Proper "rule" for pinpoint stars with M43

boxerman wrote:

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 wonder what it will be at 40mm?  That is a good middle ground to test- not wide angle but not quite telephoto either.

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.

Yes that makes sense since you need a "finer" result with a narrower field.

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)

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The BoxerMan

By the way I was inspecting my photographs 1:1 on a large monitor and I noticed something curious- there are some stars I can see at 8 sec and 13 sec exposures but can't see them at 10 sec exposures.  And there are other stars I can find in my 10 second exposures that are nowhere to be found in the 8 sec and 13 sec exposures.  The funny thing is the 8 and 13 sec exposures look more alike than either does to the 10 sec exposures.  I wonder why this is?

I also found that pictures I took after around 2 am showed star colors a lot better than pictures taken at, say, 11 PM.  Not only that but the sky was a lot darker at 2 AM and a lot "redder" at 11 PM.  That must be the effects of more light pollution earlier in the evening rearing its ugly head.