Wednesday, September 19, 2012

Tri-band solar imaging - part 2

Detuning by tilting - Coronado's T-max tuner

The Sun provides so much radiation that a tiny wavelength interval can be selected to enable viewing of certain layers of the solar atmosphere or certain elements – and still have plenty of light for fast exposures that freeze the seeing. Most notable is the hydrogen alpha (H-α) transition at a wavelength of 6562.8Å. Emission at this wavelength is a sign that hydrogen is being ionized and it comes predominately from the solar chromosphere. Narrowband solar filters can be detuned slightly to maximize contrast or to allow observation of Doppler shifted light from features that are moving rapidly along our line of sight. The tuning range is typically very limited – around one angstrom – but even such small changes lead to dramatic differences in the resulting image. I use a Coronado SM60  H-α  filter that can be tilted slightly using  the so-called T-max tuner; see the photo above.

Rates of change in brightness of various features with filter detuning and how they relate to colors in a tri-band image
(click for bigger version)
The figure above shows a typical scene, captured with this filter at three different wavelengths. The targeted object is active region 11087, which consisted of a small sunspot 20” across and a complex magnetic field which produced several low-level flares during July 2010. At the H-α wavelength of 6562.8Å (corresponding to zero tilt), the field is covered with spicules and contains two dark filaments and a bright region of plage. In all this action the small sunspot almost gets lost, with only the umbra standing out clearly as a dark, irregular spot. At 6562.3Å (half tilt) the contrast of the spicules, filaments and plage decreases. At 6561.9Å (maximum tilt) these features are nearly gone so that the sunspot with penumbra becomes clearly visible. A smaller spot, unnoticed before, becomes apparent towards the upper left.

As the wavelength is tuned away from the H-α transition we are viewing more and more light from the underlying photosphere. What is especially interesting in this regard is that the rate of change in brightness with wavelength is different for various features, depending on what they are. It is not the same everywhere. Hence, if detuned images are used as separate color channels in a composite image various colors will appear that represent different rates of change. This is evident in the tri-band image above where the R, G, B channels are chosen as most detuned, medium detuned and not-detuned, respectively, from the H-α wavelength. In such a color scheme the underlying photosphere appears yellowish, spicules and filaments are reddish, plague is white, etc.

Tri-band image of AR1087 in a quite moment. This active region had a beta-gamma magnetic field and harbored multiple M-level flares during July 2010. Click to enlarge.

Many more colors can be found in this way on the Sun and I'll be showing more later on. For example: how do you think the chromosphere and prominences at the solar limb will appear in a tri-band image as the one above? See for yourself in tomorrows blog posting!

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