Interpolating Without Changing the Mean

QUESTION: When I interpolate an array with, say, Congrid, the mean of the interpolated array is different from the mean of the original array. Is there a function in IDL that can perform the rebinning or interpolation without changing the mean or flux of the result?

ANSWER: Wayne Landsman reports that the FRebin command in the NASA Astronomy Library has been written specifically to perform this function.

Version of IDL used to prepare this article: IDL 8.2.1.