The Spectroheliograph isn’t a new invention—it’s actually one of the oldest methods used to observe the Sun in narrowbands. It was independently developed by George Ellery Hale and Henri-Alexandre Deslandres in the 1890s and later improved by Robert R. McMath in 1932 to capture motion pictures. Thanks to recent advancements in software and hardware, pioneered by Christian Buil and others, the Spectroheliograph has now entered the digital era. Image acquisition and reconstruction are done digitally with the help of specialized software.

Here’s how it works: A slit is used to capture a small slice of sunlight. This light passes through a collimator, which turns it into parallel rays. These rays then hit a diffraction grating, which spreads the light into a spectrum. The spread beam is refocused by a lens onto a camera sensor. By adjusting the tilt of the grating, you can choose which part of the spectrum is focused on the sensor, allowing you to observe the Sun in different wavelengths (such as H-alpha, Sodium D, Helium D3, or Ca K).

The Sun’s details and contrast are captured in slices, recorded as a video while the mount slowly slews, causing the Sun’s image to "scan" through the slit. Specialized software then reconstructs the full image of the Sun from this video data

Below, you'll find an excellent animation by Jérôme Bastardie that demonstrates how an SHG works. This animation is based on the Sol'Ex SHG, a DIY project by Christian Buil. While some components may differ, the animation perfectly illustrates the core operating mechanism of all spectroheliographs. The video is in French, but you can easily follow how it works just by watching the animation.

Since the Sun's image is captured in a series of sequential frames (a video) and then reconstructed by software, you won’t be able to observe the Sun directly with your own eyes. The spectroheliograph is strictly an imaging device, not for visual use (and depending on the type of ERF you’re using, it might not even be safe to attempt visual observation with the SHG).

When using a spectroheliograph, here's what you'll see through the camera. Since we need a high frame rate to record the Sun’s slices in sequence, a cropped region of interest (ROI) around the desired absorption line (in this case, H-Alpha) is used. This helps maintain a high frame rate and keeps file sizes more manageable.

After the video is captured, it is reconstructed into a still image of the Sun using specialized software, as shown in the animation below. This animation illustrates how the sequential slices of the Sun are pieced together to create a full image.

Pros of SHG

As a device that observes the Sun spectrally, the SHG offers several advantages over traditional interference-based filters like etalons.

The SHG delivers super-high contrast with an ultra-narrow bandpass. While a top-notch etalon filter might hit around 0.5 Angstrom FWHM (and those crazy 0.3A ones cost a fortune), the SHG easily resolves between 0.06 and 0.1 Angstrom per pixel with very high finesse. This means insanely high contrast images without any parasitic light leaks from the photosphere, which you get with single stack etalons..

SHGs don’t have the same “sweet spot” effect you get with etalon filters. The entire field of view (FOV) maintains the same contrast, detail, and sharpness with exceptional uniformity.

This makes SHGs perfect for full disk imaging, giving you a uniform, high-contrast view of the entire Sun.

An SHG lets you observe the Sun across multiple wavelengths with just one device. No need to buy multiple etalons for different wavelengths anymore—just tune a knob and refocus the optics. Whether you’re interested in CaK, He D3, Sodium, H-alpha, H-beta, or any part of the visible spectrum, the SHG covers it all. How cool is that? It’s not only an awesome way to learn about the Sun but also a fantastic tool for education and research. With an SHG, you’ve got a complete solar observatory at your fingertips.

Observing the Sun spectrally with an SHG provides a wealth of information. Beyond just capturing contrast and details, you also get Doppler shift data, which allows you to measure plasma speed. This means you can determine whether plasma is moving toward Earth or away, and how fast it's going—kind of like having your own cosmic speedometer. You can even pull off some clever tricks, like observing the Zeeman split in the iron absorption line to detect magnetic fields in the Sun's atmosphere. With that info, you can draw a magnetogram, giving you insight into the Sun’s magnetic activity. Or as the image above, you can plot out the Sun's rotation through Dooplergram

It's affordable. Solar imaging has traditionally been expensive, and while recent advances in etalon-based filters have lowered costs somewhat, they’re still pricey. The "cheapest" option out there runs around $1300. The MLAstro SHG, however, costs only half that for the entry model. Plus, with a simple and inexpensive upgrade kit, you can transform the SHG into a high-resolution spectrograph for nighttime spectroscopy or spectral measurements, giving you even more value for your money.

The "cheap" solar H-alpha filter comes with a hidden cost: significant unit-to-unit quality variation. This is often referred to as the "etalon lottery," where you don't know how your filter will perform until you use it for the first time. Some people end up keeping the good filters and selling the poor-performing ones on the second-hand market, which makes things even trickier for newcomers. The SHG avoids this problem entirely. It nails the top performance every single time! So you can expect consistent results, with under 0.1 Angstrom resolution and uniform field of view, from unit to unit.

Cons of SHG

While the advantages of SHGs are stated above, it is also important to consider some of their drawbacks.

Using an SHG is a different way of observing the Sun, and it takes a bit of time to get used to. Unlike the familiar process of just looking through an eyepiece with an etalon filter, SHGs require you to get accustomed to specific steps like orientation, focusing, and slewing the mount to scan the Sun's image through the slit. Calibration and focusing are also different.Luckily, with the MLAstro SHG, we've handled most of the hard work in building and calibrating it. You just need to orient your SHG, find the Sun, and start imaging. There's still a learning curve, but it's much less steep and should be enjoyable once you get the hang of it.

See those jagged edges? They happen when the Sun’s slices don’t perfectly line up due to small variations between successive frames, often caused by seeing conditions or mount vibrations while slewing. These edges are an inevitable part of SHG images with high-contrast features. To minimize these artifacts, use a high-quality mount, proper scan speed, ensure proper alignment of your scope, SHG, and counterweights, and aim for good seeing conditions. Additionally, stacking several frames can significantly reduce these artifacts, making them barely visible.

SHGs have some limitations when it comes to high magnification. Because SHG images are built from successive slices, seeing effects can be more pronounced compared to etalon-based filters, which can benefit from lucky imaging techniques. Small details often get blurred due to seeing variations. While stacking frames helps reduce these issues, it doesn’t completely eliminate them. Consequently, SHGs generally offer less spatial resolution than an etalon system of the same aperture. I’d personally recommend not using SHGs on scopes longer than 1000mm unless you have exceptionally good seeing conditions.

Wants to learn more?

Watch those instructional videos and reading materials

Here’s a fantastic video by MLAstro that covers everything you need to know about orienting, focusing, performing scans, capturing video, and processing it into an image with your SHG700. If you’ve bought the prebuilt MLAstro SHG with optics included, this is the video to watch.

If you want to read more about the SHG I suggest you to start with Christian Buil's guide on the Sol'Ex. Again, all the skills and knowledges are transferable. http://www.astrosurf.com/solex/sol-ex-observation-en.html