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Solar Astronomy

I follow solar solar astronomy for two reasons - the first is that it is easily visible during the day, the second, hat it is one of the most dynamic activities in astronomy in that the subject matter is changing second to second

The tools of the trade are also fairly simple - you have three options really:

• Simple solar disk projection
• Observation of the sun directly using a Herschel wedge and ND filters such as the Intes device from APG
• Simple solar filter using Baader or Thousand Oaks or straight filter-quality mylar film
• Monochromatic observation of the sun using
  • Lyot monochromators
  • Monochromatic interference filters of the Coronado or Daystar type
  • Prominence telescopes where at-focus solar disk stops are used to allow viewing of the low-contrast chromosphere by occluding the solar disk.
  • Scanning slit spectrometers where a spectrometer slit scans the image of the disk of the sun to build a raster image of the disk in a particular wavelength.

Without dwelling on the safety aspects too long, the first is the safest and the Herschel wedge option and the mylar filter options both vie for worst because of the possibility of the filter falling off while observing by eye. Don't observe the sun by naked eye - at your peril.

My preferences and capability extends to the Herschel wedge and the H-Alpha monochromator.

The visible structure of the Sun

The image above displays most of the visible features of the sun. The visible disk contains sunspots, faculae, sometimes visible flares. The monochromatic disk shows faculae, plages, prominences in projection, the sunspots and their internal low-flare activity.

Solar Projection

The first of the above methods is used to view the visual disk. This method is in my mind quite difficult to set up with any degree of stability - projection attachments on the telescope detract from the telescope stability because of their length. They must be long because they need to project an image onto a surface and so you benefit from having a really stable mount and telescope. You also run the risk of damaging a perfectly good eyepiece in use as the projection lens if you don't take precautions or use a simple uncemented doublet type.

The method uses a simple eyepiece in the focus tube to project a focused image onto a piece of paper, preferably in a shady location. The key benefit is use of the full aperture of the telescope for providing the image. The donside is the possible low-contrast of the image because of ambient light. However you can show lots of people at once though.

Herschel Wedge

The Herschel wedge was popularised if not invented by Herschel in the late 1700's. The idea is to make a diagonal , commonly used for rotating the image through 90º for easier viewing, bit not from a flat, parallel-sided, silvered piece ( ie. a mirror ) but from an unsilvered piece of glass whose rear surface departs from parallelism by a few degrees. This is enough to remove 95% of the light from the light path and allow ND filters to manage the rest.

The Herschel wedge I own is of Intes manufacture and uses a Baader 28.5mm ND 3 and a polarising filter after the semi-silvered mirror, attached within the prism mount and a polarising filter on the eyepiece ( for visual use ) to control the visual intensity of the solar disk. The resolution is , in my mind, the best of all the methods and provides the best contrast too. The wedge allows me to use the full aperture of my 4" Vixen to best effect.

June 01 2K2, neg #23	June 01 2K2, neg #24a		2002 July 18 neg #22	2002 July 18 neg #26	2002 July 18 neg #29	2002 July 22 neg #39

H-Alpha Monochromator

The H-Alpha line is the name given to the peak in the Balmer series of Hydrogen emission bands at 6563.8 Aº which are visible in the solar spectrum, most strongly emitted by the region of the solar atmosphere above the visible surface, the chromosphere. The emission line series, like its analogue the Lyman series in the Ultraviolet range, are widely spaced and most intense for the most energetic transitions and get closer as the transitions are between less widely spaced electron orbits in the atomic shell. The intensity of the lines are dependent on the temperature of the emitting hydrogen gas.

The chromosphere is not normally visible because of the greater brightness of the photosphere compared to the very-low contrast and optically-thin chromosphere, its like the difference between the sea and the horizon. The chromosphere temperature ranges from 10,000º to 10 milion ºC, surprisingly increasing as you move further from the sun into the Corona. The most dramatic features visible in monochromatic light ( common bands also include Calcium-K and Helium-Beta )are the prominences. Prominences are the fast moving, hot gas ejecta from the surface of the sun. This is normally associated with the active magnetic fields around sunspots, and the magnetic fields ad the speed of the ejecta determine the rapidly-evolving shape of the ejecta. The lifetime of these events is from minutes to hours and their size can be up to a solar radius before they become too thin to see.....

The H-Alpha monochromator, manufactured by Daystar consists of a narrow-band interference film stack of the order of Full-Width Half-Max ( FWHM ) of 0.7 Angstrom ( Aº) . The stack is either designed for constant-temperature use in an oven or for use around a stated ambient temperature of roughly 15ºC. Tilting the stack from 90º to the incoming beam shortens the wavelength the filter is tuned to, counteracting effects of heating by ambient heat which expand the stack and move the central point further to the red. You might also suspect if a changing view angle changes the central wavelength, then a shorter f/ration with its wider range of image angles in the image cone will cause a broadening of the passband. This is certainly the case and is why these types of filters must be used at f/30 or more in general. ~Not only that but placing behins barlow worsens the problem, surprisingly enough. I thought a barlow would reduce the light cone angular range because it is a negative lens, but apparently there is also the range of angles in the light cone into the eyepiece to consider which might be a larger effect than the barlow itself. So a Televue Powermate was recommended on various websites as an equivalent to a barlow and without these problems. And it works a treat. The on-band area is wide if not filling the eyepiece, the contrast is higher and the surface and limb activity detail is more visible due to the narrower light cone into and out of the filter receovering more of my initial FWHM. The filter is normally used on the 4" Vixen with a variable diaphragm aperture control on the objective, which also mounts the energy-rejection filter (ERF) required by this type of filter.

The key issue for me in this type of work is the apparent very low contrast of the subject from a photographic point og view. Visually there is a lot of detail, in the camera lensa at f/25, where most of these picture are taken, there is not very much contrast at all in the final image. Efforts to imprive the image by reducing camera shake show little difference. Hence I believe the effect is reveal. Scanning in the negatives requires a low contrast setting for disk details and high contrast for the limb details.

Similarly because of the great step in brightness between disk features and limb features, the typical exposure ( 100 ASA, F/25 ) for the limb is 1/30sec, that for the disk, 1/250th sec on Kodak Royal Gold.

For more information see : thedaystar filters group or look up Coronado Filters on the web.

Solar images from a Hydrogen-Alpha filter

9th March 2K2, neg #22	June 01 2K2, neg #37	June 01 2K2, neg #06	June 01 2K2, neg #33	July 18 2002, neg #33

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