the Sun

The Sun

Observing the sun:

Remember, never look at the Sun without proper eye protection. There are a variety of ways to safely observe the Sun, many of which can be found in the solar observing section of SkyandTelescope.com.
The Sun is one of the few objects that display a rewarding amount of detail without a telescope or even binoculars. The only equipment you need is an appropriate filter — a piece of No. 14 arcwelder's glass is the traditional choice. This safe filter material is available at any welding-supply store (check your yellow pages for a local dealer) in convenient 4-inch-wide pieces that allow viewing the Sun with both eyes. Although welder's glass imparts a green hue to the Sun, one of these economical filters might be all you ever need for casual observing. But while welder's glass provides satisfactory naked-eye views of the Sun, its poor optical quality makes it unsuitable for use with binoculars and telescopes.
The part of the Sun that we actually see is a layer called the photosphere. It is the nearest thing the Sun has to a "surface." The first features you are likely to notice on the photosphere are large sunspots. Spots of this size are fairly common when the Sun is active, and occasionally several groups are visible at the same time. Tracking the visibility of large spots provides an interesting project for naked-eye observers. In addition to sunspots, look for decreasing brightness toward the edge of the Sun's disk. This limb darkening is the result of looking through a progressively thicker cross section of the darker, cooler upper photosphere near the Sun's limb.
direct telescopic viewing using neutral density type filters:
- When it comes to outfitting optical instruments for solar viewing, a number of excellent options are available. However, there is one type of filter that is very dangerous: the eyepiece Sun filter. These were once commonly supplied with imported telescopes and consist of a piece of dark glass mounted in a cell that screws into the bottom of an eyepiece. The heat from the Sun concentrated by a telescope can shatter these filters without warning. Today's advice is to destroy these filters to ensure they can cause no harm.
- For safe viewing, most observers choose either a glass or Mylar solar filter mounted in a cell that fits securely over the front aperture of a telescope which cost ~$A150. Such filters are made with light-rejection coatings that allow only a fraction of a percent of the Sun's light to pass. This style of filter protects not only your eyes but your equipment too, since the potentially harmful heat of the Sun never enters the telescope.
- Glass solar filters generally produce a yellow or orange Sun, while Mylar filters usually yield a blue image. Aesthetics aside, there are other differences to consider. Mylar filters tend to offer better contrast between the solar disk and bright faculae surrounding active regions. However, Mylar's blue-tinted image also suffers more from scattered light and atmospheric dispersion than the orange image produced by a glass filter. Somewhat better sunspot detail is seen with a glass filter, but faculae are usually rendered all but invisible. Although these filters are more alike than different, it is probably best to give some thought to what you most want to see before purchasing a filter for your telescope.
- Telescopic solar observing is pretty straightforward since venders make filters sized to fit most popular instruments. Simply attach your filter to the front of the tube so that it cannot fall off, and you're in business. Don't forget to make sure that your telescope's finderscope is capped at the objective end or, better yet, removed completely. Aiming the telescope without a finder might seem problematic but it is quite simple. Just move the telescope around until its shadow is minimized, at which point the Sun should be within the field of a low-power eyepiece.
- For most visual astronomy, bigger is better — the larger the scope, the more light collected and the greater the theoretical resolution. However, when it comes to solar observing, the playing field is tipped in favor of smaller scopes. Light-gathering is not an issue since we are trying to dim the Sun's intense glare, but what about resolution? Here again, the advantages of a large instrument are essentially neutralized by atmospheric turbulence. Daytime seeing is rarely steady enough to permit the maximum resolution of even a 4-inch telescope.
- Glass and Mylar filters can also be used with binoculars. In addition to making it possible to view small sunspots, binoculars will show the limb darkening with greater ease than the naked eye alone. You can purchase filters for many binocular sizes, and you can even make your own from Mylar solar-filter material available from several venders. Make sure filters are firmly affixed so that they will not fall off or blow away in a gust of wind.
solar projection viewing:
- One method of solar observing that dispenses with filtration altogether is solar projection. (See "Observing The Sun By Projection.") An eyepiece is placed in the telescope's focuser and used to project an image of the Sun onto a convenient flat surface. Telescopes with folded light paths, such as Newtonians or Schmidt-Cassegrains, are not recommended since the converging beam of light can produce enough heat to damage internal components.
- When it comes to eyepieces for projecting the Sun's image, the much-maligned Huygenian design is a good choice because it does not contain cemented elements that can be damaged by the Sun's intense heat. Most solar projection is done onto white paper or card stock. But no matter how white the screen, it must be adequately shaded from direct sunlight and other extraneous light in order for the viewer to see the finest details in the solar image. This powerful technique enables a 4-inch telescope to produce a usable image of the Sun 30 inches across. The size and brightness of the Sun's image depend mainly on the distance between the eyepiece and the viewing surface — the farther away it is, the larger and dimmer the image.
direct viewing using the amazing H-alpha filters to allow wavelengths of only 6562.8A (H alpha):
- these provide fantastic image details of sunspots, mass coronal ejections, etc but are very expensive:
- for example, the Coronado filters require an objective filter and a special eyepiece filter in $US in 2003:
  - filters are 0.7A bandwidth but can be double stacked if buy two to increase contrast and reduce bandwidth to < 0.5A.
  - MaxScope40 40mm telescope OTA $US890, with filters $US2155
  - MaxScope60 60mm telescope OTA $US1790, with filters $US3685
  - MaxScope90 90mm telescope OTA $3990, with SolarMax90 + TMax90 filters $US9798 (<0.7A) and $US12185 (<0.5A)
  - filter packages only:
    - SolarMax40 + TMax40 + B5 $US1265 (nb. special blocking filters also available for Meade ETX60 or 70)
    - SolarMax60 + TMax60 + B10 $US2590
    - SolarMax90 + TMax90 + B10 $US5045
  - SolarMax40:
    - "Astounding! Solar prominences sprang into view, with excellent contrast. A solar flare ( an astonishing bright white feature) was clearly visible on the Sun's disk with granularity and mottling visible around sunspot groups. The Solar disk was surprisingly bright and contrast was easily on a par with filters by Day-Star (read costs much more $$$) and clearly superior cheaper to 1.5 angstrom designs.
    - Only having a 40mm aperture raised a question about the resolution of such a small aperture. This was not an issue. Daytime "seeing" was the limiting factor. The image was sharp, with more magnification showing the limits of the seeing more than the aperture. Our only criticism would be the small exit pupil of the blocking filter tends to restrict the field of view at longer focal lengths. Also as successively higher power eyepieces were used the apparent image contrast fell away quite quickly (not surprisingly, there is only so much light you can squeeze through a 40mm aperture!). This made visual observation of prominence features at high magnifications not as satisfying as the low power views through the same instrument.
    - High magnification limitations notwithstanding, H-Alpha viewing with SolarMax proved to be very addictive. Low and Medium power views clearly showed prominences: looped, hooked, fanned and continually changing shape. The view is dynamic, and a refreshing alternative to deep sky views enjoyed by astronomers of the night."
  - imaging with the SolarMax:
    - Users of smaller CCD cameras (SBIG STV, ST5c, ST237a) will find no restrictions when trying to image the sun through the exit pupil of the SolarMax blocking filter.
    - The brightness of the image is a little too high! with *CCD images are close to saturation with even the briefest exposures. A Neutral Density filter is required. SBIG ST7e owners would probably need to obtain a support bracket to hold the camera without placing too much stress on the 1.25" blocking diagonal. SBIG ST8/9/10/1001 owners would find the blocking filter field restrictive with only limited field able to be projected onto the larger detectors of these cameras.
    - 35mm camera Users: would need an eyepiece projection tube that can be inserted into the 1.25" blocking diagonal.

Sunspots:

Sunspots are relatively cool regions in the Sun's photosphere (visible surface). They form where solar magnetic fields lock onto the Sun's churning ionized gas and hold it in place long enough to cool. The photosphere's average temperature is 5,700° Kelvin, but in a sunspot's dark center (umbra), where the field is strongest, it's about 2,000°K cooler. If you could view a sunspot by itself, it would appear blazingly bright. It appears dark only in contrast with its even brighter surroundings. The umbra of a spot is typically surrounded by a lighter penumbra, where the magnetic field allows the gas more motion.
Solar activity varies with an 11-year cycle. As the cycle progresses, activity rises and falls, and with it the amount of detail visible on the Sun. At solar minimum, the Sun often appears nearly featureless, completely free of sunspots. At maximum, however, there can be hundreds of sunspots arranged in a half dozen or more groups and plenty of faculae. Obviously, the most exciting time to observe the Sun is in the years surrounding solar maximum. The last solar maximum was in 2000, and we are currently seeing the fall to the next minimum due in 2005 or 2006. Although the years ahead will see less solar activity, the occasional appearance of a large sunspot group can still take astronomers by surprise.

Coronal Mass Ejections (CME):

solar flares produced by sunspot activity cause an outburst of high-energy particles which if it is headed directly toward Earth may produce a severe or extreme geomagnetic storm. Transformer damage and widespread blackouts are possible at higher latitudes, though power outages are the most severe of the possible effects. Spacecraft are also at risk, and radio-communication disruptions are expected.
such events may also cause aurorae.
videos of the massive CME x17.2 directed at earth on 28th Oct 2003 taken by SOHO showing the snow storm effect as the high energy particles travel towards us, the bottom two videos have had the sun itself blocked so that the CME is visible:

Coronal holes:

Coronal holes are "dark" coronal regions of the sun with open magnetic field structure. During the minimum years of the solar cycle they are confined in the Sun's polar regions, while at solar maximum they can be found at all latitudes.
The fast-speed solar wind originates form the coronal holes, and accordingly they are considered the main reason for the "recurrent" type of geomagnetic activity.