With long days and shorter nights, summer is a good time to consider getting into solar astrophotography. The Sun is also heading toward the active phase of its 11-year sunspot cycle and promises to make the Sun more interesting than the featureless cueball look of the sunspot cycle minimum. Solar astronomers use the count of visible sunspots as a measure of the solar activity, and sunspot cycle #25, as counted by astronomers, looks like it will be peaking sometime around 2024.
Sunspots rotate around the Sun in 28 days on average, but because the Sun doesn’t rotate as a solid body, sunspots at the equator can take as little as 25 days to go around the Sun. Over the course of a day sunspots will be static features, but though rare, it is possible to see flares (bright regions) on the solar disk if an exceptional eruption of activity occurs. Flares usually last only a few minutes.
To begin with basic solar astrophotography, it’s easy and relatively inexpensive to start out since the only extra item you need beyond your camera and long lens is a solar filter certified safe for your eyes and camera. These filters are referred to as broadband or “white light” solar filters which will simply cut the Sun’s intensity down by a factor of at least 100,000 (ND5 or ~16.6 stops). This is so much denser than your typical landscape photographer’s set of neutral density filters that you will not be able to see anything but the sun through the filter. In addition, for the safety of your eyes a solar filter should also cut off as much of the infrared (IR) and ultraviolet (UV) as possible. Purchase filters only from reliable sources for your own safety! Note also that these filters must be used at the front end of your optics, before any magnification occurs.
To be clear, these broadband filters will not be showing you some of the dramatic shots you may have seen with flame-like glowing gas clouds hanging on the edge of the sun. Those views are reserved for specialized and more expensive solar filters to be discussed elsewhere.
Broadband filters are also sold with more attenuation (e.g. ND6), but these are generally for visual observation as the ND5 filters let enough light through to dazzle the human eye. But photographically, the ND5 filters are preferable so that exposures are as short as possible, and to allow photography near the horizon where the atmosphere significantly attenuates our view of the sun.
Solar filters are available as flexible film or as glass filters. Flex film filters (polymer or mylar substrate) are generally less expensive than glass filters, but are more easily scratched, so I prefer glass filters. Among the glass filters, you will find absorption filters (e.g. dark glass) or reflective filters (metal-film coated). My preference here is to use metal-coated glass filters since they will not get warm under the sun, though they are more susceptible to damage which may remove the reflective film and decrease the safety of the filter. On the other hand flexible film solar filter material is also sold in sheets so that filters can be customized easily.
Most solar filters may also yield a neutral image, which results 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, decreasing the contrast in solar photos.
As a final note on filters, I recommend the use of filters that are meant to be just slipped onto the end of a camera lens or telescope like a cap, rather than screwed on like standard camera filters. The reason for this is that if you ever go to photograph a total solar eclipse, you will want to be able to quickly remove the filter when you enter totality, and then quickly replace the filter at the end of totality, all without bumping your lens or scope off target. Of course, when the filter is in place, it should still be attached firmly enough to not be easily removed accidentally.
Note that the filters described above are classed as broadband solar filters. Advanced solar imaging uses costly, extremely narrow band filters which are the topic for another installment.
In theory, a solar filter is all you need to jump into solar astrophotography, but in practice, extra equipment makes it more convenient to get the best results…
Lenses and Telescopes
While 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 usually preferred since they have fewer lens elements internally. And even with the simpler designs of astronomical telescopes, these can be problems, so refractors are preferred over reflecting or catadioptric scopes. No matter what optics you use, test for internal reflections by placing the sun off-center and shoot some test shots.
As with any telescope or camera, the larger the aperture, the finer the detail can be resolved. But atmospheric “seeing” cells (including the atmosphere in the optical tube) limit the practical size of the optics to around 100mm to 150mm (diameter). Professional observatories go larger but they resort to extreme measures such as pumping the air out of their telescope tubes.
Virtually any camera can be used for solar astrophotography, but because long focal lengths need to be used, a DSLR with mirror lockup or mirrorless camera used with a remote shutter control is preferred so as to not vibrate the setup. Video mode may be worth trying if high resolution (4K or better) is available, especially if your camera is capable of recording uncompressed video. In any case, using high-speed memory cards is best, especially when shooting bursts of full-resolution stills.
Believe it or not, it can actually be hard to line your camera and scope up with the sun. The view through the camera is completely black until you actually point at the sun. So to get close to the sun, you can buy or make a solar finder. This is usually in the form of two small screens (coaxial with the main scope) a few inches apart, with the sun-ward screen including a small hole in the center. The sun goes through the hole and can be seen on the rear screen when aligned, and the sun should be in the camera’s view.
If you don’t happen to have a solar finder, in a pinch, you can line up with the sun by watching the shadow of your scope on the ground. When the shadow is minimized, you should be lined up with the Sun.
Tripod or Mount
Solar astrophotography can be done on a simple photographic tripod if low magnification is used (i.e. for whole disk shots). However, like the stars, the Sun is moving across the sky, so an equatorial tracking mount is useful to avoid the bother of having to constantly adjust your setup to keep the Sun in view. An equatorial mount is highly recommended when observing the sun over an extended period such as during an eclipse or Mercury transit. Solar rate tracking is not strictly necessary, and exact polar alignment is not required if you don’t mind making occasional pointing adjustments. Even with exact polar alignment, if the sun is low in the sky, some hand adjustment will be necessary as the atmospheric distortion affects the solar image.
Best Time and Place to Observe the Sun
The nice thing about solar astrophotography is that it can be done from our backyards. City light pollution or the presence of the moon doesn’t affect solar viewing. But two factors work to thwart our efforts. The first is atmospheric scattering from clouds or haze. The second is turbulence. The best possible solar imaging is done at the top of a mountain with a 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 traded off against the fact that the more atmosphere you are shooting through, the worse the distortion and turbulence. So mid-morning is usually best.
Even at the optimal time of day, turbulence (seeing) is the real challenge of solar astrophotography, so very short exposures are required. To get the best possible shot, modern astrophotographers shoot a large number of short exposures (video rate or faster), so that the best frames taken during moments of minimal turbulence can be selected in post-processing. This is called “lucky imaging.” Fortunately, we don’t have to manually sort through all the frames! Very capable (and free) software is available, but that is the subject of another installment.