Note that if the bias frame is "clean," and the bias value can be determined accurately, you can perform these steps using the single bias value instead of using a master bias frame. Multiply by the ratio of the image integration time divided by the dark frame integration time to obtain a scaled thermal frame:

Providing the integration time for the dark is longer than the integration time for the image, the thermal noise is reduced by scaling:

The scaled thermal frame contains the same accumulation of dark current that it would have had if the integration time were the same as the raw image.

Raw Image Dark Subtracted Image Calibrated Image

Raw Image Dark Subtracted Image Calibrated Image

Bias Frames

Raw Flat Fields

Master Flat

Flat Darks Figure 6.20

Advanced Calibration

Dark frames need not have the same integration time as the raw images.

Flat darks should have the same integration time as the raw flats.

The advanced calibration protocol allows you to use one set of dark frames for calibrating all of your images, regardless of their integration times. Considering how easy it is, take the necessary bias frames; the added flexibility in integration times is a boon to the busy observer.

The next step is to subtract the scaled thermal frame and the bias frame from the raw image. After the double subtraction, the image consists of:

• ~(tVx,vQx y^x, v) Plus the quadratic sum of the following;

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