Flat-field correction of Pixel vignetting in CMOS webcams

A similar experiment was performed to the one with the Pi Camera in the previous post, imaging distant buildings across the valley to obtain test data. Two CMOS webcams were used: the Sweex ViewPlus WC070 and the Logitech HD C525. Both of these cameras were placed in turn at the Newtonian focus of an f/5, 130mm Newtonian. The data were captured with AstroDMx Capture for Linux.Stacking was done with lxnstack.
Both cameras showed Pixel vignetting when attached to the scope. However, unlike the Raspberry Pi camera module, the vignetting was not a reverse vignetting, as can be seen in the two flat-fields made by stacking 50 flat-field frames.

Sweex camera

Sweex colour flatfield

Sweex mono flatfield obtained by mapping YUV to greyscale

It can be seen that the vignetting is classical in that the centre of the image is bright whilst the outer regions are darker; unlike the Raspberry Pi camera module which showed reversed vignetting.

Uncorrected colour image (stack of 50 frames)

Corrected colour image

Uncorrected monochrome image

Corrected monochrome image

Logitech camera

Logitech colour flatfield

Uncorrected image

Corrected image

Tests of lunar and planetary images will be carried out when data can be collected. Meanwhile, simple flat-field correction looks very promising for the use of these CMOS, HD web cameras with small sensors, as solar system imagers.

The difference between the nature of the vignetting produced by a Raspberry Pi camera module and a different webcam such as the Sweex and Logitech devices tested here, when placed at the focus of a telescope is interesting. It can be speculated that in addition to microlens displacement, the Pi camera module has a processing correction in firmware. Either way, the flat-field correction makes these cameras usable for Lunar imaging, and possibly planetary and solar imaging.

AstroDMx Capture for Linux has real-time flat-field correction, which makes viewing and outreach use of these cameras a possibility.