A deeper look into Solar Imaging

Since I began my journey into astronomy and astrophotography, I’ve always been fascinated by the idea of photographing the Sun. Yes, galaxies or nebulas are perhaps more intriguing objects with their incredible shapes and dramatic colours but think about the Sun for what it represents.

Our beautiful host star, that governs the Solar System and provides life to our planet, that seems so far away above our heads and yet is so close that we can feel its warmth. Besides, what better opportunity to learn about stars than studying one so close to us?

Below is a graphic from NASA displaying the Sun’s structure and layers:

The internal structure is not relevant for photographing purposes, but it is worth mentioning that this is where the thermonuclear fusion happens (in the core precisely) and hydrogen is turned into helium. The energy produced in the process is than released in the form of visible light (yes, lonely photons!).

The atmosphere, on the other hand, is where fun stuff happens; stuff that we can actually see, if we have the right equipment of course. As we will see, different layers emit light in different wavelengths, and each can be observed with a dedicated filter.

Choosing the right one that matches your needs (and your telescope) is therefore very important and requires some research. For instance, if you get one of those cheap yellow filters, similar to those for observing eclipses, all you’ll see is a red-yellowish disc with sunspots as black dots and virtually no details.

A high-quality filter, on the other hand, not only will ensure safety to you and your instruments but will also provide sharper images through the eyepiece or camera.

These are some of the most common filters in amateur solar imaging:

  • White light (WL): A white light filter shows the Sun’s photosphere, where sunspots are best observed, at a reasonable price. If you own a refractor telescope, the best option is a Herschel wedge or prism, which is placed at the bottom of the telescope, replacing the standard diagonal.
    For other types of optics there are white light filters, such as the Baader Astrozap or Thousand Oaks’ Solarlite film, which can be mounted in front of the telescope.
  • Calcium K line (CaK): With these filters we enter a pricier category, but the costs are fully justified by enchanting views of sunspots and granularity on the lower chromosphere, in the violet part of the spectrum.
    CaK filters work best through dedicated modules placed between the (refractor) telescope and the eyepiece or camera. For different telescope designs you can get a CaK filter that can be screwed directly into the eyepiece or camera and can be used in combination with a Herschel wedge (WL).
  • Hydrogen-alpha (H-a): These very expensive filters reveal stunning, sharp features of the Sun’s upper chromosphere with flares and filaments in spectacular detail (see the gallery at the bottom of this article).
    The best way to observe or photograph the Sun in H-alpha is through a dedicated H-a telescope, such as Lunt or Coronado, although these generally come with an extremely big price tag. A valid alternative is provided by H-a filters, such as the Daystar Quark, which are screwed in the diagonal just before the eyepiece or camera. This solution, however, works only with refractors and provides better images at long focal lengths (> F/10).

As an avid solar photographer, I own several tools to photograph the Sun in nearly all wavelengths though I find myself using mostly the H-a filter, which I believe delivers the most dramatic images.

Solar photography, particularly in H-a, works best with monochrome cameras as they are more sensitive to light compared to colour cameras. Generally speaking, for solar imaging you want a monochrome camera with a CMOS sensor (with high dynamic range) and a high frame rate capability.

This is because, like the Moon and planets, the Sun is photographed through a technique called “lucky imaging”. Some of the most popular cameras for solar photography are the ZWO ASI174MM and ASI290MM.

Now let’s see in detail the technique I use to image the Sun. For clarity, I divided the process in three phases.

Phase 1 – Capturing

In astrophotography this is the most crucial step of the process, for obvious reasons: there’s little you can do in post-processing to fix a dull image, so focus is paramount and must be spot on. Take all the time you need to reach perfect focus, or anywhere near that anyway, and your efforts will be rewarded.

The second vital aspect is “seeing” – the effect of fast-changing, turbulent airflows in Earth’s atmosphere that can make an image look blurred or distorted. Sadly, there isn’t much you can do about this apart from sitting tight with you finger ready to hit the record button as soon as the live-view image on your screen gets clear.

Imaging the Sun in the early hours of the morning (9-12am), when the Sun is high enough, helps mitigating the effects of seeing whereas places surrounded by water tend to have less turbulence in the low layers of the atmosphere.

Also, recording more than one video obviously increases your chances to get a good image out of your session, after all it’s called “lucky imaging” for a reason.

For planetary and lunar/solar photography the most widely used software is SharpCap or FireCapture. Both are free, work great and have pretty much the same functionalities. Personally, I started with SharpCap and never changed thereafter.

I record my videos in *SER format, which is more complete and easier to work on compared to *AVI or other formats. I adjust gain and exposure to be anywhere between 70-90% of the histogram, depending on seeing conditions. Over this range, the image is overexposed and will appear as a bright patch with no details.

Bear in mind that the higher the exposure time is, the lower the frame per second (FPS) gets, meaning you’ll need longer videos during which the seeing could turn bad or clouds may roll in (a classic!). On the other hand, higher gain means more noise, so it’s important to find a good balance between exposure time and gain.

For solar photography, capturing around 2000-3000 frames is generally enough, as you only need between 300-500 good frames for a quality image. In bad seeing conditions I go as far as 5000 frames per video, hoping that at least 5-10% turn out to be good.

It is also recommended to take flat frames to remove signs of dust from the optics or camera sensor and to minimise any vignetting effect.

Phase 2 – Stacking

Once I have my set of videos, I load them one by one in the stacking software AutoStakkert!3 (also free), which analyses them by evaluating each single frame and giving as a result a diagram with the overall quality.

It’s based on this graph that I choose the percentage of total frames that I want to stack (usually between 10% and 20%). The software then chooses the single best frame, the reference frame, to which all other frames are aligned. If you took any flats, this is the right moment to use them.

When the job of AutoStakkert!3 is over, the result is a single final image made of the 10-20% best frames of the video. The stacked image generally looks a little blurred and certainly nowhere near the amazing picture you were expecting; nothing to worry about, this is where post-processing works its magic.

Phase 3 – Post-processing

The first thing a stacked image needs is some sharpening to bring out as many details as possible. This step can be done in any image-processing software although (free) astronomy tools such as ImPPG or Registax do an exceptional job, in my view.

While for planetary and lunar photography I usually work with Registax, for solar imaging I prefer the functionalities in ImPPG, which uses Lucy Richard deconvolution technique.

Here I adjust the Sigma and Amount parameters to sharpen the image and based on the histogram, I carefully move the Tone curve to bring out prominences while also trying to keep surface detail.

Once I’m done with ImPPG, I do the final adjustments in GIMP, a free software that looks a lot like PhotoShop. The first thing I do is crop the image to leave out any stacking artefacts, then I adjust exposure and contrast and play slightly with shadows, mid-tones, and highlights to bring out some more subtle details.

Now, this final step may come as a surprise to you, but I can assure it is common practice. As monochrome cameras represent the best option for solar imaging, the final image is of course black and white and as such it must be coloured artificially.

To do that I convert the image from grayscale to RGB, and with the use of curves in the red, green, and blue channels I give my solar image the colour I like. As we saw before, Calcium operates in the violet part of the spectrum so I would give the Sun taken with a CaK filter a bluish, violet tint.

Hydrogen-alpha, on the other hand, operates in the red-orange part of the spectrum so my H-a pictures generally have a reddish, gold or orange hue, depending on my mood.

To achieve that I push up the curve in the red channel and move down the curves in the blue and green channels. I know, I know…it sounds like cheating, but this is solar photography!

I hope this guide will get you started into solar imaging. Bear in mind that there are many other techniques out there, my advice is to try over and over, being creative and experimenting with settings and software until you find your own method.

The Sun is a dynamic, ever-changing target that never ceases to amaze; whether in a maximum or minimum solar cycle, there is always something going on with our star!

Remember: NEVER EVER point your telescope toward the Sun without using a proper, safe solar filter. Looking directly at the Sun can and will damage your eyes and your instruments irreversibly! Also, always ensure that the solar filter is well attached and stable before pointing your telescope toward the Sun.

Solar Gallery