In astro photography and spectroscopy a flat field is often used to correct for variations of sensitivity over the image sensor. It is applied by dividing the original image by a suitable flat field image, thereby reducing or eliminating these sensitivity errors.
The flat image is obtained with the same optics by taking a short video of a white, evenly illuminated screen and averaging a number of video frames.
For meteor spectra wide angle lenses are used to cover a large area of the sky. Therefore a reduction of sensitivity away from the image centre is quite noticeable and should be corrected for quantitative (response corrected) spectra. In addition, the meteor spectrum is recorded at different positions on the detector due to the movement of the meteor over the sky. On the other hand, pixel to pixel sensitivity variations of modern detectors are quite small and averaged out by the movement of the meteor. They can therefore be ignored for the present analysis.
In addition to these sensitivity variations caused by the optics, the image transformation described in the next section changes the size of pixel areas as a function of radius from the image centre. This change of area ratio is partially included in the flat intensity. An additional correction step in the transformation to the orthographic projection can be included in the calculation of the flat. It takes into account that the orthographic projection changes the pixel area as a function of the offset from the optical axis by a factor cos(rho).
The flat correction is applied by dividing the original meteor images by the flat image (pixel by pixel), before transforming them to the orthographic projection.

The details of the processing are described in the m_spec Python manual
The Python script can be downloaded from
github.com/meteorspectroscopy/meteor-spectrum-calibration

original flat image
subtraction of offset image
final flat after edge exclusion and distortion correction

In addition, I added some buttons for easier use of the script. On the registration page, I added some buttons for fine adjustment of slant and tilt:

Tilt slant arrors for fine adjustment

On the distortion page, I added a “Stop” button, to stop the processing of images. Usually the video file is about a second longer than the actual meteor spectrum. One can stop processing if the remainder of the images is not needed. This saves some time and disk space.

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