Methods of Calibration

The purpose of calibration is to remove, subtract, and correct the "instrument signature" added to a raw image by the CCD; that is, to peel away multiple onionlike layers of unwanted signal wrapped around the signal that you want. How you carry out calibration depends on what information you want from the final image. As an observer, you must decide what end product you want before the observing session begins, and then take the necessary support frames. Here is a summary of the output from each calibration type and the support frames required:

• Basic Calibration: Pixel value proportional to the number of photo electrons created in the CCD. Requires a master dark frame made with the same integration time as the raw image.

• Standard Calibration: Pixel value proportional to the number of photons coming from the astronomical target. Requires a master dark frame made with the same integration time as the raw image, and a master flat frame.

• Advanced Calibration: Pixel value proportional to the number of photons coming from the astronomical target. Requires a master flat frame, a master dark made with the same or longer integration time as the raw image, and a master bias frame.

It is tempting for an observer to say, "I want nothing but the best. I will only calibrate images using the advanced protocol," without considering how the images will be used. The goal of imaging is, after all, to discover something about the universe—perhaps scientific or perhaps aesthetic. If the goal of your observing program is producing supernova search images, basic calibration does a fine job. Standard calibration corrects for vignetting and CCD nonuniformity, but constrains you to the same integration time for your images and darks. Advanced calibration gives you the freedom to use different integration times for your images, and provides superior dark subtraction via dark-frame matching. When you plan your observations, select a calibration protocol appropriate for your imaging goal.

6.3.1 Basic Image Calibration

The basic calibration protocol approaches a raw image as the sum of some number of photoelectrons plus a dark frame:

Raw Image Dark Subtracted Image

Raw Image Dark Subtracted Image

Dark Frames time as the raw images.

Figure 6.18 Basic calibration requires at least one, and preferably many, dark frames combined into a master dark frame. This protocol removes the bias level and dark current, but does not correct vignetting or CCD sensitivity variations. Basic calibration is adequate for simple observing tasks.

Dark Frames time as the raw images.

Figure 6.18 Basic calibration requires at least one, and preferably many, dark frames combined into a master dark frame. This protocol removes the bias level and dark current, but does not correct vignetting or CCD sensitivity variations. Basic calibration is adequate for simple observing tasks.

The goal of basic calibration is to extract, from the raw image, pixel values proportional to the number of photoelectrons generated on the CCD,

It does not correct the nonuniformity of the CCD or remove vignetting or shadowing.

It consists of subtracting a dark frame identical to the dark current information contained in the raw image; that is, a dark frame with the same integration time as the raw image. Although it is possible to perform basic calibration using a single dark frame, it is far better to generate a master dark frame by combining as many dark frames as you can conveniently shoot.

• Tip: In AIP4Win you can make a master dark frame by averaging or taking the median of many individual dark frames. Averaging multiple dark frames reduces dark noise by the square root of the number of frames combined. Taking the median eliminates cosmic ray events in the master dark, but results in a slightly noisier master dark frame.

If your raw images were made with a CCD camera without active temperature control, the dark current and bias level embedded in the dark frame may change over time. If this is the case, it is best to combine dark frames taken before and after the raw images when you use the basic calibration protocol.

If your raw images were made with a temperature-controlled CCD camera with bias drift subtraction, you can probably use dark frames shot several hours before or after your raw images were taken. Temperature control keeps the dark current constant, and bias drift subtraction determines the true bias during each readout and keeps the bias in the images you shoot at a constant value.

6.3.2 Standard Image Calibration

The standard calibration protocol treats the raw image as a photon flux, Ix^y, modified by quantum efficiency variations, Qx>y, and vignetting, Vx>y, plus a dark frame:

<RAW>^ = -g{tVx^yQx yIx,y) + (DARK)^ . (Equ. 6.18)

The goal of standard calibration is to extract the term:

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