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# Focal length star trails formula?

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### #1 jblaschke

jblaschke

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Posted 24 May 2011 - 03:11 PM

Folks, what's the formula for determining the length of exposure possible before getting start trails? I know I've seen it before, but I'm coming up empty on my searches. Thanks.

### #2 pereloup

pereloup

Sputnik

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Posted 24 May 2011 - 08:42 PM

If I remember correctly it's 600/(focal length) and that's in seconds of exposure time. So with a 50 mm lens, you should not see star trails under 12 seconds.

### #3 jblaschke

jblaschke

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Posted 24 May 2011 - 08:48 PM

### #4 luigis

luigis

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Posted 24 May 2011 - 08:57 PM

600/focal_length is wrong because it depends on the area of the sky you want to photograph. The poles are not the same as the equator.

The real formula is:

Trail length = F * E *(2Pi/T) * Cos(D)

F = Focal Lenght
E = Exposure time
T = Duration of sidereal time (same unit as E)
D = Declination of the star

Source:
http://www.wilmslowa....htm#StarTrails

A reference:
http://www.astropix....LE7/SAMPLE7.HTM

### #5 Samir Kharusi

Samir Kharusi

Vanguard

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Posted 25 May 2011 - 12:59 AM

All these formulae end up with subexposure times too short to be of much use, because they aim to have trails shorter than a pixel on your multi-megapixel sensor. For practical use one needs to determine what kind of display he wishes to end up with. e.g. for web display a 1200x800 pixel display is very adequate (one megapixel) so trying to have nil trailing on an 18 megapixel sensor is about 4x overkill (square root of 18). My suggestion is to start with 30sec subs using a 14mm lens and halve the subexposure for each doubling in focal length. Works quite satisfyingly for absolute beginners as demonstrated here, nil science required. Even a tripod is an optional, not an essential requirement Of course, if chasing large prints then one has to pay attention to the formulae.

### #6 faltered

faltered

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Posted 25 May 2011 - 01:26 PM

I like this formula -

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