Obtaining a sharp focus is critical to many aspects of amateur astronomy. Ironically, however, placing the object of attention purposefully out of focus can help us make better sense or more accurate observations of what we see. Let’s look at some examples.
Star color
The surfaces of stars radiate at different temperatures, which to our eyes appear as different colors: Blue stars are hot; red stars are cool; yellow-white stars (like our Sun) are in between. If you view a star in focus, its color can be difficult to pin down. It can change in a matter of milliseconds because of the refractive effects of our ever-moving atmosphere. When it does, it displays an array of prismatic colors as it scintillates (twinkles) like a diamond under light.
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