The Pi-computer-camera running Rasbian Linux was placed at the Newtonian focus of an f/5, 130mm Newtonian and a neutral density filter was attached to the adapter.
Python software was written to display and capture bmp images
Two overlapping sets of 50 bmp images were captured and then stacked, with flatfield correction in lxnstack. They were processed and combined into a 2 pane mosaic of the terminator.
In a separate experiment, a Raspberry Pi 3 running Ubuntu MATE Linux was used with a remounted camera module which was attached to the 130mm Newtonian and a neutral density filter was attached to the adapter.
Using the Python capture software, sets of 150 bmp images were captured of different regions of the Moon. The sets were aligned and stacked with flatfield correction using lxnstack.
Both Pi cameras suffer from Pixel vignetting and they have inbuilt chief ray angle correction. With the lenses intended for these cameras, the chief rays pass at right angles to the pixel in the middle of the sensor, but at more oblique angles and produce lower levels of illumination further away from the middle of the sensor. Moreover, there are chrominance as well as luminance effects at different places on the sensor. The inbuilt chief ray correction in the camera compensates for the expected pixel vignetting. However, if the small lens is removed from the camera and the sensor is placed at the focus of a telescope, there is no need for the inbuilt chief ray correction. But, the correction is still made.
The result of this inbuilt correction is that the centre of the image is dimmed slightly and the outer regions of the image are increased in brightness.
This problem is corrected by flatfield correction. Classical vignetting is corrected by flatfield correction. The auto-corrected pixel vignetting produces an effect opposite to classical vignetting as the two following images show.
Classical vignetting
Flat field from an auto corrected pixel vignetted sensor of a Raspberry Pi camera
Nevertheless, unsurprisingly, the flatfield can correct the telescopic images captured from the Pi cameras as shown by the following animation:
Alternating flatfield corrected and uncorrected images using the same data captured from the Raspberry Pi camera. Unsaturated images were captured in order to avoid the chrominance effects of the chief ray correction, making the camera effectively monochrome.
The problem of auto corrected pixel vignetting seems to be a problem of other webcams with very small, but high resolution sensors. Future experiments will involve using only luminance data and flatfields from these devices to see if they can be useful imagers.
