Proper "rule" for pinpoint stars with M43

OutsideTheMatrix wrote:

Giiba wrote:

...

I experimented the other night (my first seeing stars in months) with my 40-150/2.8 and found that 3.2sec was the max I could shoot without trailing @150mm when looking ENE. I have adjusted the formula on the spreadsheet to account for my findings. I am now reasonably happy with the max exposure time ouput as it gives a rough idea that can be used as a starting point for untracked exposures; increased when facing the pole, and decreased facing the equator. I have now tested 8mm (fisheye), 12mm, 15mm, 17mm, 150mm. Going to do some shots at 40mm'ish next chance I get.

How does that fit in with our findings of 10 sec at 14mm?

It doesn't. At 150 mm, the formula gives 1 second. But, as I suggested, people have different display requirements and tolerance. Maybe Giiba can comment.

I saw this formula somewhere- what do you think?

Recently, I ran across this equation:

time = 240 / (lens fl * cos dec)

Yes, correct. The formula we worked on was maximum trailing, which is in the equatorial plane. For wide-angle shots, you'll usually get some at that declination, so it's sensible. But, if you're doing a narrower field (like 150 mm), you can get a better estimate by including the declination.

where dec is the declination of the star closest to the celestial equator. This over estimates the time so I've now modified it to the following:

time = 150 / (lens fl * cos dec)