I recently took my first successful star trails photos but I have a very specific question that I hope someone can answer.
I'm happy with the shot below but I'd like the stars to be 'denser' (ie. more stars) but without adding too much noise.
I took about 230 30 second shots, f4 and ISO of 400 and then about 20 dark frames with the same settings, which I stacked using StarStax.
If I was to use a higher ISO with the same f-stop and exposure, plus using about 20 dark frames (or more), would the noise be more noticeable or would the dark frames balance that out?
The first thing I was tempted to say was "Come to Australia"we have all the stars. But I think from your profile that you are in S. Africa so we really don't have that many more. Being a tad more serious you should be able to double your ISO without getting appreciable noise. I'm not talking about a huge difference here. It really depends so much on your camera and what I don't know about Canons would fill volumes.
@swilde thanks Sue, I suppose the one way to find out is to try it...
@blightygal i was thinking that could be one way to go about it, but that'd mean about 5 hours out in the middle of the night in the middle of nowhere, best find my tent...
I always thought a decent star shot (continuous) was about 2 hours? Depending on lens, direction etc. Anyway, 5 hours for a top shot? Worth it!! Grins.
Thats my most recent star trails picture, I've got some advice. Open up that shutter as far as it will go, for these pictures, especially when you're just learning, you shouldn't be concerned with sharpness at all, just get as much light in as possible. Also, stick to as long of exposure time as possible, 30 seconds is good. With your ISO, you should be able to push up to 1600 easily without introducing much noise, you should be able to push past that, even up to the 6400 range if you're subtracting darks.
@semjaja Yeah, as long as you are subtracting the darks, I've seen people add them before, as silly as that seems. I wouldn't be very concerned about the darks as it is though, the noise won't be appreciable until you push up to ISOs past 1600 and even then you should only need a couple darks to do what you need. If you want to be real professional about it, look up flats and biases, I've never used them for wide field astrophotography though, so I'm not sure what effect they would have.
A longer exposure will have no effect on the number of stars visible, because as the Earth rotates, the stars are no longer lighting up the same area of the sensor. Once you are using a 30 second exposure, there's no benefit in going any longer. (The exception would be if you were using a tracking equatorial mount that automatically counters the Earth's rotation).
The only way to increase the number of stars visible is to collect more light during those exposures, which can be done either by using a faster (wider aperture) lens, such as an f/2.8 or f/1.4 lens, or by increasing the ISO, so that the camera is more sensitive.
Increasing the ISO is the 'free' option (doesn't involve buying expensive lenses) but will inevitably end up with more noise. Whether it becomes problematic is really something only you can decide, based on your expectations. Dark frames will help, but they are not a magic cure-all, and as you already used them in your existing shot, you're already seeing the benefit of them.
Here is an example using an f/2.8 lens at ISO 400 (so a stop more light gathering than in your shot):
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@blightygal i was thinking that could be one way to go about it, but that'd mean about 5 hours out in the middle of the night in the middle of nowhere, best find my tent...
http://www.flickr.com/photos/krisohnander/11678558446/
Thats my most recent star trails picture, I've got some advice. Open up that shutter as far as it will go, for these pictures, especially when you're just learning, you shouldn't be concerned with sharpness at all, just get as much light in as possible. Also, stick to as long of exposure time as possible, 30 seconds is good. With your ISO, you should be able to push up to 1600 easily without introducing much noise, you should be able to push past that, even up to the 6400 range if you're subtracting darks.
A longer exposure will have no effect on the number of stars visible, because as the Earth rotates, the stars are no longer lighting up the same area of the sensor. Once you are using a 30 second exposure, there's no benefit in going any longer. (The exception would be if you were using a tracking equatorial mount that automatically counters the Earth's rotation).
The only way to increase the number of stars visible is to collect more light during those exposures, which can be done either by using a faster (wider aperture) lens, such as an f/2.8 or f/1.4 lens, or by increasing the ISO, so that the camera is more sensitive.
Increasing the ISO is the 'free' option (doesn't involve buying expensive lenses) but will inevitably end up with more noise. Whether it becomes problematic is really something only you can decide, based on your expectations. Dark frames will help, but they are not a magic cure-all, and as you already used them in your existing shot, you're already seeing the benefit of them.
Here is an example using an f/2.8 lens at ISO 400 (so a stop more light gathering than in your shot):