Solar features can evolve rapidly and hence the tri-band acquisition process must be completed before these changes become significant. If you don't hurry, 'false' colors will arise that reflect movements of gas rather than changing physical conditions on the Sun.
For high resolution imaging with a webcam or similar device the quest for getting many images to catch moments of good seeing must be weighed against this requirement. Also, the more you zoom in on the Sun and the more active a feature you are targeting, the shorter the available time will be.
It is not possible to pin down a firm limit, but with my targets, preferences and setup (Coronado
SM60, f=1280mm, Skynyx 2-2M camera) I find the total available time to range from 1-5 minutes. Hence, if images are to be acquired for three wavelengths, the available time pr. wavelength is reduced to roughly 20-100 seconds. With my camera the acquisition rate is around 10-25 frames pr. second, hence each series consists of roughly 250-2500 images. Since Solar features change more rapidly in the chromosphere than in the photosphere, the time restrictions are least severe when the filter is detuned the most from the H-α wavelength. Consequently, images with the largest detuning from H-α ‘last longer’ for tri-band work.
Many filters are tuned mechanically in seconds, by tilting or by using a pressure regulator. A fast tuning mechanism is obviously best suited for tri-band imaging. Other filters are thermally tuned which is a slow process that requires several minutes to reach equilibrium between each wavelength change. With such a system, one could opt for ‘dual-band’ imaging where only two images are acquired for the red and blue channels, respectively– one at the H-α wavelength and another at maximum detuning. A green channel can then be synthesized as an average of the two other channels. An example is shown below. For this image a Daystar Quantum SE 0.5Å filter was used with 6562.8Å for the blue channel and 6563.8Å for the red channel. The result is not as rich in color variations, but I still consider it worth the effort.
|Dual-band image of AR1195 from April 25, 2011. Daystar Quantum SE 0.5Å H-alpha filter.|
Below is another shot - this time a real tri-band image - of AR 1087 on July 10, 2010. This was acquired using a Coronado filter that was detuned by tilting. The colors are obviously better.
Tomorrow I will discuss the processing flow I use for making these images. A lot of it is standard stuff for Solar work, and the tri-band part is actually quite simple.
|Tri-band image of AR 1087 from July 10, 2010. Coronado SM60 filter.|