A How To Guide to Astrophotography

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Exposure Time:

    Several factors determine what exposure time to use. Main among them are film speed, the
F-Ratio of your camera lens or telescope system and the object of your photograph. As a rule of
thumb remember that when taking a photograph at high magnifications you will be lengthening
your exposure time as compared to a lower power.

    Below is a table to get you started with the general amount of time required to photograph objects.


Subject
Camera Lens
Or Telescope

Exposure Ti me
The Sun*
Both
1/1000 to 1/30 sec
The Moon
Both
1/1000 to 1/125 sec
Planets
Telescope
1/2 to 15 sec
Stars
Both
1 min to 1 hr
Comets
Both
1 min to 1 hr
Star Clusters Etc...
Both
5 min or more

    * Never Look Directly At The Sun Without Adequate Protection!
        Use a Primary Filter or Solar Projection.

    I have compiled a table of exposure times for normal non
filtered general astrophotography into a .pdf document to help you get started.

    When in doubt of what the best exposure time is, Bracket your exposures. Take a exposure slightly
quicker than the time that you want, a normal exposure, and one slightly longer than what you want.
Look at what other people have done to get the photograph of what you are trying to get. Keep a
record of your exposures with the date and time, subject, film, exposure time, and lens or telescope
information. Including Aperture, F-Ratio, and any equipment used to magnify or adjust the image.

    A really full featured exposure calculator by the Author, Michael A. Covington can be found at:
         www.CovingtonInnovations.com/astro/astrosoft.html

    Two other factors that we will be running into are Sky Fog, and Reciprocity Failure. Reciprocity
Failure being a less of a concern to the beginner, we will look at that later. First some definitions.

Sky Fog
The light that is scattered in the atmosphere from ground Sources, and the Moon.
Sky Fog Limit
The point in time when the Sky Fog equals the background of an image, and starts
to reduce the image's contrast. It is indicated by a washed out or grayed out look.
    The Sky Fog Limit is not a fixed value, and changes from location to location by virtue of the local
light pollution, water vapor in the atmosphere, and the presence of the moon and its phase. The Limit is
also determined by what speed of film you are using and how much of the sky you are taking a picture
of. For example: The Sky Fog limit is reached faster when using a lens with a wide field of view and a
slow film, rather than a zoom lens because the camera is taking a picture of a wider area of sky, letting
in more of the background light than a higher magnification zoom lens.

Reciprocity Failure
The perceived effect that films show during long exposures. In that they lose
the ability to retain an image of a dim object in dim light over time.
    Reciprocity Failure is more a technical issue, in that with a long exposure of faint objects, the film
loses it's ability to hold an image. Resulting in the need to extend the exposure time that is required to
achieve a equally dense image of a object that is twice as bright, but was exposed for half the time.
What this means is that the usual working of exposure and time is not tied together as we might think,
but is really a curve based on time. For short exposures with bright objects in the normal working
range of film the curve is almost flat. When we start making longer exposures of dim objects, the
line of the curve starts to move upward as we need more time to achieve an equal image of a
brighter object. Films Reciprocity Failure varies from manufacturer to manufacturer and from film
to film. This technical information is available from the manufacturers, and on their web sites.

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