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I have a question about the cross-sectional profile of neon calibration lines in my LISA spectrograph. My camera is a SX-H694 which is essentially the same as the ATIK 460. I have focused carefully to get minimum line width.

When I look at the profile of the 5945A line using the Line PSF command in ISIS, I get the profile shown (below left) which has a significant tail towards shorter wavelength. By comparison, the profile sent to me by Francois Teyssier from his LISA is very symmetrical (below right).
Why do I see this tail and Francois doesn't?

David

Hi David,

I am not sure if or how these would apply to the LISA specificially but more generally,

Coma? (at the edges of the field due to aberrations in the optics or across the field due to misalignment somewhere) Is this just at the edges or all the way across? Is it always in the same direction? What happens as you adjust the spectrograph focus ?

misalignment of the neon relative to the slit, illuminating it at an angle? (Are your neon wavelengths offset relative to where you would expect them top be? Do you see the same effect with eg lines from planetary nebulae ?)

Robin

Hi Robin,

The 5945A neon line is more or less in the middle of the field. The length of the "tail" is the same everywhere along the length of this neon line. Towards the red end of the neon spectrum the effect is less noticeable because the FWHM of the lines is larger there. So this suggests the tail is probably of constant size in wavelength rather than dependent on position in the field.

The tail is always towards the blue end of the spectrum.

The tail was there all the time as I focused the spectrograph.

I can't really comment on the illumination angle of the neon source as that is built into the calibration unit which was on the LISA when I bought it and I haven't taken it off.

I don't think the neon lines are offset. When I get a good quality A star spectrum, I get a low rms for wavelength calibration with Balmer + neon lines which indicates to me that the neon lines are where they should be.

Any interesting question about PN spectra. The answer is yes. These lines also show the same asymmetry. So do atmospheric lines.

David

OK David,

Here for comparison is the line profile of the internal neon on my LHIRES (X axis in um, 35um slit, near field centre)
I think would be inclined to suspect something misaligned in the optics then (does it rattle )
Could an incorrectly focussed collimator cause this effect in a classical spectrograph? I know the distortion due to this effect can be seen across the field in a Littrow spectrograph eg
http://www.astrosurf.com/buil/lhires_eval/optique.htm
item 3

Robin

Hi David,

I had the same problem that you have, with my LISA, bought in 2012. In my case the asymmetrical shape was caused by some poor alignment of the 3 lenses in the camera lens barrel. The problem was that the interior diameter of the lens barrel was a few 100th of a millimeter larger than the lens diameter. On the recommendation of Francois Cochard I put some narrow strips of plastic tape in the lens barrel. This centered the lenses and eliminated the coma aberration and the lineshape became symmetrical. I tested also with the 19 micron pinhole to check the optical quality. Both images were taken with the 35 micron slit and a SBIG ST8300 camera with 5.4 micron pixel size.
In case you are not sure about how to fix the lens ask Francois. He was very helpful to resolve the problem.
Before taking the lens apart check the other alignments which can be done according to he LISA manual, in particular the alignment of the collimator lens.

Hope this helps,

Martin

Hi Robin and Martin,

Thanks for your comments. I've now heard from Francois and we are investigating possible ways to fix the problem. I have noticed that the objective lens does rattle in its barrel when the plastic retained screw is loosened. Tightening the screw locks it in position, but possibly not the correct position?

Anyway, It looks like I will soon be investigating the inside of my LISA, an experience I had not anticipated when I bought it.

David