Guide to the maximum exposure time to avoid star trails

[weixing]

Hi,
Seem like quite a number of people like to "shoot stars". 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

Enjoy "stars shooting" and have a nice day.

[Francis247]

Hi Weixing,

This is very good and informative. Thanks for sharing.
I will make this as a Sticky Thread.
Thank you very much.

[Redsun]

very good info
but but
for a 1st timer im not really sure what Declination is referring to?

[psychobiologist]

Originally Posted by Redsun very good info but but for a 1st timer im not really sure what Declination is referring to?

vertically above you is 0 degree, declination = angle from the direct vertical above you

[weixing]

Hi,

Originally Posted by Redsun very good info but but for a 1st timer im not really sure what Declination is referring to?

In Astronomy, the coordinate system use to specify the locations of stars and astronomical object on the sky is call Equatorial Coordinates System. The Equator of the sky is called Celestial Equator, which is projected from Earth's Equator to the sky. The Earth's North and South poles is projected on the sky as North Celestial Pole and South Celestial Pole.

Equatorial Coordinates use two values to specify the location of an astronomical object on the sky called Declination (Dec) and Right Ascension (RA). Declination is like latitude on Earth and Right Ascension is like longitude on Earth.

Declination from Celestial Equator (0 degree) to North Celestial Pole (+90 degree) is positive value and from Celestial Equator (0 degree) to South Celestial Pole (-90 degree) is negative value.

By the way, the Declination on the sky directly above you is basically same as your latitude. Our latitude in Singapore is 1 degree N, so the Declination directly above us is +1 degree.

Hope the above helps.

Have a nice day.

[mrwayne]

wow! im impressed. this must be a hell load of research!

sure useful!