Maximum exposure time for untracked mounts

[Atreta]

Math is not my strongest point but i think the error is in the pixel size, you're using megapixel not pixel size in micron

[Atreta]

Your formula sharkmelley :

star_trail_length_in_pixels = focal_length_in_millimetres * exposure_time_in_seconds / pixel_size_in_microns / 15

Is the same with :

exposure_time_in_seconds = star_trail_length_in_pixels * pixel_size_in_microns * 15 / focal_length_in_millimetres

My Nikon D40 has a 23.7mm x 15.6 mm sensor size and 6MPixel

According to the formula : Pixel_size = 1000 * sqrt (sensor length * sensor width / number of pixels) , it results that my camera has a 7850 pixel size.

The telescope has a 1200mm FL

So the exposure_time_in_seconds in order to get a 1 pixel star_trail_length_in_pixels :

exposure_time_in_seconds = 1 * 7850 * 15 / 1200 = 98.125 seconds?

I can't really believe it.... where's the mistake?

Did some search and the D40 is 7.78microns, with that formula it gave 0.097 seconds.