To take a starfield image without "streaking", the exposure time must be short enough that the star does not illuminate more than one pixel in the FINAL image.
The smaller you scale your final image, the more pixels from the sensor will be summed up and small trails will not be visible.
The calculator generates a table of maximum exposure times for a variety of focal lengths and target image widths.
The last column is the full width of the sensor. The first column tells you how many sensor pixels are traversed by a star per second.
<< Enter some values or select a camera above to get the table here. >>
Q: I don't understand at all what's going on here?
A: Ok here's some random links...