### minimising Star trails ?

Posted:

**Thu Jul 06, 2017 8:05 am**I found this from a 2008 sky at night article Shooting the stars:

Why does "cos (Declination)" also appear as a divisor?

Should the formula be?

Exposure time <= 1000 * cosD / FL

I can see that the longer "fl" requires a shorter exposure and thus appears as a divisor in the formula.Take out the star trails

The rotation of the Earth will make Orion appear to move slowly from east to west. The shorter the focal length of your camera lens, the longer the exposure you can get away with before you start getting star trails in your image. The formula for working out how long you’ve got is

Time (seconds) = 1000 / (fl x cosD)

where fl = lens’s focal length (mm) and D = declination (degrees). Assume D is 0° for Orion.

Why does "cos (Declination)" also appear as a divisor?

Should the formula be?

Exposure time <= 1000 * cosD / FL