The issue of use of filters to view the sun came up recently on this site and there followed a discussion of how these special solar filters worked and why they enabled viewing of solar phenomena is such beautiful detail. In order to understand how the filters do their job, it is necessary to have a basic understanding of the nature of the light emitted by the sun. I have attempted to meet the challenge posted by some MAPUGGERs. This has been a difficult assignment. I hope this post will be useful.
The details of how the sun works are obviously too complex to describe here. Even the brief but excellent descriptions given in the Encyclopedia Britannica take many pages. So what follows is a super brief description of the basic processes by which photons are emitted and absorbed and what we see as a result.
The body of the sun is a mass of hot gasses under great gravitational force because of its mass and with great internal pressure because of the nuclear reactions that take place deep within the sun. The internal pressure and temperature are large enough to cause nuclear fusion of hydrogen atoms into helium atoms. (Some 15,000,000 K) This process generates photons which migrate outward toward the surface of the sun. The photon pressure from the inside is balanced by the gravitational forces. The rather well defined ball we see as the sun is cooler at the surface. It radiates from its surface at a temperature of about 5800 K. The surface we see is called the photosphere. The radiation from the photosphere is characteristic of that of a black body radiating at 5800 K according to PlanckÉs Law. This is the familiar yellowish color we see . We may compare this to earthly temperatures, a common incandescent bulb radiates at about 2800 K. Thus the light bulb also has a continuous spectrum but is much more yellow/red in color compared to the sun. The important thing to realize about the sun is that its basic emission is a continuous spectrum because it is due to thermal excitation.
Immediately above the photosphere is a layer called the chromosphere. The chromosphere has an emission spectrum of spectral lines which correspond to those of the gases of which it is constituted. In this case, the emission is in discrete lines because the gas is relatively rarefied.
Still further out is a region called the corona which consists of very rarefied gasses. These gasses eventually become the solar wind. During a total eclipse, the corona can be seen as a whitish halo extending several degrees out from the occulted solar disc.
Basic to the understanding of the observed solar spectrums from the various parts of the layers about the sun is the concept of absorption and radiation of photons from atomic gasses. At low temperatures atoms are in their rest states and do not radiate significantly. But when atoms in a gas are heated thermally they absorb energy, electrons move to higher (excited) states and they can then re-radiate this energy in the form of photons. Each atom with its array of electrons radiates photons with characteristic energies. The line spectra emitted can be used to identify the elements involved. Atoms can also be excited by other photons, go to excited states and re-radiate their characteristic photons. We see these photon emissions in many lights such as sodium vapor and mercury vapor lights as well as neon signs and the like. These emissions are not continuous spectra but are line spectra. Helium was discovered first on the sun because of its characteristic radiation spectrum.
One of the effects we see in sunlight is that the continuous Planck Law radiation is interrupted with dark lines. This is caused by cooler gases at the outer surface of the sun absorbing photons of certain energies, thus diminishing the spectrum in what are called absorption lines. These dark lines tell use which elements are doing the absorption in the cooler gases.
The surface of the sun is in a constant state of agitation like a pot of boiling water. Thus there are hotter and cooler regions on the surface. Because of the strong emission of radiation from hotter regions and absorption of radiation in the cooler regions, the surface of the sun has a granular or reticulated look. Very large boiling regions are seen as sun spots. Sun spots are so large that they can be seen with a very simple filter that does no more than attenuate the total light from the sun by a factor of 100,00 or so.
Very fine filters for observing sun spots are available from several sources. One of the best is Thousand Oaks Optical (800 996 9111). I personally use their full aperture Type 2 plus filters on my LX200s.
In addition to the constant boiling of the solar surface there are much greater spurts of material that are ejected from the surface of the sun which return in gigantic arcs. These eruptions are called prominences. The are visible during a solar eclipse as arcs of flame projecting well beyond the dark disc of the occulting moon.
The granularity, the chromosphere and the solar prominences are so dim compared to the photosphere that they cannot be observed except during a solar eclipse or with very special telescopes and filters. While these features absorb and emit photons in many parts of the spectrum, one special line is of particular interest. It is the Hydrogen alpha line at 6562.8 Angstroms. This line is emitted strongly by the features we want to observe. The scheme required to form images is to use this spectral line while suppressing all other light from the sun. This is done with special interference filters. These filters are very complex, being made up of numerous thin interference layers and many reflecting optical elements. They are adjusted to cancel all wavelengths except that of the Hydrogen alpha line. These filters can be made to have passbands that are so narrow that they only pass from a few down to a fraction of an Angstrom. Such filters are often combined with other special optical attachments which are designed to obscure the image of the disc of the sun and so make prominences even more visible.
Filters for Observing the Sun
While specialized filters and optical attachments such as those required to observe solar granularity and prominences are, because of their price, usually out of reach of the amateur astronomer they never the less are enticing. The observation of stars should surely include our very own star. Ellery Hale was, for example, fascinated by the sun all of his life and made a significant reputation through his studies of the sun.
Solar filters and attachments are described in some detail in "Solar Astronomy Handbook" by Beck et al (Willmann-Bell) The design and construction of these devices is for very serious amateurs only. Excellent hydrogen alpha filters are available from a number of sources. A discussion of the daystar filters is available at <http://www.company7.com/>. These filters are very expensive and require additional pre-filtering to eliminate the massive heat load from the sun. These pre filters generally have a small stop to reduce the light gathering power of larger aperture telescopes. The stop is usually set to about f30. This feature has advantages and disadvantages. The small stop reduces the resolution of the telescope but the narrow cone of light improves the performance of the final narrow band filter which is at the eyepiece. The filters are very complex multilayer structures which are designed to reduce the bandwidth to Angstrom or usually sub Angstrom width as described below. Because the filters have such a narrow bandwidth they need to be tuned during observation. The best of the filters are tuned by controlling the temperature of the filter by means of a small oven surrounding it. Additional special optical elements are used with the narrowest of these filters to insure that the filter is in a part of the optical path where the imaging rays are parallel. Less expensive filters are tuned by tilting the filter with respect to the entering cone of light. This technique work fairly well and the design greatly reduces the cost of the filters. The cost is also determined by the narrowness of the filter. The width of the filter needs to be chosen to match the purpose of the observer.
An elegant description of the view seen with filters of various bandwidths is given in "The Manual of Advanced Celestial Photography" by Wallis and Provin (Cambridge University Press 1988). I quote:
"With a 1 Angstrom bandwidth the prominences lining the edge of the disk can be easily seen and solar flares can be viewed but the finer details over the sunÉs disc are not well displayed. With a 0.8 Angstrom bandwidth, the prominences are seen very well, with better contrast than is provided by a 1 Angstrom filter, and the details on the disc of the sun are readily seen in detail. With a 0.6 Angstrom bandwidth , the details on the disk are seen with excellent contrast and clarity however, the prominences are now difficult to see."
Thus it is important to choose exactly the right filter for the visual effect desired. The inexpensive filters which are tuned by tilting do not give a uniform appearance over the entire field of view. As with so many things, the expensive professional filters are the best. The wider, 1.5 Angstron filters will be a disappointment to many viewers.
The Wallis and Provin book is, I believe, the most useful book on celestial
photography I have found. I recommend it highly. I have appended
to this note a list of other books on astronomical photography which may be
of interest, with my notation about their suitability for various levels
Photography References - With particular attention to the Sun and Solar System
Personal comments about a few select books from my library which I have studied in some detail - Doc G (14 September 1997)
A Manual of Advanced Celestial Photography;
Brad D. Wallis & Robert W. Provin; Cambridge University Press 1988
The first among good books on astronomical photography. The book concentrates on photographic techniques rather than equipment. This is an extraordinary book and a must for anyone doing or wanting to do astronomical photography. Not for the novice but a required book after digesting one or two of the following books.
High Resolution Astrophotography; Jean Dragesco; Cambridge University Press 1995 An excellent book with emphasis on photography of the Sun, Moon and planets. Equipment is described in detail and numerous examples are shown. Not for the novice.
Solar Astronomy Handbook; Beck, Hilbrecht, Reinsch, Volker; SONNE (1982 German) (first English Edition 1995) Willmann-Bell Great detail about equipment and many examples of photograph results. Lengthily discussion of observations and recording of all solar phenomena. Definitely for those with a deep interest in the Sun. Definitely not for the novice.
Astrophotography - An Introduction; H.J.P. Arnold; Sky and Telescope 1995 A nice, and simple, introduction mainly to photography of the Sun, Moon and Planets. A fine book for the novice and the serious amateur.
Astrophotography II - Featuring the Techniques of the European Amateur; Patrick Martinez; (1983 French) (English Edition 1987) Willmann-Bell, Inc.; A fine small book jammed with important information that everyone should know about setting up telescopes for photography. An excellent book for the serious amateur.
Astrophotography for The Amateur (revised 1991); Michael Covington; Cambridge University Press A nice little book that covers the basics very clearly. The book tries to cover a bit too much ground for such a short book. Strictly for the novice.
The Cambridge Eclipse Photography Guide; Jay
M. Pasachoff and Michael A. Covington; Cambridge University Press 1993
A very nice book about eclipses with detailed tables covering eclipses through 1999. Excellent discussions of observing and photographing these events.
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