star trails!!!!!

[joshman (Josh)]

hey all i just did some photos of the sky out side, and was getting star trails after only a few seconds (i'm talking like...10seconds) is there anyway to overcome this? it's quite annoying. i was running at ISO1600, at a focal length of 70mm (didn;t wanna change down to my wider lens) at around f16

[robin]

Josh, with a 70mm focal lengh lens, you're gonna get star trails, albeit small ones, with a 10 sec exp. Are you pointing it at sigma octantis? Youve got to decrease your focal lengh or piggy back your camera on your scope

[ballaratdragons (Ken)]

Josh, if you run along with your gear to keep up with the stars you shouldn't get star trails.

Believe me. Millions do

I am the spreader of conspiracy theories, but this one is true

[[1ponders] (Paul)]

As Robin has put it, shorten your focal length or start tracking.

Longest exposure without star trails = 700/Focal Length of lens (for near 0deg Dec)
&
1000/FL for near the poles.

So 700/70=10sec

[iceman (Mike)]

Go to a shorter f/ratio so that you can get the same light in a shorter exposure.

[Ingo]

Yeah F16...I'd be surprised if you got any stars in a non-light polluted area. Try that and try 8s on 70mm. Usually works for me if you have a 1.54x or 1.6x crop camera. If you have full frame, you could easily do 10s.

[robin]

In hindsight Josh, try Opening up your f.stop to f.4 or 5.6 & see how you go. You then may be able to drop down your iso to 800 to decrease noise.

[Geoff45 (Geoff)]

Originally Posted by iceman

Go to a shorter f/ratio so that you can get the same light in a shorter exposure.

This only applies to extended objects (nebulas, planets etc). Stars are point objects, so f ratio doesn't affect exposure, it just changes the size of the field of view. In fact a longer focal length might be better, because it dims the skylight, since the sky is an extended object.
Geoff

[robin]

Assuming you're shooting on manual, then at 10 seconds, a exposure using f4 will definetely show more detail tahn a 10 second f16 exp.

[Geoff45 (Geoff)]

Originally Posted by robin

Assuming you're shooting on manual, then at 10 seconds, a exposure using f4 will definetely show more detail tahn a 10 second f16 exp.

I jumped in there without thinking too carefully.
You are right of course, because with the f16 exposure, the lens gets stopped down, so it has a smaller aperture than at f4. However the point I was trying to make it is the aperture that makes the difference, not the f ratio. A 50mm diameter lens at f4 will show the same limiting magnitude as a 50mm diameter lens at f16 for the same exposure.
Geoff

[robin]

I knew what you were getting at Geoff

[taminga16 (Greg)]

Josh, check Bill Christie's web site for details on how to a "barn door" mount for your camera.

www.zodiaclight.com

Regards Greg.

[Satchmo]

Originally Posted by ghsmith45

A 50mm diameter lens at f4 will show the same limiting magnitude as a 50mm diameter lens at f16 for the same exposure.
Geoff

Its a while since I did astrophotography, but I do remeber that you have to make a distinction between imaging extended sources ( ie nebulae ) and point sources ( ie stars ). From memory, the limiting stellar magnitude reached in a given exposure is only a function of the F ratio and has nothing to do with aperture, whereas imaging extended sources follow the inverse square law . Half the F number records four times as fast. Someone correct me if I'm wrong.

[Ingo]

F Number in lenses will limit light also? If you're imaging Jupiter widefield with Antares with F1.8...you'll get a ton of light. If you then bring it down to F16, you might only get a faint jupiter & a non-visible antares.

F Number effects the amount of light that hits the sensor.