Hi @keesscherer,
Indeed, the kappa value of 2 is way too high if you look at the rejection map.
The kappa value of 3 is much beter. Very little removal of possible good data and the outliers are gone.
In the next version, after 1. 036.0, you’ll have the option to use kappa values larger than 3.0. I have extended the range to 5.0.
But some clarification is usefull here. From a technical statistics point of view, a kappa value larger than 3 will only be effective if you were to stack more than several 1000s of frames. See also
standard deviation, sigma, 68,95,99,7 rule
Let’s suppose the following, we have 500 light frames, that we are going to stack with an outlier rejection filter with a kappa value of 3. And that we have no outliers at all, because we are interested in how much an outlier rejection filter can harm the good data, which shouldn’t be rejected, in your stack.
The question now is, how much of the good data will be removed by this outlier rejection filter, which would be harmfull for the Signal to Noise ratio of your stack.
The calculation from the 68,95,99,7 rule is:
for kappa = 3, all data (good or bad), less than the median – 3 standard deviations and greater than the median + 3 standard deviatons will be removed. The median is the central value for each pixelstack.
for 3 standard deviations, this means that 99,7 % of all the frames are included and 0,3% are excluded and thus rejected.
0,3% of 500 frames = 1,5 frame. Let say 2 frames out of 500 frames.
So setting the kappa at 3 for a very big stack will hardly have a destructive influence on the SNR ratio of your stack. Which I have actually tested in the past. This testing is the reason why I set the kappa range to 1.0-3.0.
Using kappa values larger than 3.0 will have a higher quarantee that only real outliers are rejected, but also increases the chance that it will be less effective. And kappa 3 already has no sigificant influence on the SNR of your stack. This is less than 1%, not 10%, because the SNR has the following relation to the amount of subs, it scales as the square root of the number of subs. Twice as many subs only increases the SNR by the square root of 2.
Let’s say, the 500 frames integration gives an SNR of 20.
Then the 498 frames stack would give an SNR of square root of (0,997) * 20 = 0,9985 * 20 = 19,97
This is by no means significant. It’s 1 – (19,97/ 20) = 0.0015 or 0,15 %
So feel free to experiment with the different kappa settings in the next version and check the SNR and noise values of your stack (in the fits header). If you see that using higher kappa values than 3 is significantly beneficial for the combination of effective rejection and the SNR of you stack, I would be surprised actually. But that’s always a good thing I guess ?