Exposure time

[Somnium (Aidan)]

So I just got a f3.5 lens at 8mm. I am looking to do some wide field imaging, just wanted to get an idea of how long I can expose without getting noticeable star trails

[raymo]

You should be good if you work on 30 secs @ 0 degrees Declination
40 " " 40 " "
60 " " 60 " "
180 " " 80 " "
raymo

[Somnium (Aidan)]

Interesting, I used to do 30secs at 18 mm, looking at Sagittarius, so at 8mm I would have thought I could go a fair bit longer

[xelasnave]

Trial and error.
Alex

[Somnium (Aidan)]

Originally Posted by xelasnave

Trial and error.
Alex

True, but I was hoping to set quickly and forget, I am heading up to wiruna tomorrow night, I will test it out tonight

[RB (Andrew)]

Originally Posted by Somnium

Interesting, I used to do 30secs at 18 mm, looking at Sagittarius, so at 8mm I would have thought I could go a fair bit longer

I agree Aidan.
I did my Milky Way widefield test on my new camera at 16mm at f/4.0 and was running 30 sec exposures easily.
See this thread:

MilkyWay Rising with the Canon 1DX
You should be able to bump that up I'd say to just under 60 sec on a full frame camera.

I used the 500 rule on my Full Frame camera.
The 500 Rule is:

500 Divided By the Focal Length of Your Lens = The Longest Exposure (in Seconds) Before Stars Start to “Trail”

This gives you a rough guide.
Note that this is for full frame cameras so you have to factor in the crop factor if you're using a crop sensor.

RB

[raymo]

I was erring on the conservative side. @18mm you should get perfectly round stars @ Dec-30[Sag] @ around 20 secs. Remembering that an 18mm
shot will have minute stars that would need enlarging significantly before
elongation becomes visible.
raymo

[Somnium (Aidan)]

Doing the calcs I think that if I go with 60 second exposures it is going to be a pretty short time laps, 30 seconds will be good, no star trails just a little noisier

[raymo]

At what Dec do you apply the 500 rule? At 80 Dec you can get exposures
six times longer than at 0 Dec. Imaging the few near polar galaxies you can get much longer still.
raymo

[Somnium (Aidan)]

Originally Posted by raymo

At what Dec do you apply the 500 rule? At 80 Dec you can get exposures
six times longer than at 0 Dec. Imaging the few near polar galaxies you can get much longer still.
raymo

Dec is not going to mean much for me, I am imaging with a 180deg fov

[raymo]

Then obviously only the 0 Dec figure is of interest to you.
raymo