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 Betreff des Beitrags: On our precision of rv measurements
BeitragVerfasst: 22. Dezember 2018, 22:49:02 PM 
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Hi!

For some questions in our project on the YHGs we need accurate rv data as possible. Some devices like the LhiRes III have problems because they are not stabilized. Therefor we wrote in May 2018 an ESO-MIDAS script to perform recalibration with help of telluric lines.

Since May we tested and improved the script and now had a first look at the data quality we can achieve over one year. You can watch it out here: http://spektroskopie.vdsastro.de/projec ... ments.html

We attach the script with telluric lines data file and a description.

Cheers
Christoph


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BeitragVerfasst: 26. Dezember 2018, 19:27:33 PM 
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Hello Christoph,

How is the error on the Y axis calculated ? The difference between which two values ?

Cheers
Robin


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BeitragVerfasst: 27. Dezember 2018, 09:24:49 AM 
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Hi Robin,

the error is calculated out of the differences of measured telluric line positions after recalibration to rest wavelength positions (out of HITRAN data).


We use the RMSE as error estimation:
Angstrom error = sqrt(sum of (rest wavelength - measured wavelength) / (n-1))

The km.s values are then calculated for 6412 and 6443 Angstrom.

Cheers
Christoph


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BeitragVerfasst: 27. Dezember 2018, 17:03:02 PM 
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Hi Christoph

OK, I think I understand. The graph shows the uncertainty in the measurement with time, which improves when the telluric lines are stronger. Not a systematic seasonal wavelength calibration error.

I do not know if you can use the same technique here but for high precision RV measurements with the LHIRES I have used a lamp in front of the telescope to superimpose lamp lines on the star spectrum during the exposure. This gave a very precise, stable long term wavelength calibration. (sigma ~0.5km/s with a 2400l/mm grating, example attached)

Cheers
Robin
Dateianhang:
LHIRES_RV_error.png
LHIRES_RV_error.png [ 29.43 KiB | 483 mal betrachtet ]


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BeitragVerfasst: 27. Dezember 2018, 19:38:14 PM 
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Hey Robin!
Zitat:
OK, I think I understand. The graph shows the uncertainty in the measurement with time, which improves when the telluric lines are stronger. Not a systematic seasonal wavelength calibration error.
Yes!
We now plan to have a closer look at the tellurics for improving the work with them;)


That is indeed a very impressive result! Was there a systematic error because of dissimilar light ways of the stars and calibration lamp? And if yes, do you corrected it with help of Vega as standard star?

Are the Vega data points only plotted in the graph? In Simbad Vega is listed with a rv of -20.60 ±0.2 km/s.

Mhm, I don't know wether this technique will work in our case. We might not have enough light with our 5 inch telescope in that way that we have to integrate about one hour to achieve the wanted snr. And this leads to drift problems. But it is worth to test it!

Cheers
Christoph


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BeitragVerfasst: 28. Dezember 2018, 16:06:45 PM 
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Hi Christoph,

Yes the referenced RV value for Vega in SIMBAD is very strange. I think it might actually be the rotational velocity (vsini). Even Wikipedia has a better reference for RV ;-) (13.9km/s)

Yes the transfer function is different for lamp and star. The lamp was mounted at the edge of the aperture aligned with the slit and the lines are very narrow with no aberrations compared with the star. There will always be some systematic offsets so to measure absolute RV, you need to compare with an RV standard . (In this case I was only interested in the variations in Deneb, not the absolute value so the points plotted on the graph are the uncorrected measured values) Attached is a photo showing the lamp mount. The lamp is on a pivot and moved in front of the aperture when needed.

This was a very bright target so you would probably need to reduce the intensity of the lamp (perhaps using a diffuser) for longer exposures on fainter targets. (A smaller lamp would be better, particularly for your small aperture!). The lamp lines also contaminate the spectrum but these can removed with background subtraction if a clean spectrum is needed.

The stability of the spectrograph is not the only problem with slit spectrographs. Changes in focus and guiding can also affect the way the star image is sampled by the slit. If this is not symmetric this can produce significant errors. This is another reason why fibre fed spectrographs are better for high RV accuracy.

I have also used telluric lines successfully to give high RV precision. Here in fig 6 of this paper for example.
https://arxiv.org/abs/1206.6754
The O2 telluric lines used in this case were very strong though.

Cheers
Robin
Dateianhang:
Filly_lamp_mounting.jpg
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BeitragVerfasst: 28. Dezember 2018, 17:31:10 PM 
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Zitat:
We might not have enough light with our 5 inch telescope in that way that we have to integrate about one hour to achieve the wanted snr. And this leads to drift problems. But it is worth to test it!
This is the big advantage of the method. The lamp is kept on during the exposure so any drift in wavelength is seen exactly the same in both lamp and star, just like the telluric lines, but in emission and with constant intensity. The problem is to match the intensity of the lamp with the star. This has given me an idea ---The lamp could perhaps be pulsed on and off automatically (pulse width modulation). On for 1 sec every 30 sec during the exposure for example.

Robin


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BeitragVerfasst: 30. Dezember 2018, 11:01:53 AM 
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Hi Robin!

Zitat:
Yes the referenced RV value for Vega in SIMBAD is very strange. I think it might actually be the rotational velocity (vsini). Even Wikipedia has a better reference for RV ;-) (13.9km/s)
Okay! Thank you for the info;)

Zitat:
There will always be some systematic offsets so to measure absolute RV, you need to compare with an RV standard .
Did you found errors due to flexure because of different orientations of the setup pointing at Vega and Deneb?
I assume therefor the standard star has to be as near as possible to the target star?!

Zitat:
The lamp lines also contaminate the spectrum but these can removed with background subtraction if a clean spectrum is needed.
The strong neon lines will not be a problem. Our lines of interest are on different positions, but we have to look at the weak lines.


At all, this is a very interesting technique! Thank you for sharing this idea!!! We will test it with our equip.

Cheers
Christoph


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BeitragVerfasst: 30. Dezember 2018, 14:59:54 PM 
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Hi Christoph,
Zitat:
Zitat:
There will always be some systematic offsets so to measure absolute RV, you need to compare with an RV standard .
Did you found errors due to flexure because of different orientations of the setup pointing at Vega and Deneb?
I assume therefor the standard star has to be as near as possible to the target star?!
The large effects of thermal drift and flexure in the spectrograph are removed by wavelength calibrating the target and reference spectra independently using the lamp lines in the spectra. There could still be a constant offset between the star and lamp though due to geometric differences in the light paths, which would need to be removed using an RV reference.

The long term repeatability of the Vega RV measurements shows that any offset between lamp and star is constant and independent of the telescope orientation and so there was no need to check Vega every time. (You could verify this for your setup by following the same star through a night) In practise it is probably a good idea to check the wavelength calibration against an RV standard from time to time but this should not be needed for every observation and the location of the RV reference in the sky should not matter.

Cheers
Robin


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