To process a set of images, in general, you do four things:
• calibrate the images by removing hot pixels and the like;
• de-Bayerize (demosaic) the images, converting the Bayer matrix into color pixels;
• combine the images by aligning and stacking them; and
• adjust the brightness, contrast, color, sharpness, and other attributes.
Besides the images of the celestial object, you'll need some calibration frames (Table 12.1). These enable you to measure and counteract the flaws in your camera. The most important part of calibration is subtraction of dark frames to remove hot pixels, and it's the only part I'll describe in this chapter. If you also use bias and flat frames, they enter the software process in much the same way; for more about them, see p. 185.
One important difference between DSLRs and astronomical CCDs is that DSLRs do some calibration of their own. Manufacturers don't say much about it, but apparently, some calibration data are recorded in the camera at the factory, and some measurements (e.g., of bias) are made every time you take a picture. The computer inside the camera performs as much calibration as it can before delivering the "raw" image file to you. Accordingly, in the raw image file, you are seeing not the defects of the sensor per se, but the residual errors after the camera has done its best to correct them.
The whole path to a processed image is shown in Figure 12.1. This is not as complicated as it looks; good software helps you organize the work and consolidates closely related steps.
12.3 Detailed procedure with MaxDSLR Table 12.1 Types of calibration images.
Dark frame An exposure taken with no light reaching the sensor (lens cap on and camera eyepiece covered), at the same ISO setting, exposure time, and camera temperature as the images to be calibrated. Purpose: To correct hot pixels and amp glow.
(Images of celestial objects, as opposed to calibration frames, are sometimes called light frames.)
Bias frame A zero-length or minimum-length exposure, at the same ISO setting and camera temperature as the images to be calibrated.
Purpose: To correct the non-zero offsets of the pixel values, which vary from pixel to pixel. Bias frames are needed if a dark frame is to be scaled to a different exposure time. They are not needed if the dark frames match the exposure time and ISO setting of the images from which they are to be subtracted, since dark frames contain bias information. Flat field An exposure of a blank white surface taken through the same telescope as the astronomical images, with the same camera at the same ISO setting, and preferably on the same occasion so that dust particles will be in the same positions. Ideally, the flat-field image should be dark-frame adjusted, so take a dark frame to match it.
Purpose: To correct for dust, vignetting, and unequal light sensitivity of the pixels. Often unnecessary if your sensor is clean and vignetting is not a serious problem, or if you plan to correct vignetting in other ways.
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