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.
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.
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.