SharpCap Forums

Hi Robin, a discussion has arisen in our amateur astronomy group about how and in what order calculations are performed in the live stack.

From a previous post we found this list of operations:
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The procedure for sigma clipping is as follows:
* RAW frame gets debayered
* dark/flat correction applied (if selected)
* RAW frame is aligned with existing stack (may involve fractional pixel offset and/or rotation)
* Pixels in the aligned frame are checked against the existing stack statistics and added if they lie within the sigma limit (otherwise ignored).
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It would be useful for us to understand the new interesting functions in the "Colour Noise Reduction" enhancement tab when it is applied, after completing the process described above or on each individual frame.

Thanks in advance for your availability.

Hi Carlo,

that list is close, but actually dark frame correction happens before debayering - it's much more powerful that way as once the image is debayered values from one pixel are mixed into the values from neighbouring ones. Similarly, hot pixel correction and most of the other things in the 'Processing' controls on the right happen before debayering.

Once that type of dark/flat/hot pixel processing is complete, the frame is debayered to full colour, aligned with the stack and then the sigma clipping stacking will check the pixel values fall within an appropriate range of the average for the same pixel in the stack before adding them.

All the things that you play with in the live stacking panel (histogram stretch, colour balance, enhancements like sharpening or noise reduction) are applied as the data comes out of the stack and is converted to an image ready to display - they don't change the actual stacked data at all.

The colour noise reduction is remarkably simple - transform the RGB image to be displayed into YCrCb and then smooth the Cr and Cb channels (which contain the colour info) but not the Y channel which is the luminance. Then go back to RGB.

cheers,

Robin

Hi Robin, thank you so much for your detailed response.
Congratulations for your great work on SharpCap.

Carlo