OK you asked about time exposures, long with digital. The answer is, take many images and then use Startrails to blend them. Rather than a terribly long exposure.
http://startrails.de/
Yes, if film, you are correct, one really long exposure.
Yeah, the accumulation of light and finding dark skies, is a real problem.
I realize this is about x-rays, but the same applies to photos of light.
"
The reciprocity law constitutes one of the fundamental rules of photography and of radiography. It states that the quality of a series of photographic or radiographic films will be uniformly constant if the exposure times with which the films are made vary reciprocally with the intensities of the exposing radiation. Thus, the law implies that when other things are equal a roentgenogram exposed for 1.0 second and with a tube-current of 100 milliamperes will be identical to one exposed for 10 seconds and with a tube-current of 10 milliamperes.2
The reciprocity law is based on the assumption that the density or blackening of a photographic film is dependent merely on the exposure or quantity of radiant energy which the film absorbs and is independent of the rate at which the energy is applied (
i.e., is independent of the intensity of the exposing radiation)."
Here's the article about film on Wikipedia, which quotes all kinds of physics and formulas and things, but boiled down to what we would need to know,
"
Reciprocity failure is an important effect in the field of film-based
astrophotography. Deep-sky objects such as galaxies and nebulae are often so faint that they are not visible to the un-aided eye. To make matters worse, many objects' spectra do not line up with the film emulsion's sensitivity curves. Many of these targets are small and require long focal lengths, which can push the focal ratio far above f/5. Combined, these parameters make these targets extremely difficult to capture with film; exposures from 30 minutes to well over an hour are typical. As a typical example, capturing an image of the
Andromeda Galaxy at f/4 will take about 30 minutes; to get the same density at f/8 would require an exposure of about 200 minutes.
When a telescope is tracking an object, every minute is difficult; therefore, reciprocity failure is one of the biggest motivations for astronomers to switch to
digital imaging. Electronic
image sensors have their own limitation at long exposure time and low illuminance levels, not usually referred to as reciprocity failure, namely noise from
dark current, but
this effect can be controlled by cooling the sensor."
Good Luck and happy star shooting.
North Star, always a favorite reference point.
My biggest obstacle seems to be, getting a nice clear sky. Then there's dew and airplanes, and satellites and light pollution. The last is what shows in your film exposure, as it adds up and ruins the image.
Stars are very bright, meteorites are not. I remember years ago, taking all kinds of 10 second exposures, during Perseids and wondering why all I got where stars. Oh there are probably meteorites in there, but the exposure, didn't capture them. That's why I moved up to 3200 or H (on the older cameras) and faster lenses, and 30 second exposures.
Hey thanks for asking, I wonder where those old photo are from those nights in 2010-11 or 12? I didn't know about startrails back then.