Viewing the Sun: "Danger Will Robinson!"
Of Films and Filters
That First and Later Looks
General Structure and Dynamic of the Sun
Solar Visual Observation Terminology
~T's Pep Talk!

Viewing the Sun: "Danger Will Robinson!"

Our Sun is unique. It is the only star near enough to show detail on its surface. Because it is a star, and because stars give off intense amounts of light (and heat), the Sun can not be observed directly without a solar filter. For the protection of both equipment and observer, special techniques are required. For this reason, solar observation should not be performed without a mentor.

One form of solar observation has to do with the principle of eyepiece projection. A small telescope may be turned on the Sun with an inexpensive eyepiece in place. A large piece of white cardboard can then be held several inches from the eyelens. The image of the Sun as seen through the eyepiece is then broadcast against the cardboard. A group of observers can gather around the scope and discuss various features on its photosphere.

In this case several precautions must be taken. For instance, the scope must somehow be oriented toward the Sun without reference to the finder scope. This can be done by projecting the shadow of the scope against the projection screen of the cardboard. Best alignment is achieved when the footprint of the scopes shadow is as small as it can get. To prevent "habitual" use of the finderscope, never remove lens caps from the finder during solar viewing.

Because the Sun is very bright, it is not necessary to use the full aperture of the scope. Two inches of aperture will provide all the resolution necessary to see all solar features possible by eyepiece projection. So, a second precaution is to make a cardboard mask and mount it temporarily at the mouth of the scopes dewshield. Any more radiation than this entering the scope could cause overheating and possible damage to eyepiece lenses etc. If an obstructed scope is being used, offset the opening to bypass the secondary or newtonian elliptical.

NOTE: All protective solar filters are placed before the primary. There are no filters available that screw into the eyepiece in the way a lunar filter does. Nor should any commonly available "neutral density filters" (such as welder's glasses) be used in conjunction with an eyepiece or as a pre-object glass filter unit.

And finally, because some heat will build up in the scope during use, there is the possibility that damage may accrue anyway. For this reason it is suggested that only inexpensive eyepieces (3 element Kelners for instance) be used for projection.

So by the numbers, for you're own safety and that of the equipment:

• Never look into the eyepiece when the scope is oriented anywhere near the Sun.
• Never remove the lens caps on the finder.
• Use an aperture mask based on a two inch opening.
• Use inexpensive eyepieces.

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Of Films and Filters

Most observers would agree that solar viewing by eyepiece projection is difficult and non-intuitive. It can be very awkward to track the scope manually while also holding a piece of cardboard planar to the the eyepiece. After all, there's the eyepiece, why not just look into it like everything else! The "Why not?", is called "immediate permanent blindness in one eye". "Irreparable damage to the retina." Etc. Etc.

Now of course, materials science has come a long way since the first scopes were turned on the Sun using eyepiece projection. Modern films and coatings can reject as much of the heat and light from the Sun as desired - and this before sunlight enters the optics tube. Such filters make it possible to view the Sun as though you were looking at the Moon, for instance. All manner of features - subtle and gross - on the solar surface may be resolved. In fact, it is possible to not only view the sun's photosphere, but also solar prominences thrown off the solar limb as well as that faint "aura" of luminosity that surrounds it - the corona.

However, practical solar observance begins with the simplest type - viewing sunspots, faculae, and granulation through relatively inexpensive pre-objective filters. These are the types sold by a variety of distributors such as Celestron, Meade, Orion Binocular and Telescope etc. or hand made by the observer using Baader films.

As with eyepiece projection, use of such filters requires precautions. Unlike eyepiece projection, there is no reason to mask away part of the aperture. A particular filter is pre-fitted to a particular scope and may or may not be masked. However, before installation, a solar filter should be throroughly inspected (against the Sun), to ensure that there are no pinholes or tears. Installation of a solar filter also requires care. Under no circumstances should the filter be installed in such a way that it might "fall" or "blow off" during use.

Add to this the usual precaution of never removing lens caps from the finderscope, and the picture is pretty much complete. However one modification is possible. Once you orient the scope toward the Sun based on the scopes shadow, remove the eyepiece and finalize the sun in the field of view based on what can be seen through the focuser or diagonal. This simplifies final orientation immensely...

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That First and Later Looks

Your first look at the Sun will probably impress you with its size, shape, and brilliance. Certainly contrast between the photosphere's brilliance and the darkness of visible spots will grab your attention. After awhile, you will notice that sunspots themselves show gradations in luminosity and texture. Around the larger spots and spot groups, you may see a subtle "pinprick effect". Spots approaching the limb of the Sun may take on a "sunken" appearance. The region near the limb itself may display a texture not normally seen near the center of the solar orb.

As you observe the Sun (over several days) you will come to realize that spots "drift" across the Sun's photosphere. You will get better at estimating where they may be found from session to session. You will also notice that the spots themselves change in terms of shape, and structure. Eventually you may come to predict such changes.

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General Structure and Dynamic of the Sun

Deep at the core of the Sun, there is a powerful engine of transformation at work. Hydrogen, the most common element found in the known universe, is constantly being converted to helium - the second most prevalent element. The mechanism by which this conversion takes place is known to be that of atomic fusion. And it just so happens that the fusion of hydrogen into helium results in the release of radiant energy. A release that is absolutely essential if the universe as we know it is to exist.

Although the core of the Sun lies about 400,000 miles from its visible surface, it takes some 20 million years for that energy to actually "spring from the photosphere" and some eight minutes, and ninety million miles later be collected by your telescope. During that 20 million years, each photon is absorbed and emitted countless times. Each absorption applies pressure against the unremitting clutch of gravity. A clutch that would ultimately squeeze the entire Sun into a globe comparable in size to that of our own Earth...

Meanwhile, the Sun is not entirely made of hydrogen and helium. Our Sun was marvelously fashioned out of virgin interstellar gases seeded with certain heavier elements formed in the core of a long-dead supergiant star. Thus, just like our Earth, the Sun contains trace amounts of numerous other elements besides hydrogen and helium. Among these trace elements is iron - a substance capable of generating powerful magnetic fields in response to rotary motion. And it is this magnetic field that accounts for many of the phenomena visible on the solar surface.

So even as gravity attempts to concentrate all the Sun's mass at its core. And radiation struggles mightily to liberate itself into free space. The Sun's rotational motion creates powerful magnetic fields resulting in positive and negative polarities. Polarities that drive ionized gases into space (as solar prominences), distinguish regions of greater and lesser luminosity (sunspots), and mottle the surface with other variagated features.

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Solar Visual Observation Terminology

Photosphere: The visible surface of the sun. a soild shell of gas approximately 250 miles thick with an average temperature of 11,000 degrees farenheit.

Limb darkening: The edge of the photosphere. this effect occurs because at the center of the disc we see deeper into the Sun where the temperatures are higher and the gas shines more brightly. at the limb, we are seeing cooler gas at higher elevations, therefore it appears darker.

Granules: At magnification, individual cells of gas contained within the photosphere form ~ these are called granules. each granule may last from several minutes up to a half an hour.

Granulation: Term used to describe the overall pattern of invididual cells observable at the limb.

Faculae: Large, irregular patches on the photosphere. once again, observable near the limb, faculae appear as "cracks" or "veins" within the granulation pattern.

Prominence: A "loop" of hot gas sometimes observable with a standard H-alpha filter, but requires a narrow band H/al to see filamentation.

Flare: A brilliant explosion of pent-up magnetic energy and radiation. narrow band filters are capable of revealing them, but only "white light" flares can be detected in a standard filter.

Sunspot: A dark, highly magnetic "patch" on the solar surface where the mean temperatures are about 3,600 degees cooler. here is where the sun's magnetic field has emerged through the photosphere and stopped the rising energy from reaching the surface.

Umbra: The darkest, coolest portion of a visible sunspot. these black areas vary greatly in size, shape, and magnetic field polarization.

Penumbra: A striated, lighter colored halo surrounding the central umbra. also varies greatly in size and shape.

Dispersion field: General term used to describe the umbra/penumbra region of a sunspot group.

Sunspot area: The area of a sunspot measured in millionths of the Sun's visible hemisphere.

Sunspot groups: Sunspots usually appear in groups. normally it is fairly easy to count the number of groups as they are spread out across the disk and in both hemispheres, but difficulties can occur when sunspots appear close together, so often it is wise to check SOHO information as to their given numerical designation.

"Wilson effect": Term coined by Scottish solar astronomer, Alexander Wilson to described the indented, dimpled appearance of highly magnitized sunspots when seen near the solar limb.

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~T's Pep Talk!

OK... Now we have some of the "basic" terminology down, let's talk about what we can see!

Even using standard solar projection techniques, it is very easy to follow the Sun's activity. Umbral and penumbral regions show quite clearly, and solar rotation and the number of spots can be followed quite easily!

By using the average solar filter, the Sun comes alive!! Limb darkening, granulation, faculae, sunpots in incredible detail and the "Wilson Effect" are all readily apparent. After having observed the solar surface for four years, I am continually delighted by the ever-changing face of our nearest star! There have been times when I have seen solar prominences, and white flares. The white flares are simply incredible... For they look as if a great chunk of the Sun flings itself into space!

Practice at solar observance is very rewarding. By combining "at the scope" experience with data readily available from the Solar and Heliospheric Observatory, it's not difficult to learn to predict areas where activity may occur... And now we're getting into a whole new ballgame!

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