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You can use this table to determine two things.
1) the size of star trails produced by a motionless or non-tracking camera.
2) the maximum exposure that can be used with a non-tracking camera without producing trailing stars in the image.
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The following table displays the star trail size in mm for lenses of different focal lengths.
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The left hand column displays the focal length of the lens being used.
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The column headings indicate the length of exposure in seconds in bold
and the amount in arc seconds that the earth has rotated in that period
of time.
The star trail size is given for an angular position of 90 degrees from
the pole. If you are imaging closer to the pole, multiply the figure by
the sine of the angular distance from the pole. These figures do not
take into account any atmospheric perturbations caused near the horizon.
| focal length |
1 sec
(15 arcsec) |
2 sec
(30 arcsec) |
3 sec
(45 arcsec) |
4 sec
(60 arcsec) |
5 sec
(75 arcsec) |
8 sec
(120 arcsec) |
10 sec
(150 arcsec) |
15 sec
(225 arcsec) |
20 sec
(300 arcsec) |
30 sec
(450 arcsec) |
| 15 mm |
0.065 |
0.131 |
0.196 |
0.262 |
0.327 |
0.524 |
0.655 |
0.982 |
1.310 |
1.966 |
| 20 mm |
0.087 |
0.175 |
0.262 |
0.349 |
0.436 |
0.698 |
0.873 |
1.309 |
1.746 |
2.622 |
| 24 mm |
0.105 |
0.209 |
0.314 |
0.419 |
0.524 |
0.838 |
1.047 |
1.571 |
2.096 |
3.146 |
| 28 mm |
0.122 |
0.244 |
0.366 |
0.489 |
0.611 |
0.977 |
1.222 |
1.833 |
2.445 |
3.670 |
| 30 mm |
0.131 |
0.262 |
0.393 |
0.524 |
0.654 |
1.047 |
1.309 |
1.964 |
2.619 |
3.932 |
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| 35 mm |
0.153 |
0.305 |
0.458 |
0.611 |
0.764 |
1.222 |
1.527 |
2.291 |
3.056 |
4.588 |
| 40 mm |
0.175 |
0.349 |
0.524 |
0.698 |
0.873 |
1.396 |
1.745 |
2.619 |
3.493 |
5.243 |
| 50 mm |
0.218 |
0.436 |
0.654 |
0.873 |
1.091 |
1.745 |
2.182 |
3.273 |
4.366 |
6.554 |
| 55 mm |
0.240 |
0.480 |
0.720 |
0.960 |
1.200 |
1.920 |
2.400 |
3.601 |
4.802 |
7.209 |
| 60 mm |
0.262 |
0.524 |
0.785 |
1.047 |
1.309 |
2.094 |
2.618 |
3.928 |
5.239 |
7.865 |
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| 70 mm |
0.305 |
0.611 |
0.916 |
1.222 |
1.527 |
2.444 |
3.055 |
4.583 |
6.112 |
9.175 |
| 75 mm |
0.327 |
0.654 |
0.982 |
1.309 |
1.636 |
2.618 |
3.273 |
4.910 |
6.549 |
9.831 |
| 80 mm |
0.349 |
0.698 |
1.047 |
1.396 |
1.745 |
2.793 |
3.491 |
5.237 |
6.985 |
10.486 |
| 90 mm |
0.393 |
0.785 |
1.178 |
1.571 |
1.963 |
3.142 |
3.927 |
5.892 |
7.858 |
11.797 |
| 100 mm |
0.436 |
0.873 |
1.309 |
1.745 |
2.182 |
3.491 |
4.364 |
6.547 |
8.732 |
13.108 |
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| 120 mm |
0.524 |
1.047 |
1.571 |
2.094 |
2.618 |
4.189 |
5.236 |
7.856 |
10.478 |
15.729 |
| 150 mm |
0.654 |
1.309 |
1.963 |
2.618 |
3.272 |
5.236 |
6.546 |
9.820 |
13.097 |
19.662 |
| 180 mm |
0.785 |
1.571 |
2.356 |
3.141 |
3.927 |
6.283 |
7.855 |
11.784 |
15.717 |
23.594 |
| 200 mm |
0.873 |
1.745 |
2.618 |
3.490 |
4.363 |
6.982 |
8.727 |
13.094 |
17.463 |
26.215 |
| 250 mm |
1.091 |
2.182 |
3.272 |
4.363 |
5.454 |
8.727 |
10.909 |
16.367 |
21.829 |
32.769 |
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| 300 mm |
1.309 |
2.618 |
3.927 |
5.236 |
6.545 |
10.472 |
13.091 |
19.641 |
26.195 |
39.323 |
| 350 mm |
1.527 |
3.054 |
4.581 |
6.108 |
7.636 |
12.218 |
15.273 |
22.914 |
30.560 |
45.877 |
| 400 mm |
1.745 |
3.490 |
5.236 |
6.981 |
8.726 |
13.963 |
17.455 |
26.187 |
34.926 |
52.431 |
| 450 mm |
1.963 |
3.927 |
5.890 |
7.854 |
9.817 |
15.708 |
19.637 |
29.461 |
39.292 |
58.985 |
| 500 mm |
2.182 |
4.363 |
6.545 |
8.726 |
10.908 |
17.454 |
21.818 |
32.734 |
43.658 |
65.539 |
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