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I've noticed that the height of the rectangle drawn on the calibration line has a major effect on the RMS calculation. While where and how the rectangle is drawn can result in a slight change in the tilt calculation it appears to have no major effect on the RMS. The attached chart shows the results of some of my experiments. I have seen examples of both nearly full length and short rectangles. Which is the preferred method and if the longer rectangle, do I need to adjust my pixel size to get an acceptable RMS?

I'm happy to report that I have finally gotten over the hump in processing spectra in ISIS. Thanks to Robin Leadbeater for getting me started and recommending this forum to ask for assistance for solving the inconsistent and high RMS in angstroms I was receiving that varied with the the size of the rectangle drawn to compute the smile. A special thanks to Robert Buchheim, a fellow SAS member, having read my post, contacted me off line and worked long and hard with me to identify and solve the problem.

The cause of the problem is the weak emission lines in the blue end of the calibration lamp spectrum leaving ISIS unable to successfully identify the emission lines. By using method 3 (a mixed method using a star and the ArNe lamp) in the "Summary of spectral calibrations methods applicable to the Alpy 600 spectrograph" and the calibration assistant on the calibration page I not only achieved success in processing but an improved blue blue accuracy.

Albert Stiewing wrote:The cause of the problem is the weak emission lines in the blue end of the calibration lamp spectrum leaving ISIS unable to successfully identify the emission lines. By using method 3 (a mixed method using a star and the ArNe lamp) in the "Summary of spectral calibrations methods applicable to the Alpy 600 spectrograph" and the calibration assistant on the calibration page I not only achieved success in processing but an improved blue blue accuracy.

Thanks for letting us know Albert.