Tue, 27. August 2013
Astrophotography Without a Telescope - 21
Q&A time.
I received a question from a person I have met on Panaramio, someone who takes great photographs and dabbles a bit in nightscapes. He asked:
I love night photography but find the black art of getting the right result frustrating...
If my ISO is a low number I get less noise - but need to open the shutter for longer. Corespondingly a larger ISO means less open shutter (and less star movement) but more noise. What is the magic happy zone with the ISO vis a vie the length of open shutter?. Then again I like to get a bright clear night shot with no star movement, other times Im just snapping shots off because I know there are lots of meteorite tracks....
For my (non-solar system) astro images I generally use ISO of 1600, 2500 or 3200, which is the highest I have personally felt comfortable with. Initial results at 6400 were less than satisfactory (although I may revisit that, now I have more experience). It really depends on your camera. Later models can give less noise at even higher ISO levels, or so I am told. My 60D gives far less noise than my older 300D.
To reduce star trailing, you must:
(a) shoot with a wide angle lens, with the lowest f-stop, which might get satisfactory results of around thirty seconds or so; and/or(b) use a sidereal tracker which is polar aligned; and/or(c) take several shorter images and stack them before processing them in Photoshop or other software; and/or(d) use flash or some other brief form of 'floodlighting' to capture the foreground in a long starscape exposure (waving a red torch briefly has worked for me).
It gets trickier. I believe there are also methods of stacking short exposures of the foreground with long exposures of the star field. That's not for me, as I get few opportunities to take fantastic landscape/starscape shots.
I now use a sidereal tracker between the tripod and camera. With good polar alignment I can expose starfields for several minutes without noticeable trailing. This also depends where the camera is aimed. For any given exposure, you will get less star trailing when aiming at the celestial pole than you would if aiming at the celestial equator.
Most cameras seem to expose up to 30 seconds. Beyond that you need an intervalometer attachment.
I also now use a light pollution filter, which may not be necessary in the places you visit.
That was my short answer.
A longer answer might be here at Mike Salway's excellent series of articles on Nightscapes:
http://www.mikesalway.com.au/category/articles-reviews/
Mike is probably the leader in this field in Australia and willingly gives up his secrets. He has had images posted on NASAs Astronomical Photo of the Day (APOD) and National Geographic Magazine.
Mike Salway has twice been guest speaker at the Macarthur Astronomy Forum.
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