ISIS calibration

Forums Spectroscopy Help with ISIS ISIS calibration

David Boyd

I use the File mode option under Spectral calibration on the General tab in ISIS 5.7.0. I give it a .lst file with a list of the calibration lines to use in the fit. ISIS then does fill in the calibration fit coefficients in the Dispersion window. It also highlights the 3rd order radio button in that window although the fit is 4th order.

One thing I have noticed is that these coefficients do not generate the correct residuals if you use them in a 4th order polynomial with the measured pixel positions for each calibration line listed in the Go window in the Go tab. If on the other hand you make a 4th order polynomial fit to the measured pixel positions and the correct line wavelengths using eg Excel and then use these coefficients to calculate the residuals for each line, you get exactly the residuals listed in the Go window. These coefficients are not the same as the ones listed lower down in the Go window and in the Dispersion window.

I have no idea what is going on there and where the coefficients listed by ISIS come from but they are not the coefficients of the mathematical fit which produces the residuals listed for each line in the Go window. Perhaps this is a consequence of using File mode calibration? I am only mentioning this in case anyone tries to use the coefficients listed by ISIS to reproduce the residuals and finds they get different answers.