| Description: | Making pretty pictures of deep-sky objects is a lot of hard work. The good news is that, nowadays, it is within reach of the (hardworking) amateur astronomer. The flash animation gives an impression of the workflow.
First, 40 light frames are made of M51, with 180 second integration each. They were taken through a luminance (clear) filter with a 40cm F/4.5 telescope. In addition, 10 integrations each were made through red, green and blue filters.
Also 20 bias frames were made, with a minimal integration time and the telescope covered up to capture the offset voltages of all pixels.
Then 20 dark frames were made with the same integration time as the light frames, but with the telescope covered up.
Finally, using an evenly illuminated white screen, 20 flat frames were made for each filter, to capture the vignetting of the optics, dust donuts, and differences in sensitivity between the pixels.
The dark frames are combined to a master darkframe, and subtracted from each light frame. The bias frames are combined, and then subtracted from the flat frames, which are then combined to a master flat frame (for each filter separately).
Next, the dark-subtracted light frames (for each filter separately) are divided by the master flats. The effect of this calibration is clearly visible in the animation.
Stacking images dramatically improves the signal to noise ratio: the improvement is equal to the square root of the total number of images. To this end, the calibrated light frames have to be registered perfectly to each other. A non-linear stretch is used to make all information in the image clearly visible.
The RGB master lightframes are combined to an RGB image, which looks rather pale. Then, by using a process called L-RGB combination, this RGB image is used to add colour to the high-signal Luminance image.
The sharpness of the images comes solely from the lumination information. No noise is added by the RGB image, only the colour. The final image can usually be sharpened a bit using unsharp-masking.
The final result can be viewed at: http://www.astrobin.com/full/35990/D/
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