Thread: Stacking
View Single Post
  #9  
Old 20-08-2008, 09:33 AM
jase (Jason)
Registered User

jase is offline
 
Join Date: Sep 2006
Location: Melbourne, Victoria
Posts: 3,916
How much time do you have Jeff?

In a summarised form…(there is a lot of theory and math behind this)

The basic concept is to calculate the best “mean” from pixel values of a collection of images you are combining. Algorithms, such as Sigma Reject use a standard deviation or sigma value which is based on variance. It describes the spread of values around the mean. In order to calculate the likely value of the mean in the real world (that introduces non-statistical errors), the user specifies a deviation from the mean value they deem acceptable. Any values outside this range are ignored (rejected). It is these pixels that are outside the range which are known as outlier pixels. Outlier pixels can have a profound affect on the calculation of the mean value. In short, choose a sigma or deviation value that rejects the fewest pixels, but still removes satellite trails, hot pixels, cosmic ray hits, etc. It’s best to choose a large sigma value however, if it’s too large it will not reject any values, thus you’ll simply be left with the mean pixel values. Too small a value will reject more – in other words the threshold defined to classify outlier pixels extends into the real data – not good.

Happy to provide further clarity as required. I should note that Median works is a subtly different way...and for data rejection algorithms to work effectively, the data set you are combining needs be normalised (i.e. stretched to the similar value across the data set). You specifically don't stretch it yourself, the algorithm does this for you. Actually stretching is the wrong terminology, its uses pixel math to achieve the task. It would be difficult to calculate the mean value if this was not performed.
Reply With Quote