Hi, I am moving Matlab code to IDL which uses Butterworth filtering, and see that IDL provides this functionality via its butterworth function (http://www.exelisvis.com/...s/BUTTERWORTH.html). It is hard to tell from the documentation whether this is really the Matlab equivalent (http://www.mathworks.com/...al/ref/butter.html). Furthermore, the filter order and cutoff frequency are calculated with another function in Matlab (http://www.mathworks.com/...l/ref/buttord.html), whereas there seems to be no such equivalent in IDL. I realize it may be too specific to ask here, but perhaps some other Matlab users have looked at this and have some recommendations as to how to achieve these operations in IDL. Thanks in advance, Sebastian