>> the results of statistic, particulary sigma, of the frame is very different from the three others software: Iris, Pix, Siril as you can see on the joined images. Why ?

I just took a glance in the statistics functions of siril and kstars. The algorithms used seem a bit different to me, however I think using functions like sqrt(), division etc.. are bound to cause inaccuracies with floating point numbers.

This link gives an insight about what those inaccuracies could be. So this inaccuracy in each stage of computation may or may not get exaggerated, and this exaggeration will be different for different algorithms. Sometimes choosing to do some mathematical operations before or after a stage in the same algorithm may work out on paper but not on computers.

I'd love Jasem's take on this too.

>> - the way the square must be set on the image is not very convenient. It's best to draw the frame on the image at the desired location and size. As all the other software do: Pix, Siril, Iris

Drawing the frame is possible with shift+click if thats what you are looking for. However if you want to specify x,y and size for the square thats a new thing

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