Mr_P
Mr_A_or_M_Only
Reged: 31/05/2009
Posts: 427
Loc: On my back soaking up the rays...
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Evening All,
I was wondering if someone could give a little technical help....
I have a great shot in mind but need to include the stars. I wondered how long an exposure I would need to be able to show the earths movement...
Any help appreciated
Cheers
Nick 
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beejaybee
Marvin
Reged: 18/07/2007
Posts: 6283
Loc: Really Here In Name Only
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Quote:
how long an exposure I would need to be able to show the earths movement...
You will start to get trailing of the stars near the celestial equator with an exposure time of about 500/F seconds, where F is the focal length in millimetres. So the maximum exposure without trailing for a fixed camera with a 100mm lens would be 500/100 = 5 secs.
Nearer the celestial pole, the movement is less & longer exposures are needed.
These notes are for astrophotographers who want to show stars as points. If you actually want to show trails, I'd start at 5 minutes ... You may find a B+W 491 "Redhancer" filter, or an astronomical broad band anti-pollution filter, useful for reducing the pervasive effects of light pollution. And you'll need to wait for it to go properly dark, in latitude 55N this means until the last week in July as there is perpetual twilight at this time of year.
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Mr_P
Mr_A_or_M_Only
Reged: 31/05/2009
Posts: 427
Loc: On my back soaking up the rays...
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Fantastic, thanks for that!! I figured as much for the exposure time but hadnt heard of these filters.
Once again, many thanks for the advice.
Cheers
Nick 
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tonycro
veteran
Reged: 27/09/2004
Posts: 1274
Loc: UK, Northamptonshire
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for longer trails, use a seris of exposures and merge them together with http://www.startrails.de/html/software.html - freeware
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FRIPN Tony
Learn from the mistakes of others, life isn't nearly long enough to make them all yourself!
tony on flickr 
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