|
Available as PDF:  Or
Microsoft Word: 
The Earth's rotation makes the stars
appear to move from east to west in the sky. For a stationary,
unguided camera of focal length F, a star having
declination d, will photograph as a trail of
linear length, L, in an exposure time of t
seconds:
t * F * cos ( d )
L = --------------------
13750
Note that the units of length, L, will be
the same as the units of F.
Shorter exposures will result in shorter
trail lengths. Assuming an 8x enlargement of a print
and "normal" viewing distances, a trail of 0.1 mm or
less will be virtually unnoticeable. The following table
indicates maximum unguided exposures which will meet
this criterion with several common 35 mm camera lenses:
| Lens |
|
Maximum exposure in
seconds at declination |
| Focal Length |
35 mm field |
(degrees + or -) |
| |
|
0° |
30° |
45° |
60° |
| 28 mm |
49 x 73 |
49 |
56 |
70* |
98* |
| 35 mm |
39 x 59 |
39 |
45 |
55 |
78* |
| 50 mm |
25 x 37 |
26 |
31 |
37 |
52 |
| 100 mm |
14 x 21 |
14 |
16 |
20 |
28 |
| 135 mm |
10 x 15 |
10 |
12 |
14 |
20 |
| 200 mm |
7 x 10 |
7 |
8 |
10 |
14 |
| 300 mm |
4.6 x 7 |
4.6 |
5.3 |
6.5 |
9 |
| 400 mm |
3.5 x 5 |
3.5 |
4 |
5 |
7 |
*Note: exposures over 60 seconds with
"fast" lenses (f/2 or better) will show sky fog.
The angle of sky coverage, B,
of any film having linear dimension s used
with a lens of focal length F is given by (s
and F must have the same units):
( S )
B = 2 * arctan |-------|
( 2F )
Instrument Requirements for Various
Objects
| Object Type |
Lens |
Focal Length |
F/ ratio |
Comments |
Star
Trails |
as available |
50mm - 400mm |
f/2.8 - f/8 |
f/4 lens having 40° coverage best. |
| Moon |
telescope |
50" or greater; use amplification |
f/8 - f/80 |
35mm SLR on telescope with projection. |
| Sun |
telescope |
same as Moon |
same as Moon |
Use proper filters to avoid eye and equipment
damage |
| Planets |
telescope |
100" or greater |
f/16 - f/80 |
Projection necessary to enlarge image. |
| Stars (general) |
25mm
diameter minimum |
any, but 100mm - 300mm best |
f/4 - f/8 |
Use medium speed f/ ratio for best definition. |
| Star Clusters |
50mm diameter minimum for open clusters |
400mm or longer |
f/4 - f/16 |
Use longer focal length for smaller clusters |
| Nebulae |
50mm diameter minimum |
150mm lens or short focus telescopes |
f/2 - f/8 |
Need greatest possible light collection. |
| Comets |
25mm diameter minimum |
100mm lens or short focus telescopes |
f/2 - f/5.6 |
Wide angle for tail coverage; long focal length
for coma. |
| Aurora |
25mm diameter minimum, fast |
28mm - 100mm |
f/2 - f/4 wide open |
Wide angle; short exposures to minimize motion. |
| Meteors and Satellites |
as available |
50mm - 100mm |
f/2 - f/4 |
40° - 60° sky coverage |
|
