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.