The following section walks you through the basics of taking basic exposures in black and white (the next section covers color). For specific steps with certain software packages, visit the Software Instructions section.
Short Shots – The First Step
The easiest way to get a picture from your CCD camera is simply to take a relatively short exposure in black and white, just to see what you can get. Start by aiming to a relatively bright object and frame the object as desired using quick exposures (1 second or so) in the focus mode.
The longer the exposure, the more detail you will see. However, there are certain limitations to how long an exposure you can get. Many CCD images are unguided, and since this is the Basics section we will not deal with guided images here (visit the Advanced Imaging section for details on guiding). With unguided exposures you will be limited to about 30-90 seconds depending on the accuracy of your telescope mount. Simply start by taking a 30 second exposure to see what you get.
Above: A 30-second, unguided exposure of M20, the Trifid Nebula.
Now, you will probably want to determine how long an unguided exposure you can take. Begin by taking longer and longer shots until you start to see trailed stars. Elongated star images indicate that you have exceeded you mount’s capability to track precisely. Don’t worry, in the next section we will see a technique for overcoming this limitation. (Taking guided CCD exposures will also eliminate tracking errors, but this is a more advanced technique covered later.)
Above: A sequence of increasing exposures of NGC 2467 at 30, 60, 90, and 120 seconds. Trailing is evident in the 90-second shot, at least in the original full-size image. Even in this reduced size web image, though, definite trailing can be seen at 120 seconds.
From this quick test you can determine the proper exposure time to use for unguided exposures. In the case of the setup used here, 60-second exposures are appropriate. By combining multiple short images, an effect similar to a single longer exposure can be achieved.
Above: A combination of six 30-second exposures of M17, the Swan Nebula.
Many CCD cameras will require dark frames. CCD cameras generate electronic noise from heat within the camera (called dark current). For this reason, most CCDs are cooled, which minimizes this effect, but does not eliminate it. The resulting noise needs to be removed and this is done with a dark frame. A normal exposure detects both the dark noise and the object being imaged. A dark frame is taken with the shutter closed and detects only the dark noise. This is then subtracted from the normal exposure to remove the noise and leave only the subject.
Note: Many newer cameras have very low inherent noise levels, and may not require dark frames.
Above: M33 before dark subtraction and an equivalent exposure dark frame.
Above: M33 after dark subtraction.
For starters, most CCD imaging software will allow automatic dark frame subtraction. This method takes a dark frame and automatically subtracts this from the normal image. When just starting out, this technique is ideal.
Later, you will want to take images without automatic dark frames and just take separate dark frames which will be removed later in the image processing stage.
Taking Dark Frames
Remember, dark frames must be equivalent to the regular light frames in every respect. The exposure length, temperature, and binning must all be equal. The noise characteristics of a camera are dependent on exposure length, temperature (colder = less noise), and pixel binning. Changing these parameters between lights and darks will cause a mismatch between images and the noise removal will not be ideal.
Dark frames must be the same exposure time as the main image. Also, the camera temperature must be the same. The usual technique is to capture all your images and only take dark frames at the end of the night. Since darks do not require the telescope, many imagers take darks at the end of the night while they disassemble their telescope and begin packing up.