Calibration question

[tloebl]

This is probably normal buy could someone explain why it happens...if I choose the raw images and do manual calibration, the images are indeed calibrated. If, however, I star align the raw images before running manual calibration, the aligned raw images will not calibrate.

[pfile]

the point of calibration is to fix up the per-pixel artifacts that a given sensor has. subtracting bias/darks removes bias/thermal signal. each pixel is slightly different, having different response to thermal signal. if you register a frame, you rotate and translate the pixels around such that a given pixel in the registered sub no longer falls on top of the same pixel in the corresponding calibration frame. further, some interpolation is done which mixes pixels together.

same goes for flats - they correct for minute differences in quantum efficiency and for optical train issues like vignetting. if you shift the pixels in the lights around by registering, they no longer fall on the right pixels of the flat.

rob

[tloebl]

Thanks for that simple but perfectly logical explanation.