With long days and shorter nights, summer is a good time to consider getting into solar astrophotography. The Sun is also heading into the active phase of its 11-year sunspot cycle and promises to make the Sun more interesting than the featureless aspect of the sunspot cycle minimum. Solar astronomers use the number of visible sunspots as a measure of solar activity, and sunspot cycle #25, as counted by astronomers, appears to peak around 2024.
Sunspots orbit the Sun in an average of 28 days, but since the Sun does not rotate as a solid body, sunspots at the equator can take as little as 25 days to orbit the Sun. Over the course of a day, sunspots will be static features, but although rare, it is possible to see flares (bright regions) on the solar disk if an exceptional flare of activity occurs. Eruptions usually only last a few minutes.
Getting started with basic solar astrophotography is easy and relatively inexpensive to get started since the only additional item you need beyond your camera and long lens is a certified solar filter that is safe for your eyes. and your camera. These filters are called broadband or “white light” solar filters which will simply reduce the intensity of the Sun by a factor of at least 100,000 (ND5 or ~16.6 stops). It’s so much denser than your typical landscape photographer’s set of neutral density filters that you won’t be able to see anything but the sun through the filter. Additionally, for the safety of your eyes, a sunscreen should also cut off infrared (IR) and ultraviolet (UV) as much as possible. Buy filters only from trusted sources for your own safety! Also note that these filters should be used at the front end of your optics, before any magnification.
To be clear, these broadband filters won’t show you some of the spectacular shots you may have seen with clouds of glowing, flame-like gas hanging from the edge of the sun. These views are reserved for specialist and more expensive sunscreens to be discussed elsewhere.
Broadband filters are also sold with more attenuation (eg ND6), but they are generally intended for visual observation because ND5 filters pass enough light to dazzle the human eye. But photographically, ND5 filters are preferable so that the exposures are as short as possible, and to allow photography near the horizon where the atmosphere considerably attenuates our vision of the sun.
Sun filters are available as flexible film or glass filters. Flexible film filters (polymer or mylar substrate) are generally less expensive than glass filters, but scratch more easily, so I prefer glass filters. Among the glass filters, you will find absorbing filters (eg dark glass) or reflecting filters (coated with a metallic film). My preference here is to use metal coated glass filters as they won’t heat up in the sun, although they are more susceptible to damage which can remove the reflective film and decrease filter safety. On the other hand, flexible film solar filter material is also sold in sheets so that the filters can be customized easily.
Most sunscreens can also produce a neutral image, resulting in a white sun. I prefer filters that have a yellow-orange tint both for aesthetic reasons and because the blue end of the spectrum suffers more from atmospheric scattering, which lowers contrast in solar photos.
As a final note on filters, I recommend using filters that are meant to be simply slipped over the end of a camera or telescope lens like a cap, rather than screwed on like camera filters standard picture. The reason is that if you ever go to photograph a total solar, you’ll want to be able to quickly remove the filter when you go into totality, then quickly replace the filter when you’re done with totality, all without knocking your lens or scope off target. Of course, when the filter is in place, it should always be attached firmly enough that it cannot easily be accidentally removed.
Note that the filters described above are classified as broadband solar filters. Advanced solar imaging uses extremely narrow and expensive band filters which will be the subject of another article.
In theory, a solar filter is all you need to get started with solar astrophotography, but in practice, additional equipment makes it more convenient to get the best results…
Objectives and Telescopes
Although virtually any long camera lens or telescope can be used for broadband solar imaging, scattering and internal reflections can be a problem, so astronomical telescopes are generally preferred because they have less d lens elements internally. And even with the simpler designs of astronomical telescopes, this can cause problems, so refractors are preferred over reflective or catadioptric refractors. Whatever optics you use, test the internal reflections by placing the sun off center and taking a few test shots.
As with any telescope or camera, the larger the aperture, the more detail can be resolved. But atmospheric “seeing” cells (including the atmosphere in the optical tube) limit the practical size of the optics to about 100mm to 150mm (diameter). Professional observatories are getting bigger but resorting to extreme measures such as pumping air out of their telescope tubes.
Virtually any camera can be used for solar astrophotography, but since long focal lengths must be used, a DSLR with mirror lockup or a mirrorless camera used with a remote shutter control is preferable so as not to vibrate the configuration. Video mode may be worth trying if high resolution (4K or higher) is available, especially if your camera is capable of recording uncompressed video. Either way, it’s best to use high-speed memory cards, especially when shooting full-resolution burst photos.
Believe it or not, it can be difficult to line up your camera and aim with the sun. The view through the camera is completely black until you point at the sun. So to get closer to the sun, you can buy or make a solar sight. This usually comes in the form of two small screens (coaxial with the main bezel) a few centimeters apart, with the sunscreen including a small hole in the centre. The sun passes through the hole and can be seen on the rear screen when aligned, and the sun must be in the camera’s field of view.
If you don’t have a sunsight, in a pinch, you can line up with the sun by looking at the shadow of your scope on the ground. When the shadow is minimized, you should be aligned with the Sun.
Tripod or mount
Solar astrophotography can be done on a simple photographic tripod if low magnification is used (i.e. for entire disk planes). However, like the stars, the Sun moves across the sky, so an equatorial tracking mount is useful to avoid having to constantly adjust your setup to keep the Sun in view. An equatorial mount is highly recommended when observing the sun over a long period, such as during an eclipse or transit of Mercury. Sunspeed tracking isn’t strictly necessary, and exact polar alignment isn’t necessary if you don’t mind making occasional pointing adjustments. Even with exact polar alignment, if the sun is low in the sky, manual adjustment will be required as atmospheric distortion affects the solar image.
Best time and place to observe the sun
The good thing about solar astrophotography is that it can be done from our backyard. City light pollution or the presence of the moon does not affect solar observation. But two factors contribute to thwarting our efforts. The first is atmospheric scattering by clouds or haze. The second is turbulence. The best possible solar imaging is done on top of a mountain with smooth, steady airflow, but backyard solar astrophotography can still be rewarding.
For solar astrophotography, the best time to observe the sun is in the morning, before the sun has heated the landscape much. This, of course, has to be offset by the fact that the further into the atmosphere you shoot, the greater the distortion and turbulence. So mid-morning is usually best.
Even at the optimal time of day, turbulence (seeing) is the real challenge in solar astrophotography, so very short exposures are required. To get the best possible shot, modern astrophotographers take a large number of short exposures (video frame rate or faster), so that the best images taken during moments of minimum turbulence can be selected in post-processing. This is called “lucky imagery”. Luckily, we don’t have to manually sort all the images! Very capable (and free) software is available, but that’s the subject of another episode.