Raw Mode

In recent years a technique known as "raw mode" has been popular among webcam planetary imagers who like to push their equipment to the limit. To understand raw mode we need to investigate in more detail how webcams transfer so much data so quickly. Most webcams use the USB 1.1 system to transfer their video streams from the webcam to the PC. USB 1.1 has two data rates, namely, 1.5 megabits/second and 12 megabits/second. The slower rate is reserved for simple hardware like keyboards and mice whereas webcams use the 12 megabit/second rate. Webcams typically have CCD arrays of 640 x 480 pixels, i.e., 307,200 pixels and they use a color-encoding technique called YUV to compress the color data from the filtered pixels. This compression technique works on blocks of 4 x 4 pixels and the letters stand for the following: Y = luminance, U = red-luminance; V = blue-luminance. The YUV technique is very efficient at transmitting acceptable color images of everyday scenes from a webcam to a PC. A quick calculation shows that at 8 bits of data per pixel all of the monochrome luminance information from a 640 x 480 grid could be transmitted at 5 frames per second without any data compression. In fact, with YUV, color images look remarkably uncor-rupted even at 10 frames per second. However, even at 5 frames per second, the YUV compression does take subtle liberties with an image and the data is not as clean as it would be with a monochrome camera. In addition, control of the webcam's gamma response and artificial sharpening algorithms are used to give a more acceptable picture at high data rates for daylight scenes with the webcam lens. However, the planetary imager is not using the webcam in normal mode; he or she wants the most accurate transmission of data from webcam to PC. For many planets, especially Jupiter, the webcam gamma setting is best turned down to a low value and a frame rate of 5 frames per second is best when seeing is excellent. The planetary imager is interested in low noise frames, too. Because of all these considerations, various software "gurus" worked out how to run a routine that would reset the standard domestic webcam to its default factory settings in manual mode with a slow frame rate, low gamma, and sharpening algorithms turned off. The end result is a much smoother raw planetary image but with some additional processing required. In most variants of the raw mode, the webcam images download in monochrome. They are much cleaner images than the color ones, but are, essentially, the raw output with no attempt to reconstitute the color information. Thus, they will be corrupted with the filter grid (Bayer Matrix) pattern. However, your PC can reconstitute the color for each frame (and the thousands of frames in each webcam video) with the right software. Various amateur software enthusiasts offer freeware to download and make the raw mode possible on your PC, but with a disclaimer that the risk of damaging your webcam is yours! (See http://www.astrosurf.com/astrobond/ebrawe.htm) Is raw mode worth it? This is a tricky question. Certainly, for the perfectionist, who has squeezed every last drop out of his or her camera, the raw mode will make a subtle improvement to the highest quality images. But, for the beginner, the extra hassle involved in reconstituting the color image will not make any noticeable difference until all the other issues like collimation, focusing, and prolific observing have been addressed. Many monochrome webcams are, by default, working in raw mode, so if you use these, switching to raw mode is not necessary.

CHAPTER NINE

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