Measuring radial velocity with LISA

[Martin Dubs]

Hello Jacques,

Vh refers to the helicentric velocity correction, which you can calculate for the time of the observation on the neighboring tab in ISIS. As the two stars are close together, they have a similar heliocentric correction. The cross correlation then measures the radial velocity difference of the two stars from a heliocentric viewpoint, which is compared with the data in SIMBAD. Normally you compare a star with unknown radial velocity with a reference star with known radial velocity, preferably with the same spectral class. Further details you can find in the release notes of ISIS:
http://www.astrosurf.com/buil/isis/new/release.html

Regards,

Martin

[Jacques Montier]

Thanks very much for your response, Martin !
It seems clearer now.


Cheers,

[Paul Luckas]

Hi David,

I think it gives a good indication of what I would say is the excellent (sigma ~1/40 of the resolution) repeatability of the measurement (a combination of the equipment performance and the procedure used), which is what counts at the end of the day. Taking lamp spectra at the time of each observation and using them for calibration takes care of any long/medium term spectrograph stability issues. The only additional suggestion I have would be to take lamp spectra immediately before and after each target measurement to check for and quantify any drift during the observation. ISIS also has a rather clever system for tracking and correcting for any drift in multiple exposures using cross correlation which could be useful if you are not already using it. The ultimate precision/accuracy will depend on the spectral content of the actual target(s) measured/compared but these results bode well I would say. Do you have project involving RV measurement in mind ?

Robin

Question/Sanity check: when using the wavelength registration feature in ISIS, is it best to calibrate using only the neon calibration frame taken before the target sequence, or do you still average 'before and after' neon calibration frames (assuming some drift during the sequence)? I've assumed that you would only use the first (if wavelength registration aligns everything to the first spectrum in a target sequence). Not fully understanding ISIS' cross correlation function as it pertains to the wavelength registration feature I thought it a valid question as a sanity check.

Thanks,

Paul

[Robin Leadbeater]

Hi Paul,

Yes in this case it is important to use a lamp spectrum as closely matched to the first (ie reference) exposure. (I suppose ideally the sum of two, immediately before and after the first exposure)

Cheers
Robin