Sunspot Group 652

    The images below were obtained on 18 July 2004 Pacific Daylight Time at a local park. Fifty 3-minute videos were taken sequentially at prime focus of my Takahashi FS-102 using a Coronado 60mm H-alpha filter and a Philips ToUcam Pro with an IR blocking filter. The sequence was interrupted for 8 minutes after video 22 because the telescope had to be shifted to the other side of my German Equatorial mount. Still images are located below the movies. Check out a rapidly developing flare and an associated filament that I captured on 25 July 2004.
    I really lucked out, capturing transient filament activity for the duration of the photo session. These were the most active and largest filaments I have captured to date. Filaments are prominences that are seen in absorption against the solar disc. According to Moore and Hunt (1997: The Atlas of the Solar System, Chancellor Press, London) the temperature of this material is about 10,000-30,000K, 100 times cooler than the surrounding material of the chromosphere and corona. Additionally, these regions are about 100 times as dense as the surrounding material. At the lower temperature and the higher density there is an enhanced concentration of non-ionized hydrogen relative to the background material that gives rise to the emission at the H-alpha wavelength. Despite appearances, most of the material in prominences is the result of condensation of material from the corona that flows downward into the chromosphere. In the vicinity of the sunspots the brighter regions are known as plages. Plages are regions of enhanced density and higher temperature that are overlaid by hot regions in the corona, and they are associated with strong magnetic fields. See Moore and Hunt (1997) for a more detailed discussion.
    In this sequence the ToUcam shutter speed was set to 1/100 second. This is faster than my previous solar imagery when I used 1/50 second (e.g., 25-26 October 2003). I selected the faster shutter speed because the main portion of the prominences lacked detail because the pixels were light saturated with the longer exposure. As you can see below, the faster shutter speed resulted in fantastic detail within the prominences. However, to extract this detail, and that on the disk, additional image processing was necessary. The original image was pinkish with low contrast even after setting the linear wavelets 1-6 to values of 1, 3, 15, 20, 15, and 10 in RegiStax2. I then accessed the histogram tool, in which the histograms of each of the red, green, and blue channels are displayed over the bit range 0-255. I moved the right slider to the left to 150, and stretched the 0-150 bit information to the full bit range, which gave greater contrast. I then tuned off the blue channel, which contained little if any useful information, and it contributed to the pink color of the initial processed image. I set the weight of the red channel to 0.8 while leaving the green channel weight set to 1.0. These changes resulted in the more pleasing yellow-red color balance.
    After reading the article "Channel-Surfing to Better Solar Images" by James Weightman in the June 2004 issue of S&T, I decided to examine the green channel to see the information it contained. To my eye, this channel captured more surface detail than the red channel alone and compared to the sum of the green and red channels as discussed above. The reason this works is that the response functions of the red, green, and blue channels are not mutually exclusive. The tails of the green and blue response functions of the ToUcam chip overlap the H-alpha line, and their reduced response relative to the red channel at the H-alpha wavelength effectively acts as a shorter shutter speed. Thus, in the green channel the prominences are virtually absent except where the H-alpha emission is very intense. Also, I've been told that the filters that cover each pixel of the ToUcam are composed of one red, one blue, and two green elements. This would contribute to the greater spatial resolution of the green channel compared to either the red or blue channel alone.

Prominence and Transient Filaments (43kb, 38kb) 18 July 2004, 20:16UT
Prominence and Transient Filaments (43kb, 38kb) 18 July 2004, 21:08UT
Prominence and Transient Filaments (43kb, 38kb) 18 July 2004, 22:08UT
Prominence and Transient Filaments (43kb, 38kb) 18 July 2004, 22:35UT