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Journal

No. 48 - June 2005

Sun graphic

See the Sun in a different light

The Sun is something we take very much for granted. It rises in the morning gives us heat and light throughout the day and then drops below our horizon, heralding the cold and darkness of the night. The Sun supplies the power to drive our atmosphere (weather) and is the life support system for the biosphere. The Sun is a dynamic object, constantly changing but at the same time, showing patterns in the way it behaves.

There are several ways of looking at the Sun, but safety is of prime importance. You must never look directly at the Sun, especially with any kind of binocular or telescope unless the instrument is equipped with proper filters which not only block out the glare of the visible light but will also block the damaging UV and infrared rays which we cannot see.

Unless the correct precautions are taken, you will quite literally burn your eye. I remember being shown in school how, if you take a piece of paper and focus sunlight onto it, using a lens or eye glass, the paper will start to smoulder and burn. The same thing happens if you focus unfiltered sunlight onto the retina of the eye. Unlike other parts of the body, the retina doesn't have pain receptors, so there is no way of knowing that you are irreparably burning your eye, until too late.

The main safe methods used by amateur observers to look at the Sun are:

  1. Solar Projection
  2. Direct viewing through a specialized solar filter
  3. Hydrogen alpha telescope

1. Solar projection

This method is good for tracking and measuring the size and position of sunspots and plotting them on a grid. It is a very safe method of observing as the observer stands with his/her back to the Sun and looks at sunspots projected onto a sheet of paper. Accurate plotting of the position of sunspots can be made but no other solar detail is visible by using this method of observing. Sunspots can change in size and shape. There can be lots of them, but sometimes the Sun can be totally blank. However, this does not happen in a random fashion.

sunspots
US National Solar Observatory, Sacramento Peak

Sunspot activity follows a set pattern with the numbers rising and falling over an 11 year cycle. This is known as the sunspot cycle and plotting the latitude and numbers of sunspots seen against time produces the "Butterfly" diagram. This shows that sunspots do not appear at random over the surface of the Sun but are concentrated in two bands on either side of the equator. These bands first form at mid-latitudes, widen, and then move toward the equator as each cycle progresses.

Early records of sunspots indicate that the Sun went through a period of inactivity in the late 17th century. Very few sunspots were seen on the Sun from about 1645 to 1715. This period of solar inactivity also corresponds to a climatic period called the "Little Ice Age" when rivers that are not normally frozen, did freeze up and snow fields at lower altitudes, did not melt. There is evidence that the Sun has had similar periods of inactivity in the more distant past. The connection between solar activity and terrestrial climate is an area of on-going research. A very topical area for debate. The plotting and observation of sunspots is an interesting study and a useful contribution to our knowledge about the Sun and the possible effects on the Earth. These observations can be made relatively easily and with very modest equipment.

2. Direct viewing

Direct viewing of the Sun can be done by using a metal-coated, purpose made safety filter, usually Mylar or Baader film. The filter is then securely attached to an ordinary optical telescope, the filter covering the full diameter of the instrument which is then used to look directly at the Sun. (Be sure to cover up any finder scope too. It's only too easy through force of habit to unthinkingly use it.) The metal coated solar filter blocks out 99.999 % of the visible light as well as blocking the harmful UV and Infrared rays.

Sunspots are still the only features observable using this method, but fine detail surrounding the sunspot can be seen, as in the picture. This was taken using a small 80 mm refractor telescope, using a "Thousand Oaks" white light filter. The direct viewing method shows the detail in the sunspots but it's not so easy to plot the positions, whereas the projection method is a convenient way to plot the positions of sunspots on the solar disc but doesn't show the detail.

Neither of these methods shows the Sun's features which are normally hidden from our view by the intense glare. We can only catch a glimpse of them during a solar eclipse. However, the third method of looking at the Sun reveals its hidden treasures.

3. Hydrogen alpha telescope

The Hydrogen alpha telescope allows viewing of the solar disc in a very specific wavelength of light. Seen in hydrogen-alpha light, the Sun appears alive, with rubyred prominences jutting from the limb, dark filaments crossing the disk, and bright flares erupting around sunspots. Viewing the Sun for the first time through a hydrogen-alpha telescope can be every bit as exciting as the first time you saw the rings of Saturn.

H alpha Sun
H alpha Sun, US National Solar Observatory, Sacramento Peak

The Ha telescope looks just like any ordinary optical telescope, except when you look through it you see .......... absolutely nothing! So much light is cut out by the filter system within the telescope that, nothing, apart from the Sun produces enough light to penetrate through to the eye. You will also notice that this telescope, for obvious reasons does not have a finderscope. For this reason it can be extremely difficult to locate the Sun. The Sun appears as a large object dominating our sky, but in this case, not large enough! Just try it for yourself and see how long it takes you to find it. (Be careful not to look at the Sun with the naked eye when attempting to aim the telescope ...... it's really tempting, but not worth the risk). Happily, there is a solution to the problem. Hold a piece of white paper behind the telescope so that the shadow of the shape of the instrument falls onto the paper. Move the telescope until you have the shortest possible shadow on the paper, lock out the telescope, look into the eyepiece and the Sun should be in the near vicinity. Use as low powered an eyepiece as you have, to make it easier, a 40 mm eyepiece is a good option.

So what are we looking at when we eventually locate the Sun and what is a Ha telescope? Hydrogen alpha (Ha) is in the red portion of the visible spectrum. The Sun is viewed by isolating the Hydrogen-alpha wavelength at 656.3 nm and rejecting all other wavebands of light.

The brilliant light coming from the photosphere which is the equivalent of the Sun's surface (although a gas ball doesn't really have a surface), washes out all the relatively feeble light of the Sun's delicate outer atmospheres, the chromosphere and the corona. This makes it impossible to see any features because of the glare. Many solar features most prominent in red light get drowned out by the intense yellowwhite light of the photosphere.

If we can isolate the specific narrow band of light and observe the Sun only in this very narrow wavelength, most of the blinding light is eliminated allowing us to observe features which are otherwise not observable in continuous or white light. This reduces the intensity of the sunlight to a safe level. We can now cut out the glare and observe the Sun in red light to best show up the features we are looking for. Observing at this wavelength, we are able to view all the features of the photosphere. These features include prominences, filaments, flares and active regions. The passband around the Ha line must be extremely narrow for such details to be visible - typically 0.1 nm. Because of this very tight tolerance, Ha filters and telescopes are very expensive. A few years ago the ASE purchased a Hydrogen alpha telescope to allow members to see the Sun in a different light. It's a fascinating sight and we are really fortunate to have this facility, so why not make use of it? The H alpha telescope waveband can be finely adjusted on the telescope, to show surface detail like bright areas and filaments at the expense of the limb prominences. If on the other hand if we altered the waveband again, we would have a clearer image of the prominences but less detail on the Sun's disc.

It is fascinating to make drawings of the Sun using a combination of all three methods of observing. An incredibly detailed picture can be built up to illustrate the ever changing face of the Sun. Even during solar minimum when there are few sunspots to record, don't assume that the Sun is quiet. There can still be amazing displays of dancing prominences on the limb and long dark filaments snaking across the disc for unimaginable distances. It is also interesting to record bright active areas and look out for solar flares. Make a record of these features and see if you link them to any reported aurora borealis.

Further information on how to observe the Sun can be found on the BAA website at http://www.britastro.org.

If you prefer to read about the Sun rather than make any observations then have a look at the Nasa Spaceweather website at http://www.spaceweather.com. Here you can see daily pictures of the Sun taken in many different wavelengths of light by large telescopes and orbiting satellites. There is also an aurora alert on this web page, so you need never miss the aurora borealis again.

There are also many excellent books on the market, one of my favourites being "How to Observe the Sun Safely" by Lee MacDonald ISBN 1-85233-527-0.

With so many resources easily available to us it seems a great shame not to make more use of them. There's no excuse .... well ... apart from the weather!

Lorna McCalman


Contents

Cover page

From the President

Cooling the Earth

See the Sun in a different light

The 2005 Scottish Astronomy Weekend

Z Ursae Maioris

The Televue Radian eyepiece

The Society at work and the City Observatory

About the ASE Journal


This journal as a single web page


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