Under this topic Radial Velocity will be discussed.

TonyAngel wrote:
Under this topic Radial Velocity will be discussed.

Was:exoplanet transits

TonyAngel wrote:

Thank you Robin. Yes I had read Christian Buil's page. As I already had the use of the SGS I was hoping that I could at least see if it could have spotted the larger doppler shift exoplanents, or failing that spectroscopic binaries just to do a proof of concept before seeing if a more expensive spectrograph could be justified. Perhaps I can just use the SGS for confirmation of exoplanet's star spectrum, (allowing this could be contamininated by the planet). As you can guess I am rather new to spectroscopy from a practical angle.


SBig claim an RV precision of +-6km/s with a 600l/mm grating for the SGS which is probably realistic (I can get down to +-1km/s with my LHIRES III under favourable conditions but that is with a 2400l/mm grating which gives ~10 higher dispersion. Entry level to just detect the exoplanet round Tau Boo which I think is the easiest is ~200m/s and Buil's precision was around +-50m/s (Which is just 0.001A at 6000A !) so quite a tough target.

Actually measuring binary star component RV can be tricky as the two spectra blend and can be difficult to separate. Here is a nice easy example, Beta Aurigae using the LHIRES at 0.3A resolution.
www.threehillsobservatory.co.uk/astro/spectra_35.htm
Exoplanet measurements are more straightforward (though not easier!) as there is only one component (The contribution from the planet either from planet temperature, reflected starlight or absorption during transit due to an atmopshere can be safely ignored at visible wavelengths at least. This is the task of a future generation of exoplanet hunting space telscopes, looking for signs of life)

I would say a good way of measuring the capabilities of a proposed system would be to repeatedly measure the RV of a radial velocity standard for several nights and see how much scatter you get. Then you can start beating down the variability until you are at least well below +-1km/s. You would then be ready to try for an exoplanet.

Cheers
Robin

Thank you Robin.

Very good advice which I will endevour to follow. I have been reading your webpages for advise on spectroscopy. I am just waiting for the clouds to disapear - it is the rainy season in Southern Spain.

Best Wishes,

Tony.

Hi Tony,

If you (or anyone else) are looking for useful things to do with a spectrograph then good groups to keep an eye on are

The French based ARAS group
www.astrosurf.com/aras/
and their forum
www.spectro-aras.com/forum/

The German based VdS group
spektroskopie.fg-vds.de/index_e.htm
and their forum
spektroskopieforum.vdsastro.de/

There are no British/US forums working at a similar level unfortunately but both these groups are Brit friendly ;-)

Cheers
Robin

Christian Buil has updated his spectroscopic data reduction program ISIS
www.astrosurf.com/buil/isis/isis_en.htm
to include the cross correlation algorithm he used to detect exoplanets by the radial velocity method.
www.astrosurf.com/buil/isis/new/release.html
There is also a link there to an interesting presentation
www.astrosurf.com/buil/exoplanet/exo_buil.pdf
(in French) that outlines how he made the exoplanet detections and some speculations on how amateurs might be able to discover new exoplanets

Robin

Very interesting. I was at an exoplanet meeting over the weekend and rv of exoplanets was discussed. I am going to forward the link to them.