Telluric line inaccuracy
Posted: 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.
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.