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Even for the most seasoned and experienced astronomer, the question of the Sun and how to properly view it is as confusing as it is baffling. B&H is increasing its solar viewing offerings, so we’re here to give you a primer on what there is to see when looking at the Sun, and some of the tools you’ll need to view it safely and properly.
First, let me start by stating that you should NEVER look at the Sun without taking the proper precautions, and using only those optics SPECIFICALLY designed and certified for solar viewing. Failing to take the proper safety precautions will result in irreparable damage to your eyes—including blindness. Now, with that unpleasantness out of the way, let’s get to the fun stuff: The Sun and how to view it. For further reading, see the B&H Explora article A Buying Guide for Solar and Solar Eclipse Viewing. And don't forget to read about the next solar eclipse, in the B&H Explora article, Mark Your Calendars: North American Solar Eclipse 2017.
The Sun is anything but a featureless white or yellow disc in the sky. The surface is easily one of the most dynamic regions of our solar system. What follows is a summary of the features of the Sun’s hot and violent surface.
Filaments are dense, cooler clouds of material that are suspended above the surface by loops of magnetic fields. Viewed in H-alpha (see below) they appear as dark, thread-like features.
Prominences and filaments are the same things, except that prominences are seen projecting out above the edge of the Sun.
Filaments and prominences can remain quiet or completely dormant for days or weeks, but as the magnetic loops that support them slowly change, filaments and prominences can erupt and rise from the Sun over the course of a few minutes or hours.
Spicules are small, jet-like eruptions seen throughout the chromospheric network. They appear as short dark streaks when viewed in H-alpha and last only a few minutes. When they occur, material is violently ejected from the surface and outward into the hot corona at speeds of 45,000 to 67,000 mph.
Sunspots, which appear as dark spots on the surface when viewing in white light, are essentially cool pockets on the Sun’s surface caused by localized magnetic fields that interfere with the natural convection currents. Sunspot activity is cyclical with periods of intense activity where many form, and others when they are virtually absent from the surface. Because of the factors that cause their creation, you will generally see one or two large sunspots surrounded by smaller ones.
The most interesting thing about viewing sunspots is that they can change drastically over the course of a few hours—so viewers are treated to a constantly changing sight.
Plage are bright patches surrounding sunspots that are best viewed in H-alpha. They are also associated with concentrations of magnetic fields and form a part of the network of bright emissions in the chromosphere.
Faculae are bright areas that are usually most easily seen near the limb, or edge, of the disk. While they are also magnetic areas like plage or sunspots, the magnetic field is concentrated in much smaller bundles.
Granules are small (relatively speaking) features, measuring about 625 miles, that cover the surface, except where there are sunspots. They are the tops of convection cells where hot fluid rises up from the interior, spreads out across the surface, cools and then sinks. Individual granules last for about 20 minutes, so the granulation pattern is continually changing as old granules are pushed aside by new ones. The flow within the granules can reach supersonic speeds of more than 15,000 mph and produce sonic booms and other noise that generates waves on the Sun's surface.
Supergranules are much larger versions of granules that can measure upwards of 22,000 miles across. For comparison, the Earth is 7,926 miles in diameter—so a supergranule is almost three times the size of Earth! Very similar to their smaller counterparts, these last for one or two days and travel at slower speeds of around 1,000 mph.
Most easily viewed in H-alpha or Calcium K-line (see below), the chromospheric network is a web-like pattern that outlines supergranules and is made by pockets of magnetic field lines that are concentrated by the fluid motions in the supergranules.
Coronal loops are found around sunspots and in active regions. They are associated with the closed magnetic field lines that connect magnetic regions on the solar surface. Many coronal loops last for days or weeks but can change rapidly.
A solar flare occurs when magnetic energy that has built up in the solar atmosphere is suddenly released. Radiation is emitted across the entire electromagnetic spectrum, from radio waves, through the visual spectrum to x-rays and gamma rays.
A CME is a giant cloud of solar plasma with magnetic field lines that are blown away from the Sun during strong, long-duration solar flares and filament eruptions.
When choosing HOW to view the Sun, you have many choices, but the basic decision is between general white light viewing or viewing through an extremely narrow wavelength of the spectrum.
White Light viewing is what most of us think of when we consider getting something with which to look at the Sun. Basic Mylar solar viewing glasses and binoculars or glass objective filters on telescopes or camera lenses simply block more than 99.9% of light so you can safely look at the Sun. Both media transmit all wavelengths of light so they are generally considered “white light” filters, but Mylar tends to tint images blue and glass yields orange images.
From an observer’s point of view, Mylar filters tend to offer better contrast between the solar disk and bright faculae surrounding active regions; but the blue tint scatters light and shows atmospheric dispersion more than the orange image produced by a glass filter. Sunspot detail is seen somewhat better with a glass filter, but faculae are usually rendered virtually invisible.
Targeted Wavelengths simply means using special filters that block all but a specific wavelength, versus all wavelengths like White Light viewing, to improve the image and contrast. There are four primary wavelengths involved with solar viewing: Hydrogen, Calcium, Sodium, and Helium. Now, it might sound odd to be calling out wavelengths based on elements, but in this context we’re referring to the emission spectrum of the specific elements found in the Sun. When they move from a higher quantum state to a lower one they emit photons (light) at a specific and predictable wavelength. It is this wavelength to which solar filters are tuned.
Hydrogen-alpha (H-alpha) Hydrogen makes up more than 70% of the gas in the Sun and is the primary fuel to keep it going. It is in the red end of the visible spectrum at 656.28nm, and is the easiest to isolate—it’s also the most “general purpose” wavelength in terms of what you can see when viewing through it.
Within that wide wavelength, precision filters can be focused even further for research on specific regions of activity, expressed as bandwidths and measured in Angstroms (Å):
• 0.3Å increases surface contrast, and reveals thin details on surface and prominence features.
• 0.4Å improves surface contrast and enhances fine chromosphere details.
• 0.5Å is best for surface contrast and views of prominences.
• 0.6Å offers middle-of-the-line viewing capability that allows you to observe solar surface contrast as well as prominences.
• 0.7Å reveals prominences in high contrast, and, under the right conditions, surface texture. Prominences appear larger than when viewed with narrower bandwidth filters.
• 0.8Å reveal prominences in high contrast.
Calcium There are two popular bands within the Calcium wavelength—K-line and H-line—and are at the other end of the light spectrum than H-alpha in the near-ultraviolet.
The blue K-line, at 3933.7Å, is sensitive to magnetic fields, so magnetically active areas and structures show in high-contrast against the surrounding chromosphere; K-line filters reveal complex temporal variations, also in the chromosphere.
The H-line, at 3968.5Å, is closer to the visible spectrum than K-line, and therefore easier to see. It provides improved quantum efficiency, higher glass transmission, and improved performance from anti-reflection optical coatings; and offers slightly more visible features. Views through the filter show the chromosphere, plages/faculae, sun spots, and solar flares in a vibrant violet color with high contrast.
Sodium The Sodium D-line (Na-D), at 5895.9Å, is an extremely thin but bright emission line from the Sun's surface continuum. Filters tuned to this wavelength need to be extremely delicate and precise, but produce highly detailed views of granulation, supergranulation and P-modes (acoustic waves). Sodium also shows impulsive-phase flare eruption kernels. Visual and photographic images reveal a similar image as Calcium, revealing granulation, but are much brighter and more easily visible.
Now that we’ve seen what to view and how filters affect what we see, we need to discuss the tools that are available to view through or with.
Viewing As we’ve seen, the most basic optics are simply Mylar or glass filters that transmit the full spectrum of light and allow you to see basic structures such as sunspots during full-disk solar viewing, and some prominences or corona during an eclipse. You can get these for a few dollars; they are very similar to the old cardboard 3-D glasses but instead of red and blue lenses, will come with Mylar lenses. More sturdy takes on this include glasses (think sunglasses but with solar-filter lenses) and binoculars.
A step up from these are objective filters. Mostly made of glass these days, these blocking filters fit over the objective bell of conventional binoculars, spotting scopes, or telescopes that allow you to take advantage of their magnification capabilities to tease out more detail than when viewing with no or low magnification. While conventional binoculars and solar binoculars will perform equally—with their filters being similar—you have the advantage of being able to take the solar filter off and use the optic as you normally would. Solar filters for camera lenses will either sleeve over the front or thread onto the lens like conventional photo filters. The objective filters can be more complicated versions called etalons (see below).
A more specialized type of filter moves to the back of the scope and goes into the focusing drawtube, and your eyepiece fits into the filter. Again, this is to adapt a conventional telescope for solar viewing. The primary advantage to these is that you can use them on a wide variety of OTAs with different apertures, whereas objective filters require you to get a filter for each aperture. These come tuned for broad-spectrum, white-light viewing or can be tuned to specific wavelengths like Hydrogen, Calcium, or Sodium.
On the other side of the ledger is that with the filter put far back in the optical path, you are essentially focusing the power of sunlight into a very narrow point—think of an ant and magnifying glass here. Anytime you use one of these filters, you must use a UV/IR filter to dissipate the heat and stop interference from those wavelengths on either side of the visual spectrum. This filter becomes especially important if you’re imaging the Sun as camera sensors tend to be sensitive to UV and IR so the filter helps to prevent blurring, glare, and color abnormalities, while improving contrast in your images. These come as front objective or rear eyepiece filters. I prefer objective filters for this application (if you have a rear primary solar filter) because you lessen the heat load right at the beginning, instead at the end when it’s been focused to a tight spot.
Next up are dedicated solar scopes. These are exactly what they sound like. All the filters are integrated into the optical path and offer the best views, because all the filters are designed to be full aperture for the system, so light transmission isn’t blocked or vignetted anywhere that can cause a reduction in image quality.
There are several terms you’ll see associated with these such as etalons, single-stacked or double-stacked. Etalons is simply a technical term for the primary solar filter with some sort of tuning system (more on that below). The stacking refers to the number of blocking filters in the optical path. A double-stacked system will give you a narrower bandpass to reveal more detail and improve contrast.
Tuning Tuning a solar system is a way certain scopes and filters adjust the wavelength filter to compensate for variables that interfere with your ability to see all you are capable of. When you are viewing specific wavelengths, you may need to tune the wavelength up or down a little to counter that interference. Atmospheric disturbances such as humidity and variations in the electromagnetic field can affect our views.
Another issue is Doppler shift. Think of a siren or train coming toward you and then passing you. The pitch of the sound changes because when the train is approaching the sound waves are compressed, and after it passes and moves away from you the sound waves stretch. Since light moves as a wave, this similar effect is seen when looking at the Sun because of the rotations of the Earth and Sun, plus our orbit around the Sun. Tuning the solar filter to compensate for Doppler shift allows you to focus on a specific region, then move to another and tune it again. Experienced solar observers with precision tuners can look at a prominence and slowly adjust the tuning as they focus on the different areas from the surface out.
There are three primary tuning systems: Tilt, Pressure, and Temperature.
Tilt tuning, as the name implies, allows you to simply tilt the etalon filter, thus shifting the angle at which the wavelength hits it. The advantage of this system is that it is fairly basic and low-tech, so tilt-tuning systems rarely need service and come in at a lower cost. The downside of tilt-tuning is that distortion to the image may occur—especially if you’ve tuned it far from the base wavelength and the angle is becoming drastic.
Pressure tuning uses a chamber system that seals air inside the tuning system. The etalon is tuned by moving one of the chamber walls in or out, which increases or decreases the pressure in the chamber. This change in pressure affects the light moving through it—thus tuning it up or down from the base wavelength. Pressure tuning is more precise than tilt, plus it solves the distortion problem that tilt can experience. Depending on how much you use and/or abuse the tuning system, you will need to get the system serviced over time.
Temperature tuning is similar to pressure tuning in that it heats the air in the etalon chamber. These systems are by far the most precise and most expensive. Since the system relies on electronics to control the internal heat, the tuning systems are digitally-controlled, whence the precision comes.
Solar observing is an emerging market as events like eclipses capture the public’s fascination, coupled with NASA’s increased attention on studying our closest star. As more products become available, interest will only increase. With such diverse and varying pieces of equipment out there, experienced astronomers can easily adapt their existing rigs with solar viewing gear, while amateurs can start small and work their way up from glasses to binoculars and beyond as they grow in the field. Don’t be intimidated by the Sun—as long as you take the appropriate precautions, viewing and learning about it can be as awe-inspiring… and you don’t even have to stay up late to do it.
For the quickest way to get your solar viewing and solar eclipse gear, click on this link!
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