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BeitragVerfasst: 06. Dezember 2017, 14:04:57 PM 
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Dauernutzer

Registriert: 23. Februar 2008, 19:33:52 PM
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Hi all,
have you seen the news? The ESPRESSO spectrograph at ESO has made its firstlight on a star.
https://www.eso.org/public/unitedkingdom/news/eso1739/

The echelle spectrum looks nice :) Notice use of image-slier and calibration reference next to it.

How to make such well-spaced calibration source? is it at a constant in CCD-space or is it constante distance in wavelength?
What do you think?

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BeitragVerfasst: 06. Dezember 2017, 14:24:06 PM 
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Hi Fil, they use a Laser Frequency Comb (see chapter 12.9.2 on page 461 in "Spectroscopic Instrumentation"). It is a constant distance in wavelength. You can use it if you have about 100.000 Euros. Cheers, Thomas


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BeitragVerfasst: 06. Dezember 2017, 14:39:29 PM 
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Registriert: 17. Dezember 2006, 15:08:21 PM
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Hi,
Zitat:

How to make such well-spaced calibration source? is it at a constant in CCD-space or is it constante distance in wavelength?
The frequency comb is equidistant in frequency:

https://en.wikipedia.org/wiki/Frequency_comb

As noted by Thomas, this is an expensive calibration source. Another option for a calibration source with equidistant lines is a Fabry-Perot

https://en.wikipedia.org/wiki/Fabry%E2% ... rferometer

This is a cheaper (but not cheap..) solution, but needs careful stabilisation.

https://arxiv.org/abs/1404.0004

Best regards,
Otmar


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BeitragVerfasst: 06. Dezember 2017, 14:57:12 PM 
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We were fortunate in 2016 to visit the assembly hall of Espresso at the Geneva Observatory with its designer Francesco Pepe.

In addition to laser calibration unit, for the flats they use a 10 000 K light source. LDLS : Laser Driven Light Source.

The spectro is also regulated in temperature at 1/100 degrees Celsius in a double enclosure.
Bild

Bild


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BeitragVerfasst: 07. Dezember 2017, 02:34:22 AM 
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Dauernutzer

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Zitat:
The frequency comb is equidistant in frequency
The spectrum looks so nice! I asked bout the frequency comb "spacing" because there appears to be little change in dispersion throughout the complete spectrum! It seams only 25% greater in red than blue... is this natural in an Echelle, possibly because of different orders instead of only a singe order in traditional spectrographs?
Of course, I do not know how they got a color pitcure out of the spectrograph (color camera or post processing to simulate wavelength?) so it is hard to see what wavelengths are shown.. but I am curious to find out what the distance between the dots is in wavelength :) Tau Ceti is like our sun, so I will compare this with a solar spectrum and hope to identify some Fraunhoffer lines :)

It should be forbidden to show such nice output images to people who have not yet completed building their first [echelle] spectroscope! :)

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BeitragVerfasst: 07. Dezember 2017, 09:15:52 AM 
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Zitat:
Of course, I do not know how they got a color pitcure out of the spectrograph (color camera or post processing to simulate wavelength?) so it is hard to see what wavelengths are shown.. but I am curious to find out what the distance between the dots is in wavelength :)
It's not a color CCD, (Color CCD is not the good solution for spectroscopy because of the Bayer matrice and the poor efficiency of a color CCD) but 2 larges monochrome CCD (90x90mm) one for the red part of the spectrum, the second for the blue part. In fact the color spectrum is process like a color picture by mixing the 2 composants R and B together, so it's not an RGB image, but an RB image.

See the optical design below :
Bild


Zuletzt geändert von Olivier GARDE am 07. Dezember 2017, 09:33:30 AM, insgesamt 2-mal geändert.

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BeitragVerfasst: 07. Dezember 2017, 09:20:38 AM 
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Hi,
where is the problem? You need only 1975 C14 to feed this nice echelle.
Christian


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BeitragVerfasst: 07. Dezember 2017, 11:08:52 AM 
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Registriert: 10. Juli 2016, 13:53:59 PM
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Zitat:
Hi,
where is the problem? You need only 1975 C14 to feed this nice echelle.
Christian
Maybe a C8 would do to keep the cost down.

Regards Andrew


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BeitragVerfasst: 07. Dezember 2017, 12:28:31 PM 
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Zitat:
You need only 1975 C14 to feed this nice echelle.
Well, not really... F/2.5 focus at seeing always less than 0.5 arcsec :)


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BeitragVerfasst: 07. Dezember 2017, 18:27:21 PM 
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Hi

see, e.g., the shift-detection with a Fabry-Perot-Etalon on PEPSI:

https://arxiv.org/pdf/1505.06492.pdf

It is very important to stabilize such a device, since peak position and separation depends on the distance between the two mirrors. You can simulate a Farby-Perot output with SILENT.

In my echelle FLISES I glued a second fibre at the entrance directly beside the science fibre. This fibre provides a simultaneous calibration signal (Th/Ar at the moment).
I attached a pic of a test with arcturus and the Th/Ar signal. Here you would also need to have a blue filter to suppress the strong Ar lines (as done for flatfielding to pronounce the blue orders of the halogen signal).

The good thing: Th/Ar lamp is stable!
The bad thing: Lesser lines and blended, more difficult to detect small shifts.

best,
Daniel


Dateianhänge:
arkturus_sim_90.png
arkturus_sim_90.png [ 356.34 KiB | 501 mal betrachtet ]

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