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Astrophotography and Sketching >> DSLR & Digital Camera Astro Imaging & Processing

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jblaschke
super member

Reged: 03/07/08

Loc: New Braunfels, Texas
Focal length star trails formula?
#4600556 - 05/24/11 04:11 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.

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pereloup
member

Reged: 02/10/11

Re: Focal length star trails formula? [Re: jblaschke]
#4601058 - 05/24/11 09:42 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.

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jblaschke
super member

Reged: 03/07/08

Loc: New Braunfels, Texas
Re: Focal length star trails formula? [Re: pereloup]
#4601068 - 05/24/11 09:48 PM

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luigis
sage

Reged: 07/27/10

Loc: Buenos Aires, Argentina
Re: Focal length star trails formula? [Re: jblaschke]
#4601091 - 05/24/11 09:57 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.wilmslowastro.com/software/formulae.htm#StarTrails

A reference:

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Samir Kharusi
Carpal Tunnel

Reged: 06/14/05

Loc: Oman
Re: Focal length star trails formula? [Re: luigis]
#4601444 - 05/25/11 01:59 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.

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faltered
professor emeritus

Reged: 04/01/05

Re: Focal length star trails formula? [Re: Samir Kharusi]
#4602415 - 05/25/11 02:26 PM

I like this formula -

Press Button - If you like what you see - Keep - If not - Try Again.

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