Image Processing
The QC grayscale, like any other webcam device, is a video camera, and is intended to make repeated short duration exposures. These cameras are optimized for speed. Although not counter to the qualities required for astrophotography, the simple demands placed on the camera allowed the designers to ignore some qualities we desire in a camera. The two biggest problems are:
ADC Resolution
The resolution of the converter in the QC grayscale is 6-bits. The typical astronomical camera has 12-bits to 16-bits of resolution. This tends to limit either the dynamic range, the detail, or both.
Thermal Noise
The camera is not cooled. The dark signal generated by the CCD is typically much greater than the signal for extended exposures.
The existence of these two properties individually could compromise the quality of the final image, but together, they virtually assure failure, unless they are considered during every step. The choice of objects which can be photographed, the settings required, the maximum and minimum exposure times, the method of dark signal subtraction, flat fielding, and stacking are all dependent upon the severity of these factors.
Object Selection
It is natural to think that because of the noise level in longer exposures, bright objects are the best subjects. At the simplest level, this is true. Jupiter is a target which is a favorite for the QC, because it is bright enough to capture in a short exposure. The reason for this is that the entire range of the object exists above the entire range of the camera generated noise. The noise may be ignored, because the settings required to capture Jupiter are such that the noise cannot also be captured. Neither of our problems effect these objects.
Beyond the planets, most bright objects are difficult to photograph. Brighter extended nebulae (M42) are a challenge because they need the highest dynamic range to get the full extent of their nebulosity. With my camera/telescope, the Trapezium begins to bloom in a two second exposure, while it takes at least 14 seconds to capture a reasonable amount of nebulosity. These are the objects which suffer most from the limited range of the ADC. Any of the bright planetary nebulae, such as NGC3242, or NGC7662, are both bright, and extended. To keep from blooming on the bright portions, the extended areas must be left out. The relatively short exposure times required do not allow enough signal accumulation for the extended areas to reach a reasonable level for digitization.
The second category in the list includes faint extended objects, such as M51 and The Horsehead Nebula, which due to their lack of contrast are not subject to blooming under normal conditions, and require long exposures, which they handle very well. These objects suffer most from the excessive thermal noise of the camera.
The third category contains the most difficult objects. These are objects which are dim, extended, high contrast objects. The absolute worst case I have encountered is M31. It has a bright central core, which is easy to get, yet the periphery extends to well below the noise floor of the camera. It requires a mosaic to get its full extent. The frames which comprise the mosaic will vary from a nearly saturated core area, to empty space. The exposure settings must be selected to maximize the information in the image. Does this mean setting them on the core, or setting them on the periphery? This type of object is like M42, but exists in the same realm as the thermal noise.
Now that we've covered the major problems, let's look at some solutions. There are several topics to be covered: