Telluric line inaccuracy

Telluric line inaccuracy

Postby Terry Bohlsen » Mon Jul 16, 2018 11:43 am

Hi All
I have come across an interesting problem.
I was using my LHIRES with 2400l/mm grating recently and found that there was a problem with the telluric lines.
It is winter here at the moment and getting quite cold by Australian standards (down to -2degC). I started imaging after dark in the early evening. I process my spectra as I go and use the telluric lines to check the accuracy of my calibration. For a 10min exposure taken just after a neon spectra the calibration is usually within 0.01A. This is consistent over many hours usually. Later that night I found that the accuracy had dropped and the telluric lines were inaccurate by up to 0.8A. I thought that this was strange and decided to stop imaging. Nothing had changed with the LHIRES since earlier in the evening. I have never had this problem before.
When I parked my scope I noticed that dew had formed on 1 side of the corrector plate as my heater strip had slipped on that side of the telescope. I assume that this asymmetrical dew had created the inaccuracy in the spectra. I had always felt that the telluric lines were a "gold standard" but this has shown that error can be introduced.

Now there is a different problem also. The last few nights have been very cold and we also have a drought. There is very little water to make lines. ;)
Attached are 2 spectra of the same star (e Lup) taken on 11th and 15th July. Neither have had the telluric lines removed. The earlier one has clearly visible lines but the 15th was very dry and I can barely make out any lines. It makes it difficult to check the calibration. :D
Attachments
elup_20180715_499_TBohlsen.png
elup_20180715_499_TBohlsen.png (30.67 KiB) Viewed 255 times
Terry Bohlsen
Armidale NSW
Australia
Terry Bohlsen
 
Posts: 124
Joined: Thu Sep 29, 2011 7:40 am

Re: Telluric line inaccuracy

Postby Robin Leadbeater » Mon Jul 16, 2018 9:36 pm

Interesting. Were the Telluric lines shifted relative to the star spectrum or just relative to the neon lamp spectrum ? ie was the star spectrum including the lamp lines shifted due to part of the aperture being obstructed by the dew ? In which case the Telluric lines would give the correct calibration

Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
Posts: 1036
Joined: Mon Sep 26, 2011 4:41 pm

Re: Telluric line inaccuracy

Postby Terry Bohlsen » Tue Jul 17, 2018 1:26 am

Robin Leadbeater wrote:Interesting. Were the Telluric lines shifted relative to the star spectrum or just relative to the neon lamp spectrum ? ie was the star spectrum including the lamp lines shifted due to part of the aperture being obstructed by the dew ? In which case the Telluric lines would give the correct calibration

Robin

Bobin.
The telluric lines were shifted compared to the neon calibration. It is difficult to know whether the star spectrum was shifted as well because the target is an spectroscopic binary. I feel that the dew on 1 side of the corrector plate should not have an effect on the star spectrum. The water causing the absorption lines is very local to the scope rather than in the atmosphere above. This shift is a similar effect to holding a neon calibration light in front of the scope but not in line with the slit. This gives a spurious calibration so can't be used. The neon calibration I used was the internal one in the LHIRES so not effected by any dew.
Terry
Terry Bohlsen
Armidale NSW
Australia
Terry Bohlsen
 
Posts: 124
Joined: Thu Sep 29, 2011 7:40 am

Re: Telluric line inaccuracy

Postby Robin Leadbeater » Tue Jul 17, 2018 12:53 pm

Hi Terry,

My suspicion is the the star spectrum would have been similarly effected by the partial obscuration of the aperture. (The telluric lines are generated from water vapour in the air column, not from the liquid water in the dew.) You could perhaps test this by masking off part of the aperture.

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
Posts: 1036
Joined: Mon Sep 26, 2011 4:41 pm


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