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Computerized telescopes are becoming increasingly popular. These
telescopes make finding celestial objects extremely easy. However, there
are still many non-computerized scopes out there, and even for users of goto
scopes there are some tricks necessary for getting the exact picture you want.
Finding Objects Manually
Finding targets for CCD imaging is very much like hunting down objects
visually. The only exception is that looking through an eyepiece is not
always an option when you are imaging. One solution is to use a
flip-mirror finder. This is a device which allows you to look through an
eyepiece to center an object, then flip a mirror out of the way to allow the
light from the telescope to fall onto the CCD chip.
Above: The layout of a typical
flip-mirror finder.
Using a flip-mirror finder can help for some objects, but not all. The
Horsehead Nebula, for example, is not visible through an eyepiece is most
telescopes. Centering an invisible object in the eyepiece is not an option.
Probably the easiest way to find objects in the field of view is to use the
CCD camera itself. Many deep-sky objects are visible in short exposures
(less than about 3 seconds) with a typical CCD camera. Even the Horsehead
can be seen faintly with a 5 to 10 second exposure.
Once you have located the right part of the sky using your favorite hunting
method (star-hopping, setting circles, etc.), you can begin a continuous
sequence of images, using the camera's focus mode, to display what the camera is
seeing. This will allow you to easily locate your target.
Tip: Use low-resolution mode
(binning) to decrease exposure time and increase sensitivity.
Framing the Shot
Once you have found your target (or your scope has found it for you), there
may still be some adjustment necessary. What you see in your brief
"finder" exposures may only be the brightest part of an object.
The core of the Orion Nebula, for example, appears easily in a 1-second exposure
with most CCDs. If you center the core and take a longer exposure, you
will notice that most of the nebulosity extends to the south of the brightest
portion instead of being evenly distributed around it. In this case you
should move the bright core toward the top of the field of view. For many
objects this is a trial and error situation.
Above: The Orion Nebula imaged with the brightest portion
centered.
Above: The Orion Nebula re-centered to frame the entire
object.
Another common situation arises when trying to center very faint
objects. Let's take the Horsehead Nebula as an example again.
Suppose that you need a 15-second exposure with your CCD and telescope in order
to see the Horsehead at all. Trying to center the object can be
frustrating if you have to wait 15 seconds for each exposure (plus a few seconds
for download time).
Instead, try taking one longer (30-second, or so) exposure and note the positions of
bright stars in the field. Then, take a sequence of shorter exposures
(say, 1 second) which will capture only the bright stars. If you know that
the Horsehead appears to the right of one bright star and below another, you can
center that location in the field of view. Once you take a longer exposure
you should see the object properly centered.
Above: Bright stars near the Horsehead Nebula can be used to
center the object in a shorter exposure.
Above: Note the positions of the bright stars. Even
though the nebula cannot be seen now, we know where it is based on the longer
exposure and can move it wherever we desire based on the bright guide stars.
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