Spectral calibration by using low cost optical fibers

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Christian Buil
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Joined: Mon Sep 26, 2011 6:59 pm
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Spectral calibration by using low cost optical fibers

Post by Christian Buil »

I'm developing the following precision spectral calibration :

(1) The spectral calibration flux is permanent at the entrance of the telescope/refractor or semi-permanent (low intensity miniature lamps or low cost plastic optical fibers (1.5 mm diameter, 25 euros for 100 meters - see Amazon - see photos + 3D print). So, these illumination is permanent (here neon lamp for calibrate Halpha region) during the exposures. The spectral lines do not saturate the exposed image.

(2) Each image therefore contains a mixture of the star spectrum and the calibration spectrum.

(3) processing (specINTI software for exemple) to precisely calibrate each image (compute the first order shifting, for exemple) and removal of calibration lines in the final profile.

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Obstruction in the entrance pupil is negligible (less than 0.04 magnitude with a 100 mm diameter refractor). This new approach is effective in eliminating the effect of mechanical flexure (calibration error), economical and easy to made. Valuable for all spectrograph.

Christian Buil

Sol'Ex/Star'Ex web: http://www.astrosurf.com/solex/
Chaine YouTube Astro-Spectro: https://www.youtube.com/channel/UCdlVj1 ... g/featured
Hamish Barker
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Joined: Thu Jun 20, 2019 12:11 am

Re: Spectral calibration by using low cost optical fibers

Post by Hamish Barker »

that is very clever! I guess one could also install a variable polarizer or other variable (graduated neutral density?) filter between the lamp and fibers for tuning brightness down for fainter targets.

do you have a 3d print file somewhere for the holders?

Cheers,
Hamish
Christian Buil
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Joined: Mon Sep 26, 2011 6:59 pm
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Re: Spectral calibration by using low cost optical fibers

Post by Christian Buil »

Hamish, for the adjustment of the intensity, the solution is quite simple: I modify the distance "d" between the source (a simple E10 neon lamp, see down to the page http://www.astrosurf.com/solex/specinti.html) and the input of the fibers. The intensity of course depending on the "d x d" value.

The telescope interface is tricky because no two telescopes are alike. But I will post generic schematics. I will inform you.

Christian
Hamish Barker
Posts: 226
Joined: Thu Jun 20, 2019 12:11 am

Re: Spectral calibration by using low cost optical fibers

Post by Hamish Barker »

thanks. I guess in a pinch, duct tape on my spider vanes will do.
Rainer Ehlert
Posts: 135
Joined: Thu Sep 29, 2011 9:36 pm

Re: Spectral calibration by using low cost optical fibers

Post by Rainer Ehlert »

Hi,

Interesting and simple.

Would this also work for using a Relco starter lamp ?

Thanks and regards Rainer
regards Rainer____Observatorio Real de 14_____MÉXICO_____N 22° W 101°
Simon de Visscher
Posts: 48
Joined: Sun Dec 29, 2019 5:48 pm

Re: Spectral calibration by using low cost optical fibers

Post by Simon de Visscher »

Hello,


To vary the intensity you can also get a cheap potentiometer that will vary the current and therefore the glow intensity of the lamp.

Simon
--
Simon
LISA/Alpy600/LHIRES N400f5, AP130EDT
Hamish Barker
Posts: 226
Joined: Thu Jun 20, 2019 12:11 am

Re: Spectral calibration by using low cost optical fibers

Post by Hamish Barker »

Rainer Ehlert wrote:Hi,

Interesting and simple.

Would this also work for using a Relco starter lamp ?

Thanks and regards Rainer
I find the relco is pretty faint even when it is directly in the calibration lamp hole of my L200 (needs 30-60 second exposure), so suspect multiple lamps would be required if using fibers like this.
Christian Buil
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Joined: Mon Sep 26, 2011 6:59 pm
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Re: Spectral calibration by using low cost optical fibers

Post by Christian Buil »

The technique I show is only suitable for relatively intense spectral lamps. For example, here the calibration of the red part of the spectrum by using an easy to find neon lamp + fibers on a 4-inch refractor - this gives a good reference for very precise calibration (powerful for high resolution H-alpha evolution survey, for example), simple to perform and elegant:

Image

Usable also with fluo-compact fluorescent lamps, mercury vapor tubes, .... The light from the RELCO lamps seems really too faint for me.

Here, what does not work: use of a powerful flashlight (150 Lumen) for tentative of flat-field acquisition:

Image

Because the spectrum is approximately continuous, the intensity in the spectral image does not give a flat-field at the correct SNR. In addition, the spot lighting in the entrance pupil is not appropriate to obtain a photometrically correct flat-field (fringes, inability to remove the shadow of dust, etc.). For flat-field (and high spectral résolution) I use LED panel (a Star'Ex spectrograph working at R=20000 is at the focus of the refractor):

Image

Christian
Rainer Ehlert
Posts: 135
Joined: Thu Sep 29, 2011 9:36 pm

Re: Spectral calibration by using low cost optical fibers

Post by Rainer Ehlert »

Thank you
regards Rainer____Observatorio Real de 14_____MÉXICO_____N 22° W 101°
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