on 16/07/2003 07:14, Carlos J. Santisteban Salinas at cjss@xxxxxxxxxxxxx
wrote:
> 15 degrees = 1 hour. (something about 10 seconds less, really ;-)
on 16/07/2003 05:10, Gareth.J.Martin at g.j.martin@xxxxxxxxxxxxx wrote:
> As for
> calculating the exposure time from a star trail. If the earth rotates
> 360 degrees in 24 hours (for quick calculations sake we'll ignore that
> it is slightly quicker than this!) then it rotates 15 degrees in 1 hour
> and 0.25 degrees in 1 minute.
Not very precise mesures on a white wall now drawed with some pencil lines
diverging from a loosely defined South pole, indicates exposure time was
0.533333 hours (8 degrees, averageing on 5 measures)
on 16/07/2003 05:10, Gareth.J.Martin at g.j.martin@xxxxxxxxxxxxx wrote:
> A quick formula for calculating the exposure time BEFORE stars trail is
> 500/f (f = focal length of the lens).
500/21mm = 23.80952381 (minutes, I suppoose).
A little more than 1/3 of an hour: .39682539683
Thought it was about an hour. Took this photograph some 5 years ago so this
might count for my estimation of exposure time. Too lazy to keep exact
control, I was just playing. Or, as Matt BenDaniel pointed out some time ago
"perhaps I (Matt) underestimate the power of the OM".
These two exposures were obtained @f8 using a 21/3.5 MC, +2 compensation on
OM2; during a moonless very dark, clear sky with unusually low humidity.
Temperature about 10 Celsius.
Batteries OK before and after the two consecutive exposures of a little more
than half an hour each.
I must say that the sky rendered black, absolutely; and trails on a variety
of colors on Ektachrome100. FWIW, found an earlier (perhaps 7 years earlier)
KR 64 on which slide trails rendered white, but it was a different
composition with shorter exposure time (perhaps 10 minutes using a 28/2.8
@4) and sky rendered dark blue.
Please correct my arithmetics if necessary.
Rgds,
Fernando
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