this interview, Trong discusses his background, the excitement of solar imaging, and how his product stands apart from traditional solar filters and imaging systems. He delves into the importance of achieving ultra-narrow bandpasses for higher contrast, the advantages of his ready-to-use SHG over DIY alternatives, and what enthusiasts can expect from his next product release.

SCOPETRADER: Can you tell me a little about your background?

Minh: I’m a civil engineer by profession with a deep fascination for astronomy. I built and operate a remote imaging observatory in the suburbs of Hanoi, Vietnam. My journey into solar imaging began about four years ago, and I eventually discovered the Spectroheliograph (SHG) through Christian Buil’s DIY project. While his project is impressive, I noticed certain limitations, such as the lack of durability in 3D-printed housings for outdoor use, challenges with precision optical alignment, and optics that restricted the usable focal length for achieving full-disk imaging in one pass. These limitations drove me to redesign and improve upon the concept, leading to the creation of the SHG 700, a robust and versatile solution.

You can see my image gallery here https://www.astrobin.com/users/minhlead/

SCOPETRADER: What is it about solar viewing and photography that excites you the most?

Minh: I vividly remember the first time I observed a solar prominence with my scope, it was awe-inspiring. Unlike other celestial objects, the Sun is incredibly dynamic, constantly evolving, and displaying changes within minutes. Observing the Sun with an SHG adds a new dimension to this experience. It enables wavelength shifts to measure plasma velocities, allows post-processing adjustments to narrow or widen the bandpass for specific events, and provides the ability to switch wavelengths to explore different solar layers. The possibilities are truly endless, making each observation a thrilling scientific and creative endeavor.

SCOPETRADER: How is your product/approach different than what’s on the market today?

Minh: Solar imaging has always been an expensive pursuit. Better performance has traditionally come at a steep cost, narrower bandpasses, more uniform fields of view, larger telescopes, and more specialized equipment. And if you want to explore multiple wavelengths, the price simply shoots through the roof. For many enthusiasts, especially newcomers or those on a budget, these barriers make solar imaging feel unattainable. I remember saving for a very long time until I can afford my first solar scope.

I don't want to be a part of the cycle. The SHG 700 is built to break these barriers. My goal was to design a tool that delivers consistent, high-quality results without the frustrations of the dreaded “etalon lottery” yet still priced reasonably. With the SHG 700, you can scan the entire visible spectrum, including H-alpha, CaK, Sodium, and more, with a single device. It eliminates the need for costly, wavelength-specific etalons while ensuring reliable, top-tier performance every time. 

Whether you’re a hobbyist, educator, or researcher, the SHG 700 opens new doors for solar imaging. It’s a tool built to push the boundaries of what’s possible while keeping costs in check. Everyone deserves the opportunity to explore the Sun’s breathtaking beauty and complexity, without breaking the bank. 

One of the biggest hurdles for beginners in approaching solar imaging with an SHG is the DIY nature of most commercially available SHG . With projects like the Sol'Ex, you need to purchase an optical kit or source the components yourself (often with uncertainties about the quality of individual components), 3D print the housing, or buy a commercially printed version, and then spend countless hours assembling everything. The process doesn’t stop there; calibrating and focusing the optics can be just as time-consuming. Many enthusiasts purchase the optical kit only to leave it untouched for months, waiting until they have enough time and energy to begin building (that day may never come). Even when everything is done right, a faulty print or a misaligned thread insert can derail the entire effort, forcing you to restart the process from scratch. 

That’s where the SHG 700 changes the game. It comes prebuilt, with all the optics installed, calibrated, and ready to use. This removes nearly 70% of the heavy lifting traditionally involved in setting up an SHG. All that’s left is aligning it with your telescope and fine-tuning the focus, a process that gets you imaging in a fraction of the time. For beginners and busy hobbyists alike, the SHG 700 eliminates frustration and uncertainty, letting you focus on what matters most: capturing stunning solar images.

Not just that, the SHG 700 is built for solar imaging from an experienced solar imager. The unibody aluminum design ensure the high rigidity yet light weight and zero deformation under solar radiation and ambient heat. The high precision machining ensure perfect optical alignment. The micrometers help the focusing of the optics quick and effort less, you can adjust the focus on the fly without shifting the optics. The optics also good for longer telescope (the SHG 700 accept scope up to 730mm in FL for fulldisk in 1 pass). And the Quartz slit ensure you do not need ERF on scope up to 4 inches.

I can confidently say that the MLAstro SHG 700 is truly one of a kind and the most cost-effective option available for those passionate about solar imaging

SCOPETRADER: What exactly is a Spectroheliograph?

Minh: A Spectroheliograph is a device that uses diffraction and scanning to capture images of the Sun at specific wavelengths. Unlike traditional imaging systems, which rely on filters to isolate a single wavelength, the SHG scans the solar disk, recording narrow slices of light. These slices are then reconstructed into a full-disk image, revealing stunning details at wavelengths like H-alpha or Calcium K. It’s a versatile tool for observing solar features across the visible spectrum.

SCOPETRADER: Talk a little about the importance of 0.3 Angstrom contrast

Minh: Achieving a narrow bandpass is essential for solar observation. A narrower bandpass reduces parasitic light from the photosphere, isolating the hydrogen emission (or absorption within the disk) to produce higher contrast images. While it’s challenging to quantify the exact FWHM of an SHG image due to its unique operational method, it can surpass even the contrast of a 0.3A etalon.

The SHG works by reconstructing images from single-pixel slices of the spectrum. For example, using a small-pixel camera like the 678M (2-micron pixel size), one pixel corresponds to approximately 0.087 Å of the spectrum, resulting in an effective FWHM closer to 0.1 Å." Under new light from real-life experiments and calculations from experts, the FWHM of a spectrograph can be estimated as two times the spectral resolution, meaning a 0.087 Å resolution spectrograph will have an effective FWHM of 0.174 Å (so nearly 0.2 Å), still insanely high but not 0.1 Å as previously stated. Furthermore, unlike etalons, the SHG’s transmission curve is nearly vertical, delivering impeccable contrast without the “sweet spot” or uniformity issues.

Press-tuned etalon filters often suffer from uneven pressure in tuning, leading to variations in FWHM and brightness across the field. Achieving a uniform 0.3A etalon is costly. With the SHG, these challenges are eliminated, you achieve consistent top performance across the entire field every time.

SCOPETRADER: When do you expect your next batch of products?

Minh: We’re targeting late February for the next batch, though there’s a possibility of earlier shipping by late January. Based on user feedback, we’ve introduced updates like foam cases for safe storage and improved compatibility features, and we’re excited to share these with our customers soon.

An SHG lets you observe the Sun across multiple wavelengths with just one device

Dylan O'Donnell's coverage: Scanning the Sun like a FAX - Spectroheliographs

Check out the video below, where Dylan dives head first into the fascinating world of solar astronomy using the MLAstro Spectroheliograph on a SkyWatcher 82 ED doublet. His results are jaw-dropping.