Guide to the maximum exposure time to avoid star trails

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weixing
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Guide to the maximum exposure time to avoid star trails

Post by weixing »

Hi,
Below are a guide to the maximum exposure time allowed in order to avoid star trails when using a tripod. To get perfectly sharp stars, use only 1/3 of the maximum exposure time.

When aiming at celestial equator (Declination 0 degree)... basically overhead in Singapore.
Lens Focal Length: Maximum exposure Time
18mm: 55s
24mm: 40s
28mm: 35s
35mm: 30s
50mm: 20s
100mm: 10s
135mm: 7.5s
200mm: 5.0s
300mm: 3.3s
400mm: 2.5s

When aiming at Declination +/- 30 degree... around 60 degree above north/south horizon in Singapore.
Lens Focal Length: Maximum exposure Time
18mm: 65s
24mm: 50s
28mm: 40s
35mm: 33s
50mm: 23s
100mm: 12s
135mm: 8.5s
200mm: 5.5s
300mm: 3.8s
400mm: 3.0s

When aiming at Declination +/- 45 degree... around 45 degree above north/south horizon in Singapore.
Lens Focal Length: Maximum exposure Time
18mm: 80s
24mm: 60s
28mm: 50s
35mm: 40s
50mm: 28s
100mm: 14s
135mm: 11s
200mm: 7s
300mm: 4.7s
400mm: 3.5s

When aiming at Declination +/- 60 degree... around 30 degree above north/south horizon in Singapore.
Lens Focal Length: Maximum exposure Time
18mm: 110s
24mm: 85s
28mm: 75s
35mm: 60s
50mm: 40s
100mm: 20s
135mm: 15s
200mm: 10s
300mm: 6.5s
400mm: 5.0s

When aiming at Declination +/- 75 degree... around 15 degree above north/south horizon in Singapore.
Lens Focal Length: Maximum exposure Time
18mm: 220s
24mm: 160s
28mm: 140s
35mm: 110s
50mm: 75s
100mm: 40s
135mm: 30s
200mm: 20s
300mm: 13s
400mm: 10s

Please note that the above focal length is for 35mm film.

Have a nice day.
Last edited by weixing on Thu Aug 31, 2006 11:09 am, edited 1 time in total.
Yang Weixing
:mrgreen: "The universe is composed mainly of hydrogen and ignorance." :mrgreen:

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ariefm71
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Post by ariefm71 »

Great stuff.
Wxg, are these from your hands-on experience?

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weixing
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Post by weixing »

Hi,
Not from my hand-on experience... I only just started in Astrophotography and didn't had many chance to try wide field Astrophotography. It's taken from some astrophotography book.

Have a nice day.
Yang Weixing
:mrgreen: "The universe is composed mainly of hydrogen and ignorance." :mrgreen:

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cataclysm
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Post by cataclysm »

These are really good information. But does the book mention its for 35mm format camera or digital slr? Croppped factors from smaller dslr sensors will alter the focal length of the lenses.

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weixing
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Favourite scope: Vixen R200SS & Celestron 6" F5 Achro Refractor
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Post by weixing »

Hi,
These are really good information. But does the book mention its for 35mm format camera or digital slr? Croppped factors from smaller dslr sensors will alter the focal length of the lenses.
It's for 35mm, so you have to apply the crop factor. Actually, it didn't alter the lens focal length, it just making the lens angle of view equivalent to that of a longer lens on a 35mm cameras.

Anyway, even you buy lens that design for APS-C digital camera, you still need to apply the crop factor, right?? :wink:

Have a nice day.
Yang Weixing
:mrgreen: "The universe is composed mainly of hydrogen and ignorance." :mrgreen:

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