CCD Imaging Primer
	Imaging Made Easy and Fun!

Image calibration can be a hobby in itself (a rather boring one if you ask me!). My advice to beginning CCD imagers (at least if you are using a Starlight Xpress CCD camera) is to ignore image calibration altogether for the first few nights! Of course, the purists will be having a major hissy-fit right about now, but since we're just here to take pretty pictures and have fun so we'll ignore them!

When to Do It, and Why.

Other than dark frames (which I do every night), I have found that image calibration is unnecessary most of the time with the Starlight cameras (even from suburbia). Of course, there are situations where your images may benefit greatly from using flat frames as well as dark frames. The two that I have run into are:

• use of a tele-compressor (which often causes vignetting)
• dust motes on the CCD window (which causes dark doughnuts in the image)

Flat frames will remove these imperfections. It is better though, if your only problem is doughnuts, to clean the camera so that you don't need to do or use flats on a regular basis.

On rare occasions you may encounter a noisy image from your camera (the noise appears as speckles), in which case a dark frame will remove this noise. Dark frames also remove vertical background gradients that sometimes are evident in uncalibrated images. Such a gradient may or may not be obvious on the individual frames, but if you are combining large numbers of frames it will tend to be more obvious in the final image!

How to Do It, Part 1:
Getting the Calibration Frames

When I take dark frames for a night's imaging, I usually take ten of them (or only five if I'm in a hurry) at the beginning of the night. If the temperature changes by more than a four or five Celsius then I will do dark frames once or sometimes twice more during the night. Since the Starlight cameras have no mechanical shutter, you must cap the telescope to take dark frames.

On the rare occasion I do flat frames, I do them at twilight (evening or morning) by pointing the telescope at the zenith. Flat frames produced by this method are called "sky flats". Some people do flats by using a homemade "light box" and pointing the telescope at that. Still other people will evenly illuminate the inside of their dome and point their scope at that. Flat made in this fashion are called "dome flats."

Here are the procedures for taking calibration frames:

• Dark Frames: cap the telescope, take a sequence of exposures (exposure should be the same as your "light" frames)
• Flat Frames: point the uncapped scope at the zenith during twilight (before the stars are out in the evening or as they start to disappear in the morning), set exposure to half a second or so and take a dozen or so frames that have an average brightness level of around half the saturation level for your camera. See the table below for camera-specific information on saturation levels. What time exactly does this happen? The only way to find out is to keep shooting every couple of minutes and watching the levels! Sometimes it's hard to keep stars from showing up in the flats. One way to avoid this is to slew the telescope slowly back and forth across the zenith while you expose. Note that flats should only be done with a cloud-free zenith.
• Bias Frames: cap the telescope, take half a dozen zero second exposures.

The MaxIm "sequence" feature can be helpful for taking multiple calibration frames, especially with the flats, since you can take a whole bunch and throw out the ones you don't like later.

How to Do It, Part 2:
Setting Up the Calibration Frames

Calibration frames should be the same size as your light frames (i.e. pictures). For this reason, I recommend you not square any of your images (or calibration frames) until processing is completely finished.

The first step is to configure your calibration settings. There are two ways to do this (both on the "Process" menu): Set Calibration or Calibration Wizard. Let's do the former. This brings up the dialog shown below.

Assuming your light and dark frame exposures are the same (this is a good practice, I think) you will not need to worry about dark frame scaling. In this case, all you need to do to set up your calibration is click on the "Select Files" buttons and choose one or more bias, dark, and flat frames. Then press the OK button. (That wasn't too bad, was it?)