# Interpolating Flux to Regular Wavelengths

QUESTION: I have my data in two columns. The first column is the wavelength in angstroms and the second column is the flux. The data looks like this.

wavelength flux 5201.720215 0.0097 5205.345215 0.0088 5208.970703 0.0080 5212.596191 0.0094 5216.221680 0.0133 5219.846680 0.0167 5223.472168 0.0169 5227.097656 0.0156 5230.722656 0.0145 5234.348145 0.0138 5237.973633 0.0130 5241.599121 0.0118 5245.224121 0.0110 5248.849609 0.0116 5252.475098 0.0134 5256.100586 0.0147 5259.725586 0.0139 5263.351074 0.0126

I want to create a new data set from this data that is sampled or re-binned on 1 angstrom intervals,
with the flux linearly interpolated from the data I have collected. I am confused about
which routines in IDL I should use (*Rebin*, *Congrid*, *Interpolate*, etc.) to do this.
Can you show me how this can be done?

ANSWER: Today's answer is provided by Wox on the IDL newsgroup.

Since your data is already ordered in a monotonically increasing way and is the same data type, we can just create a two-dimensional array of the data.

data = [ [5201.720215d, 0.0097],$ [5205.345215d, 0.0088],$ [5208.970703d, 0.0080],$ [5212.596191d, 0.0094],$ [5216.221680d, 0.0133],$ [5219.846680d, 0.0167],$ [5223.472168d, 0.0169],$ [5227.097656d, 0.0156],$ [5230.722656d, 0.0145],$ [5234.348145d, 0.0138],$ [5237.973633d, 0.0130],$ [5241.599121d, 0.0118],$ [5245.224121d, 0.0110],$ [5248.849609d, 0.0116],$ [5252.475098d, 0.0134],$ [5256.100586d, 0.0147],$ [5259.725586d, 0.0139],$ [5263.351074d, 0.0126] ]

Next, we need to create a vector representing the wavelengths that we want to include in the new data set. In this case, we need a vector going from 5201 to 5264 angstroms.

start_wl = Floor(Min(data[0,*])) end_wl = Ceil(Max(data[0,*])) wl_vector = cgScaleVector(Findgen(end_wl-start_wl+1), start_wl, end_wl)

Next, we simply linearly interpolate the flux at the locations given by this new wavelength vector.
We use the *Interpol *command in IDL to do this.

interp_flux = Interpol(data[1,*], data[0,*], wl_vector)

We can plot the data to see how we have done. The original data is drawn with a blue line. The newly interpolated flux data is overplotted in red symbols. The original data points are overplotted in green symbols.

cgPlot, data[0,*], data[1,*], Color='dodger blue', XStyle=1, Thick=2, /Window cgPlot, wl_vector, interp_flux, Color='red', PSym=16, /Overplot, /Add cgPlot, data[0,*], data[1,*], SymColor='light sea green', PSym=17, SymSize=2, /Overplot, /Add

You see the results in the figure below.

The original and interpolated flux values. |

*Version of IDL used to prepare this article: IDL 8.1.*