This is true for lower apertures, but at higher apertures they become much more noticeable especially at high magnification. This is especially annoying for tel

This is true for lower apertures, but at higher apertures they become much more noticeable especially at high magnification. This is especially annoying for telescopes (it tends to look like this https://farm9.staticflickr.com/8679/16584213212_5b33222d6e_c.jpg https://farm9.staticflickr.com/8679/16584213212_5b33222d6e_c...), which why astrophotographers use a trick called flat calibration to remove any artifacts from dust or smudges in the optical train. The trick is surprisingly simple: take a picture of an evenly illuminated surface (usually a flat box or a clear sky) and make sure your ADU is in the linear phase of the sensor (ergo, don't clip the highlights or darks). Then divide your original image by the flat ($T * mean(flat) / flat). This is of course easier with a monochrome sensor and works best with RAW data, but it can definitely be done with a color CMOS as well. In case you ever find yourself with hard to correct dust on the sensor: this trick might save an otherwise poor image.