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When you first start imaging, you will probably want to take pictures of the
objects you are familiar with from visual observing. This will allow you
to see your favorite objects in spectacular new detail. Eventually, you
might also want to image the things you cannot see visually through your
telescope. A perfect example of this is the Horsehead Nebula. A
popular photographic target, the Horsehead is well-known by amateur astronomers,
but it is a very faint object. Seeing it visually with less than a
16" telescope is extremely difficult. However, a short CCD exposure
through even a small telescope will easily capture this elusive subject in
amazing detail.
Size Matters
One of the most obvious considerations when selecting a target is the
apparent size of the object. Most descriptions of a celestial object
(whether in a book, computer program, etc.) give the apparent size of the
object. For the most common deep-sky targets, galaxies, nebulae, and
clusters, this size is almost always given in arcminutes. An arcminute is
1/60 of a degree. For reference, the moon is 1/2 degree (or 30 arcminutes)
across.
Knowing the size of your potential subject, all you need now is the size of
the field of view of your CCD chip.
Use the following calculator to determine the field of view with your
telescope and CCD camera.
Pixel Resolution & Field of View
Calculator
Use this information to determine if your selected target will fit in the
field of view. If a subject is too large, it might be best to avoid
imaging it for now. But don't put it out of mind entirely, because when
you are ready for more advanced techniques, you can make
mosaics
composed of smaller images to capture a large object.
What's Out Tonight?
Once you have some ideas about what you would like to image, you need to know
which targets are available when you will be imaging. The best resource
for this is a planetarium program, such as
Starry
Night or
The Sky. These computer
programs can show you the sky on any given night from your location. If
you plan on imaging from 8:00 PM until 11:00 PM on October 28th, this type of
software will allow you to find out what will be visible.
Other important considerations concerning an object's location in the sky
include light pollution, obstructions, and atmospheric conditions. If you
live on the outskirts of a city, you may have dark skies in one direction, but
not another. Determining if an object will be in the direction of the
light pollution at the time you will be out is critical. The position of
an object in the sky can also indicate whether it will be hidden by an
obstruction such as that big tree next door! An object's position at a
given time also can determine how sharp the image will be. If an object is
low in the sky, you must look through much more atmosphere than if it is high in
the sky. Sometimes waiting a few hours for a target to climb higher in the
sky can increase your chances of a good shot.
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