You have probably seen various photos where the Sun always has different color. There is no mystery about it, it all depends on the filter you have, and a filter you must use.
The most obvious and desired target are the sunspots. They vary in sizes and numbers. When you photograph the Sun during its 11-year solar cycle, the number of sunspots could be quite different. During the maximum of the solar activity, the Sun will be covered in sunspots, many of which large enough, and worthy of your interest. On the other hand, during the solar minimum, you may not find even a single sunspot.
Photographs are probably the most unbiased method of calculating the Wolf number. For more information about sunspots, groups and Wolf number calculation, see How to draw the Sun.
Pick the right lense/telescope
When you photograph the Sun, you are rarely concern about the aperture of your instrument. Let’s face it, the target you are interested in is quite bright, and you don’t really care about collecting power. Granted, the aperture affects the resolution, but any decent size instrument will do. So, what you would mainly care about is the focal length of your telescope, and here is why:
The image above shows how the Sun would look like in your camera with different focal lengths. As you can see, to have a decent shot of the sunspots you would need to use at least a 500 mm (36 x 24 mm or even a 1000 mm) lens. Keep in mind that these images are for a 35 mm film camera (or as it’s nowadays known in the world of digital photography, a “full-frame” camera). When using a digital detector, the relative size of the Sun would depend on the size of the camera chip. Most 2/3 DSLR (Digital Single Lens Reflex) cameras have slightly smaller chip size, so to get the 35 mm focal length equivalent you would have to multiply roughly by 1.5-1.6. For 4/3 DSLRs you would have to multiply the focal length by 2. Remember that the absolute size of the Sun in mm will always be the same, no matter what camera you are using!!!
Additional magnification is often needed to capture only a group of sunspots or a large one. There would be a separate section about Barlow lenses, eyepiece magnification, etc.
After you settle on optical system and final focal length, you can calculate the Sun’s size in mm using this simple equation:
d = 0.0093 x F
where d is the projected on the detector size of the Sun in mm and F is the focal length of your instrument in mm. Knowing the resultant size in mm and the size of your detector, you could determine how big the Sun would look, or what fraction of it would be on your photographs.
Go to The Sun