1969-2019 - 50th Anniversary Year
The Sun in White Light and Calcium K Light by Ron Mansfield
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The CaK photograph (upper) was taken with a Coronado PST CaK telescope using a mono video camera at prime focus piggybacked on an 8-inch Celestron SCT. Recorded at 25 fps, stack of 600 frames.
The white light photograph (lower) was taken at prime focus through the same 8-inch Celestron SCT. Using a Canon 350d and a Baadar solar filter. 100th sec at f10 ISO 400.
These two shots taken at the same time in the Spring of 2011 clearly show why it is necessary to study the Sun in a number of different ways in order to fully understand what is happening on its surface. Whilst the pair of sunspots is clearly visible in both photographs, it is only in the upper photo that a huge area of activity streaming away from them can be seen.
The lower photo was taken in "white light." That is to say, by means of a filter that simply reduces the amount of light at all wavelengths that gets through to the eye or in this instance, to the camera. Typically these filters reduce the amount of light by around 99.9% making them safe for visual use. However, the residual light that penetrates the filter is still more than bright enough to not only see the Sun visually, but also to photograph it without lengthy exposures – just 1/100th of a second in this case. Such filters can be attached to any normal telescope.
In contrast, the upper photograph was taken using a dedicated telescope, an integral part of which is a specially designed narrow band filter that only allows the passage of light in a very restricted rage of wavelengths centred on 393.968 nanometres. This is the wavelength of the so-called "K" Fraunhofer line due to ionised calcium.
Although the telescope can be used visually, many people have some difficulty seeing this particular wavelength which is right at the extreme blue end of the visible spectrum. However, as can be seen from Ron’s photograph, the camera is able to pick up on a good deal of detail of which there is no sign in the white light photograph.
As with the Sun’s hydrogen alpha emission, the emission due to calcium emanates from the chromosphere, which is the layer immediately above the photosphere from whence most of the Sun’s light comes.
Another Fraunhofer line due to calcium, the "H" line, occurs very close by at a wavelength of 396.847 nanometres. Although the wavelength difference between the two lines is less than three nanometres, (1 nanometre = 1 billionth of a metre) this nevertheless means that it lies outside the extremely restricted range of wavelengths that can pass through the calcium K telescope’s filter.