I have recently purchased the typical compact CCD fixed-slit fixed-wavelength-range spectrometer, and the emission lines captured from a wavelength-calibration HgAr lamp are VERY assymetrical.
An example of two emission lines can be seen in these images:
http://i63.tinypic.com/14kh9xl.png
http://i64.tinypic.com/174q4x.png
Optical efficiency and FWHM seem to be Ok, but lines are not symmetrical. What could be the reason?
Thankyou in advance!
It looks like coma optical aberration in your Czerny-Turner type spectrometer with spherical mirrors occured due to off-axis optical path. You may try to affect slightly the line shape by turning an input optical fiber through some angle around the axis. If it have no effect - the problem is inside the box. In the last case the best solution could be a request to manufacturer.
Thankyou Mikhail for your answer. In my experience, I had a simmilar effect with another spectrometer when the adjustable slit was off-axis horizontaly. In fact, the manufacturer has replied that asymmetry is not an specification for them and it is not checked during quality control, but they are willing to fixed it.
Thankyou!
The emssion lines of Hg are a complex of lines of different isotopes. For example the Hg line at 253.652 consists of ten lines of Hg isotopes from 198 to 204. I am not sure whether the lines you measured are a mix of varous Hg lines or not, but if they are not the pure lines of single isotop, it may be the reason for the observed asymmetry.
The isotopes suggestion is certainly worth exploring. If the asymmetry is of the same magnitude for all lines along the dispersion axis, then that would rule out line blends from different isotopes, and optical aberration would appear to be the culprit.
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