There are actually three important calibration routines that all CCD observers should be familiar with.Dark Frames

Every camera has some amount of noise that shows up as dots on the image taken in the dark. To remedy this we take an image with the shutter closed. This "dark frame" is subtracted during the integration process to reduce the noise.

The Handbook of Astronomical Image Processing recommends the "Image-Times-Five" rule. The more dark frames you take, the more accurate the frame and the lower the noise. For a sample of 100 electrons, the uncertainty is 10%, for 10,000 it is 1%. A good rule of thumb is to make sure the total exposure time of all your dark frames equals five times that of the image you are calibrating. So if you are taking a 2 minute image, you can do five 2-minute darks or ten 1-minute darks.

This is where your bias frame comes in. The dark frame also contains readout noise. Readout noise does not scale over time, so your dark frame right now is unsaleable. That is a pain if you plan to take exposures of different lengths during your observing session because then you must take a ton of dark frames to match the integration time (times five!) of each image. If you could yank the readout noise then the dark frame only consists of thermal noise, which is scalable. So let's do it! All you need to do is subtract the master bias frame from each dark frame you took. It's that simple.
 

Bias Frames

Bias Frames help compensate for read-out noise and interference from the computer. A bias frame is basically an image taken with the shutter disabled. The image will consist only of read-out noise and noise caused by interference of the computer. What a bias frame does is set the zero point of the CCD output and the pixel scales to the same value. This makes the final image more accurate since the zero points are equal and no nonlinear pixel values exist.

Flat Frames

For the absolute highest quality photometry flat frames must be used to calibrate your image. A flat frame compensates for obstructions, reflections, and other problems in the light path. This is the path light travels from the time it enters the telescope to the moment it strikes the CCD chip. Dust on optical surfaces, reflections from baffles or poorly aligned optics, vignetting, and other noise sources can interfere with your final data.

The first thing you have to do is take flat darks. These are dark frames taken to be applied to the flats. So you want to match the integration time with that of your flat, not that of your final image. These darks will be separate from your image calibration darks. Other than that, the procedure is exactly the same as above.

The goal is to take an image of a uniform light source (the "flat" field). So the first thing you need is that uniform light source. This is the most difficult part of taking flats. The good news is that once you find a uniform light source that works you can use it forever. There is no fool-proof method of creating the uniform field. How you do it will likely depend on your physical location, mechanical ability (handyman factor) and your level of patience!

Once you have a uniform field, expose your CCD to about 1/2 of the full well depth of your pixels. Take at least 16 flat field images, this is the lowest number required in order to avoid adding noise to your final calibrated image. For .01 mag accuracy photometry keep your signal to noise ratio to 500:1 or better. The exposure time will differ based on what filter you are using since each will pass a different fraction of the light source.

Every time you change an element in the light path, such as removing a filter, you change the light path so you have to take new flats. So you can't take a flat with a V filter, then take it off and put on an R. When you do that the V filter flats should be discarded. (If you are using a high quality filter wheel then you can do all the flats at once since their orientation in the light path will be the same when the filter wheel changes it.)

Applying Flats

  1. Average all your flats
  2. Average or median combine all your darks made specifically for the flats
  3. Subtract the averaged dark from the averaged flat

What you have left is your master flat. Congratulations! This flat will be good as long as you don't change anything in the optical configuration of your system.

Now you can begin taking data (images). Divide the flat into each image after you have dark subtracted it. Most software programs have a way of automatically doing this. Now your images are fully calibrated.