INTRODUCTION
Perhaps the most fascinating aspect of observing the sun is the dynamic nature of the subject. Our sun is a celestial body whose features are in constant turmoil, one can watch and record many features including spot groups develop and decay, the gaseous sphere rotate, the prominences come and go; this and more all on a relatively short time scale. As such, each individual observation is a truly unique perspective that cannot be duplicated at a later time. For many of us this is the appeal of solar observing.
Most recently amateur astronomers have had the tools and know how to acquire images of these activities rivaling results of professionals just a few decades ago. For many amateur solar observers producing aesthetically accurate photos can be an attractive end in itself. However, for the amateur that desires to do more with this hobby, analyzing his images with the intent of extracting scientific data can be an even more pleasing activity. By adding to the databank available to the professional, the "amateur researcher" will no doubt come away from the experience having developed the skills and discipline to produce consistently accurate results.
The digital revolution has made the extraction of such information no more difficult than the click of a mouse and a few strokes on the keyboard. One particularly suited software is a freeware, available for download at http://rsb.info.nih.gov/nih-image/ titled NIH Image. "Image" is written for the Macintosh platform but recently a PC version has become available. NIH Image is a user friendly software that permits limited image manipulation; its real value may be in its analyzing abilities. Possibilities include the conversion of gray values into calibrated units, thereby densities into brightness and brightness into temperatures. Slices of density may be viewed permitting observation of features not easily seen. It provides tools for measuring geometric dimensions of images; such as a sunspot's apparent length, width, perimeter or area. With the click of a mouse one can create a density.
IMAGE ACQUISITION
Photos are obtainable through several mediums today. Digital cameras, video, or the standard film camera comes to mind immediately as the most often means of obtaining images. Whichever mode of imagery you utilize, to use the techniques described here, three factors should be considered. First, an image should be of reasonable scale to show detail. Whole disc photos do not have sufficient resolution to be very useful. Of course the higher the quality image one has the better; but data can be obtained from less than ideal photos though sacrificing accuracy accordingly. Secondly, the image has to be digitized. Working from a digital camera this is a given. Video images must be "grabbed" and converted to a digital format while film or hardcopy must be scanned to produce a digital file. Lastly, you must know the image scale of your photos to "calibrate" your measures. The simplest way to work may be to standardize your system. Regardless of the recording medium you choose—select a focal length that provides reasonable scale at the image plane and calculate the arc second per pixel resolution of your images. Then use this standard system repeatedly for your photography, minimizing any future variables.
ANALYZING THE IMAGES
The pixel is the basic picture element in digital photography. By knowing the pixel resolution of your photographs NIH Image can calculate the geometric proportions of a selection of pixels from your files. These measurements may be viewed on screen, saved to a file or printed as hardcopy. Keep in mind that you are analyzing apparent views at this point. Corrections for foreshortening would need additional computations. Similarly, NIH Image measures gray values from selections you provide to acquire the various density profiles obtainable; if your interest is say—comparative studies of a feature over time—you should calibrate or provide images that have identical density values so as not to skew your results.
Since user manuals are downloadable from the NIH Image Home Page there is little need to go into highly detailed instruction here. Suggestions and minimal guidance should be sufficient to get most technophiles off and running. It will be simpler if working from grayscale images, so it may be advisable to first convert a copy of any multi-channel pictures to a grayscale image and work with it. Creating a surface plot of an area is as simple as making a selection with the rectangular selection tool, going to the analyze menu and selecting surface plot; then clicking the OK button. The surface plot will illustrate density variations graphically in wire frames or as a grayscale rendition. Similarly the density profile option will allow one to graph density variation within a line segment selection. From the illustation you can see that features not readily visible, such as the bright inner ring between the penumbra and umbra, can become apparent with this analyzation technique. The apparent length, perimeter, or area of selected features can be computed quite easily if you have the pixel resolution of your digital images available. Again remember that foreshortening and the varying earth-sun distance can alter measurments if you don't additionally compute corrections for those variables when needed. After setting the parameters as the number of pixels per second of arc in the set scale window under the analyze menu, it is simply a matter of using one of the selection tools to determine the area you wish to measure; then instruct the software to compute the measure and to show the results. Results are visible in two locations, either the "results" window or the "info" window. Area may be computed in square seconds of arc and length or perimeter in seconds of arc. When setting the parameters in the set scale window you will see that you have a choice of a number of output scales including kilometers. The selection tool for measuring length would be the straight line tool found in the tool pallet. The illustrations show simple measurement of the length of a sunspot group and the height of a prominence above the solar limb.
NIH Image also has an interesting feature that permits graphically depicting slices of density superimposed on your original photo. To use this option first open an image, the select the Enable/Disable density slicing tool from the tools pallete. In the LUT window adjust the density range of the slice you wish to view with the E/D tool by noting the upper and lower values in the info window. Then obtain the density slicing by clicking and dragging on the range bar in the LUT window. A bit of experimentation will show what is possible with this tool. The animation depicts a series of slices of AR 8297. The index window has a readout informing the viewer the relative density of the slice portrayed; the greater the number the denser the slice. This technique is a simple method to determine and compare relative brightness values for different regions of a spot or group.
In conclusion, the "step by step" instructions included with the illustrations are by no means the only method to obtaining data, but rather are intended as a seed to help your own observing program grow. Perhaps with a bit of further experimentation you can develop an observing program utilizing these "freeware" tools, extracting even more information from the images you obtain of our "nearest star". Back to the ALPO Solar Page