B/W webcam in raw mode

Influence of the acquisition rate on the image quality

Introduction :
A 640 x 480 pixel frame from a webcam takes 640 x 480 x 3 / 1024 = 900 ko = 0.879 Mo. The data flow from a webcam to a PC is limited by the USB 1.1 connection (1.5 Mo/s). This limitation implies some compression of data when "high" acquisition speed is used. This compression means a reduction in the image quality which becomes more noticable when a webcam is used in a raw mode. The following tests show these effects on an indoor target.

Data acquisition system

Optics :
The following images were taken at the prime focus of a Fluorite 55 mm F/8 refractor (same focus and amount of light for all images).

Webcam :
Philips Toucam with BW detector - raw acquisition mode
Settings : Gain  = 50% - Luminosity = 60% - Gamma = 50% - 1/50s exposure

First set of comparison with the some number of frames

Each picture is the addition of 100 frames
No additional processing is done

5 frames per second x 20 s = 100 frames
10 frames per second x 10 s  = 100 frames
20 frames per second x 5 s  = 100 frames
 

Comments :
Switching from 5 fps to10 fps results in a slight decrease of resolution, while the reduction of resolution becomes dramatic at 20 fps.

 Second set of comparison with the some duration of acquisition

This comparition is more relevant to actual astronomical observations where the duration of acquisition is usually limited by the target (rotation of planets).

Each picture is the addition of the frames acquired in a 5 s time lapse
No additional processing is done

5 frames per second x 5 s = 25 frames
10 frames per second x 5 s = 50 frames

Comments :
A 5 s time lapse at 10 fps gives a picture a bit less noisy than a 5s time lapse at 5 fps, but there again, the resolution achieved at 5 fps is definitely greater in spite of the two fold reduction in the number of  acquired individual frames.

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