Nterms is the order of the DIGITAL_FILTER (the order of the BESSEL functions within this routine to be more precise). The amount of coefficients returned by DIGITAL_FILTER is equal to (2*Nterms + 1). I am afraid I am not an expert at signal processing so I am uncertain about how changing the value for Nterms will affect your result. There is a reference included in the code (you can view the code by entering '.edit digital_filter' into the IDL console) that might be able to provide you with more information:

; DMS, April, 1985.
; Adapted from:
; "Digital Filters", Robert Walraven,
; Proceedings of the Digital Equipment User's Society, Fall, 1984.
; Department of Applied Science,
; University of California, Davis, CA 95616.

David
-HGS