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athornett

Spectrum of Vega

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This is an analysis using RSPEC software of the Vega spectrum I took 4/8/2018 using my Sky Watcher Equinox 80mm Pro telescope & CCDSPEC spectroscope, hand guided on Manfrotto mount.

I have now taken out a trial subscription to RSPEC amateur astronomy spectroscopy software https://www.rspec-astro.com/

The spectrum taken using Nebulosity 6 software/QHY6 camera. The x-projection was generated in CCDSPEC's own PCSpectra software.

Using RSPEC, I calibrated the x-axis using Vega spectra from the internet, using two points. I have created the following Vega spectrum for calibration purposes based on this internet data. Note that I have annotated the lines using Angstroms as RSPEC uses Angstroms on its x-axis and it all gets rather confusing if one graph is in Angstroms and the other in nanometres as it is not obvious that the difference in scale exists on the x-axis and the graphs do not line up.

RSPEC allows reference spectra to be compared to user spectra. I loaded up a reference spectrum in blue (from RSPEC reference library) at the same time as my own Vega spectrum in red.

Having calibrated my spectrum against two lines on the reference spectrum I created above from other internet resources, there is excellent correlation between the lines on my spectrum and the RSPEC reference spectrum (no – I did not use the RSPEC spectrum to create my calibration Vega spectrum!)

Another interesting feature in RSPEC is to use it to identify elemental lines – I used this on my spectrum. I think that the closest elemental match to the lines seen in my graph was found when both Star Type A is ticked (Vega is A0V) and also Hydrogen Balmer series.

Andy

Vega-6-0s-Neb.bmp

Vega-040818-best-match-lines-identified-in-RSPEC.png

Vega-100818-XAxisProjection.bmp

Vega-spectrum-040818-displayed-in-RSPEC.png

Vega-spectrum-040818-plus-A0V-reference-spectrum-in-RSPEC-230918.png

Vega-spectrum-040818-with-hydrogen-Balmer-series-identified-by-RSPEC.png

Vega-spectrum-annotated-line-wavelengths-in-Angstroms (calibration graph based on internet data).png

Edited by athornett
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Thanks Neil. I am amazed at how accurately the lines on my very amateur spectrum matched those on the reference spectrum- I thought there would be at least some difference! However, I suppose that gases emit at a particular wavelength only and this gives the same results time and again.

Andy

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45 minutes ago, athornett said:

Thanks Neil. I am amazed at how accurately the lines on my very amateur spectrum matched those on the reference spectrum- I thought there would be at least some difference! However, I suppose that gases emit at a particular wavelength only and this gives the same results time and again.

Andy

Yes you can use these lines to calibrate your spectrograph.  Note that they will not be exactly at the published wavelengths for the Hydrogen Balmer series because of Doppler shifts.  Vega is moving towards  us at 14km/s and our orbit round the sun also affects our relative velocity relative to Vega but the shift is small and at the limit of detection at your resolution (~ 0.3A). 

It is very obvious using a high resolution spectrograph though. Here for example are measurements of Deneb and Vega with my LHIRES spectrograph.  Here we are seeing Vega moving towards us at 14km/s and pulsations in Deneb.  (Deneb is effectively breathing in and out and we are seeing the surface moving towards and away from us.)

RobinVega_Deneb_RV.png.5ec19258e968892bd4d7b14ef4de2990.png

Edited by robin_astro

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Note that your linear calibration using 2 lines will not be precisely correct as your spectrograph dispersion is not quite linear. (If you zoom in, you will find small errors in your Balmer line positions. )  If you use a higher order calibration using more of the Balmer lines, your calibration will be more accurate.

Robin

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Thanks Robin for both the preceding comments. Looking at your LHIRES spectrum of Vega and Deneb showing radial velocity shifts, I have to say "Wow!" I did wonder whether I would ever be able to demonstrate radial velocity shifts - I guess I would need to take multiple spectra and average over time as I don't have the resolution and even then would be very difficult? The Science Surplus DIY Spectrometer has  grating with 1800 lines/mm - http://www.science-surplus.com/products/spectrometers - do you think that would have enough resolution to demonstrate this radial shift if I can find successful way to get enough starlight into the optical fibre?

Yes I am aware of the limitation of 2 lines fit for the graph - I have had a go at creating a polynomial equation generating Excel spreadsheet which I attach to this post (this is my own creation).

Andy

Calculation spreadsheet for higher order polynomials Excel Andrew Thornett generic Sept 2018.xls

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Hi Andy,

RSpec  etc can also produce a non linear fit function to multiple lines.   

My 1 km/s precision here is nothing compared with Christian Buil's work measuring exoplanets to a 20x greater  precision of ~50 metres/sec by simultaneously measuring hundreds of lines using a very stable fibre fed high resolution wide spectrum range echelle instrument (scroll to the bottom of this page to see his results)

 http://www.astrosurf.com/buil/extrasolar/obs.htm

The key to radial velocity measurements is the stability of the instrument. If your instrument is stable enough it is relatively straightforward  to measure shifts of  ~ 1/20 of the resolution on bright targets which for example works out at ~30km/s for the ALPY with a resolution of 12A.  Here are some measurements I made when I was testing my ALPY for example.

http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=618

Of course if the velocity is high enough you can use very low resolution, as in this high redshift Quasar measured with a Star Analyser where lines normally in the UV have been shifted 4.9x into the visible region

http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=618

Cheers

Robin

Edited by robin_astro

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