Wavelength Shift with Lhires 2400

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Wavelength Shift with Lhires 2400

Postby Albert Stiewing » Tue Nov 27, 2018 6:01 pm

Before Submitting data to the Be database I check and adjust the wavelength using water as my benchmark, ensuring that the Ha region is spot on. I have noticed that at both ends of the spectrum a shift remains. I at first thought maybe the pixel size in ISIS might need adjusting, but have not seen a noticeable difference in my results. Playing with the slant seems to make a difference. I checked the slant on the three calibration line and find that the slant is different on all three. I use the middle neon line for my calibration as it is closest to the Ha.Is it normal to have three different slant angles? If not what should I be looking at to correct it. Attached is a recent Be submittal with the difference show.
agabclcadifnfhmd.png
agabclcadifnfhmd.png (8.33 KiB) Viewed 481 times
Albert Stiewing
 
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Re: Wavelength Shift with Lhires 2400

Postby Robin Leadbeater » Wed Nov 28, 2018 3:00 pm

Hello Albert,

I think it is normal that the slant angle varies slightly across the field. Mine changes by approximately 0.2 deg and you can see the same size variation in this image from Christian Buil's LHIRES tutorial here.
http://www.astrosurf.com/buil/isis/guid ... /fig29.png

In theory, this should not affect the calibration though as ISIS should measure the position of the lamp lines at the same Y position as the star spectrum. You can check the calibration is working correctly by processing a lamp spectrum like a star (with "sky not removed" ticked) and measuring the wavelength of the lines

I checked some of my recent LHIRES H alpha spectra and I see individual telluric line errors up to 0.1A but nothing obviously systematic. I think the errors depend on the SNR of the line. (Weak telluric lines in a noisy spectrum are hard to measure accurately) The BeSS validators (for example Valerie Desnoux, Olivier Thizy) would perhaps have a better idea of the expected accuracy of the telluric lines.

Cheers
Robin
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