Solar Observing - How to Look at the Sun Safely
Introduction and Safety Notice
Events such as an eclipse or a transit (in photo) can't be observed with unaided eye
The Sun is the closest and the brightest star in our sky and it is a rewarding observing target. Depending on the method and the instrument used - it can reveal beautiful features such as sunspots, granulation, corona (during full solar eclipses), prominences and filaments (observable with a higher end instruments).
However being extremely bright - 400,000 times brighter than the full Moon - the Sun poses a significant risk to our eyes if observed improperly. Normally we can't look at the Sun for more than a few seconds since our eyes are forced to close to protect themselves. However using improvised and improper filters (such as a floppy disk or a negative film) can reduce the visible glare but let enough harmful IR radiation pass. This way we can "fool" our eyes and allow them to stay open long enough for this radiation to harm our eyesight. Using optical aids such as binoculars or telescopes without dedicated solar filters is even more dangerous, since they focus and multiply the amount of radiant energy which enter the pupil (A l 60mm scope will multiply it by ~1000 times) and can cause damage even during a brief exposure.
Unlike other improvised methods - a welder's glass number 14 is safe for solar observing
Therfore if you wish to stay on the safe side - you should avoid improvised solutions (such as CD, floppy disk, smoked glass, negative films, etc.) and only use filters and equipment which was specifically designed for visual solar observing. You can also use the projection method (described below) or look through a number 14 welder's glass which is considered safe. If you still wish to read about improvised solutions - you can look into one of these articles: NASA - Eye Safety During Solar Eclipses, Eclipse Filters - Time for an International Standard.
Direct Solar Observing Methods
You may obtain "eclipse glasses" at public observing events or in internet stores
The Sun is safe to observe directly only at sunrise or sunset (when it touches the "true" horizon such as the sea) or during a totality phase of a full solar eclipse. In all other cases you must use some kind of a solar filter. "Eclipse shades" or "eclipse viewer" are cheap and allow you to view the Sun safely during long periods of time. You can also use a welder's glass with a shade number 14 which is considered safe for direct solar observing and can provide an acceptable image quality.
Direct viewing is useful for observing solar eclipses, but it won't show you any sunspots or other details (except the largest of sunspots, which are quite rare). In 2012 this method could also be used to observe a spectacular event of a Venus transit, but sadly the next one will occur only at 2117.
Solar Projection Methods
Pinhole projection of the Sun can be performed with the simplest tools, but the image will be quite small
If you aren't able to obtain a proper eclipse viewer - the pinhole projection method is the simplest, safest and allows more than one person to view the image. This method is only useful for observing phases of a solar eclipse, since the image you get will be small and won't reveal solar surface features.
The simplest projector is a piece of a thin cardboard with a small pinhole punctured in it. Behind the cardboard you should place a sheet of a white paper on which the solar image will be projected. The further the distance between the pinhole and the projection screen is - the larger and the fainter the image will be. The same goes for the pinhole diameter - the smaller it gets, the sharper and the fainter the image will be. Note that if it's too small - the diffraction effects can become dominant and the image will be blurred in addition to being faint. You may start with a diameter of 0.5mm and then experiment. To get higher contrast you can project the Sun in a dark room through a pinhole located on a window rather than outdoors.
Projection box is an improvement over the simple pinhole projection method which provides higher contrast
Even further improvement can be achieved using a long, fully enclosed cardboard box with a round hole in which the observer should insert his head. One of the inner sides of this box should be covered with a white paper - a projection screen. At the opposite side a pinhole should be punctured (near the top, so the observer's head won't interfere). For higher quality image - instead of puncturing a hole in the cardboard you can make a square cutout and cover it with an aluminum foil, in which you should accurately puncture a pinhole with a needle. The observer should be facing the projection screen and there is no need to insert the whole head, only up to the eyes level.
Another variation of the projection method is a masked mirror with an exposed ~5mm spot. The mirror should be placed outside and reflect the solar image onto projection screen located in a darkened room. The method will produce a much larger image and it is described in this article.
There are commercially available projection devices which are usually cheap and include some kind of optics (i.e. Solarscope). They allow receiving a much brighter and a largrer view.
Projection with a Telescope or Binoculars
A projection using a small telescope or binoculars can provide much larger, brighter and sharper image of the Sun
A small telescope or a pair of binoculars without a solar filter can be used instead of a pinhole to project the Sun's image onto a white screen. Obviously it is strictly forbidden to look through the eyepiece while aiming the telescope. Just move it around until you see the Sun's image pop at the projection screen. A cardboard blocking screen can be used in order to block the direct sunlight and increase contrast (see illustration).
It is highly recommended to use only small telescopes or binoculars for solar projection (under 80mm), since the gathered sunlight may easily damage the internal components of a larger instrument. I'd also avoid using this method with expensive, higher end instruments regardless of their aperture. Also such a telescope shouldn't be left unattended since someone may accidently look through it, or a fire might start from the focused sunlight.
The resulting image will be much brighter, sharper and larger than from a pinhole, since the telescope gathers more light, magnifies the image and allows to focus it. You might be able to see some solar surface details (such as sunspots) or even silhouettes of the lunar mountains during an eclipse - see examples below of projected solar images made during a partial eclipse of 2005 with a cheap Bushnell 60mm spotting scope and a sheet of white paper.
An example of image projected by a 60mm spotting scope during a partial solar eclipse
Hints of lunar mountain's silhouettes visible on the projected image
Observing the Sun with a Telescope
Even a small telescope gathers enough sunlight to harm your eyes
If you own a telescope and wish to observe the Sun directly through its eyepiece - you must use a full aperture solar filter which is mounted on the front side of the instrument. Looking directly into the eyepiece even briefly is dangerous and can result in permanent eye damage. A photo at right shows what even a small 60mm spotting scope can do to a piece of paper in a matter of seconds if pointed into the Sun. Also avoid using ocular (eyepiece) filters since the energy with is absorbed by an unfiltered objective can easily cause these filters to crack and expose your eye to concentrated sunlight. This doesn't include high end oclular filters which are used in combination with aperture energy blocking filters. It is also important to use only filters or films which are intended for visual and bot photographic ones.
A cheapest option is to obtain a sheet of a white light solar filter film (such as Baader Astrosolar) and make the filter yourself. You can use the instructions which manufacturers usually provide or just improvise yourself - either is fine as long as you make sure that there are absolutely no gaps in the filter. The result can be checked for gaps in a dark room against a light source. Also you should ensure that the filter is tight and secure when mounted on a telescope. See an example of such DIY white light solar filter below.
Astrozap film for making a white light solar filter
Home-made solar filter made from a film
A refractor telescope with a white light filter is an excellent solar observing tool
Wrapping the telescope can prevent overheating
While observing the Sun - make sure that the finder is covered with its cap (unless it has its own filter). You can aim the telescope by pointing it into the Sun's general direction and then searching with a wide angle eyepiece. Another alternative is making or buying a some sort of solar finder, such as the televue sol searcher.
Also, during a hot day and prolonged observing sessions, consider covering the body of your telescope to prevent it from overheating. It can be as simple as wrapping a towel around.
Generally - for solar observing you shouldn't have a clear aperture of more than 80-100mm. If you are planning to use a large reflector - use an aperture mask with an 80mm round cutout for a solar filter (some telescopes have it included in a dust cap). The mask should be off-axis and cover a clear section of your aperture at its side, otherwise the secondary mirror will obstruct a large portion of the aperture and reduce the contrast.
Commercial solar film filter
The reason why we shouldn't use large instruments for solar observing is because during the day there is much more atmospheric turbulence than at night and larger telescopes are more susceptible to effects of the turbulence then small ones. To make a complex explanation simple - the heat rising from the ground causes convection and creates areas with different refraction indices (which act as "lenses"). Large aperture is affected by such "lenses" at larger scales, while smaller one "ignores" them.
Note that if you are using a large aperture anyway - occasionally there may be moments when you can see finer details than with a smaller scope. This is because during occasional moments of a stable atmosphere the superior resolution of the larger aperture "kicks in". But in average a smaller instrument will provide steadier and sharper view.
Commercial glass solar filter
White light solar filters are also commercially available and are relatively inexpensive. There are filters which are made from a solar film and glass ones. The glass filters are considered to be more durable and arguably provide a higher quality image, but they are more expensive. As mentioned above - make sure the filter is intended for visual and not photographic use.
Photographing the Sun with a White Light Filter
While most of the photos of the Sun you see in the internet are made using expensive narrowband filters (see further section) - you can still make stunning images using a visual or a photographic white light filter. "White light" photos can reveal fine details in sunspots and even show features such as the granulation, if performed in good conditions and with a sufficient focal length.
The solar photography is quite similar to lunar astrophotography, except for the fact that we aren't using the largest available aperture. If you are using short focal lengths (a small refractor telescope or a telephoto lens) - a single frame image may suffice. At longer focal lenghts the blurring effects of atmospheric turbulence are more evident and you should consider making a series of short exposures or a movie clip. Individual frames can be combined using a software tool like Autostakkert, Registax or Avistack. The exposures should be as short as possible.
A photo of the Sun made with ASI120MM camera, 80mm refractor and a home-made white light filter
A DSLR camera attached to the telescope's prime focus or via eyepiece projection can be used to take a series of large individual frames (a result of such method can be seen in this image) or record a movie clip using a "cropped" mode or an external software (i.e. EOS Movierec).
Another, more advanced option, is to use a dedicated planetary camera to capture a video clip for further processing - see an example above.
H-Alpha and other Advanced Solutions
All of the filters mentioned above are so called "white light" filters which simply absorb most of the sunlight, evenly through the solar spectrum. This method allows us to view the Sun's inner layer called photosphere with its features (sunspots, granulation). It is characterized by so called "black-body" radiation which spreads over the whole visible spectrum. The view we get is quite static, and changes only over a period of hours and days.
The Sun's more outter later, the chromosphere, contains much more interesting features such as filaments and prominences, which can erupt and change in front of our eyes in a matter of minutes (or even less). The chromosphere's spectrum is dominated by emission lines, meaning it shines only in very specific colors (Hydrogen-alpha being the strongest, which gives it a reddish color). Since the chromosphere is also many magnitudes weaker than the photosphere - in order to observe it we need to block all the photosphere's energy and only let the desired wavelength in.
This is performed by using a specialty narrowband filters (usually H-alpha) or telescopes which have them built-in. The prices for these instruments starts from 600-1000$ for the cheapest models (Daystar Quark and Coronado PST). In addition to H-Alpha there are wavelengths such as Calcium-K which allow us to see different Sun's layers, and different settings for these filters which allow you to focus on specific features.
If you need to know whether there are solar features which are currently worth observing, such as a large sunspot group, you may use the Spaceweather site which provides a current image of the Sun. And no matter which method you choose - you should remember that a telescope pointed to the Sun always requires caution and should not be left unattended if there are children around.