Measuring FWHM of neon lines in ISIS

[David Boyd]

I've noticed that the value you get for the FWHM of a neon line using ISIS depends on how you measure it. The FWHM and Line PSF tabs often give different answers, close but different, and it depends on where exactly you draw the rectangle around the line - how long you make it and how wide. Variations of more than 10% are not unusual for bright lines and for faint ones it can be even larger.

When you are focusing the spectroscope and making repeated measurements of the neon line FWHM, this lack of consistency can be confusing.

What is best practice when measuring the FWHM of a neon line? Is it best to use the FWHM tab or the Line PSF tab? And is it better to draw the rectangle short or long and very tight to the line of a bit further away?

Thanks,
David