Help learning how to measure RV (using ISIS).

[Paul Luckas]

Hi all,

I'm looking for a step by step guide on the practice of making velocity measurements (e.g., RV) in ISIS, including when, where and how to include the heliocentric correction process. The beginners texts that I've been reading (Robinson, Walker, et al) provide a great introduction to the theory - but I still find myself lost in the practical aspects of actually undertaking test RV measurements on some standard RV stars and the like. I'm missing a lot of the 'how' and 'when' process steps that others more advanced than I clearly view as 'basic'. Frustrating as it is, I am just not "getting it".

I know it's a big ask, but I figured nothing ventured nothing gained. I'm also pretty sure other beginners would benefit from anything offered up on this topic.

Assume that I know how to acquire spectra and calibration frames, have a very good understanding of how to process spectra in ISIS, and understand the practice of IR correction and continuum removal (if required). Now, how to I quite literally undertake even basic RV measurement in ISIS on a set of, let's say, spectra of an RV standard or other target star. What do I need, and how to I proceed?

Many thanks to anyone willing to walk me (and others) through this.

Merry Christmas to all,

Paul

[JP Nougayrede]

Hello Paul, there is a nice tutorial on Christian Buil website :
http://www.astrosurf.com/buil/redshift/demo.htm
Clear skies,
Jean-Philippe

[Robin Leadbeater]

Hi Paul,

The cross correlation method will give you the difference in RV between the measurement and the reference (which might be an actual measured standard star, an artificial template or the measured star itself, for example measured at a different time. If you need an absolute measurement you can use ISIS to measure the wavelength of a particular line (or lines) compared with the at rest laboratory value, making sure that the line(s) chosen is isolated and not a blend. (You might also need to consider the impact of any asymmetry in the line if present) The RV is then c* (delta lambda/lambda)

All measurements need to be heliocentric corrected and the simplest way to do this in ISIS is to tick the "heliocentric radial velocity correction" box in the general tab when processing. Provided the target name is recognised in SIMBAD, your observatory location is entered in ISIS and the correct date and time is in the fits header, ISIS will calculate and apply the correction. One caveat - if you remove telluric lines, this has to be done without helicocentric correction and the correction then applied. In this case I prefer to start with the corrected spectrum, shift it during the telluric line removal procedure by the amount needed to optimise the removal, note the shift applied and then finally remove this shift, returning the spectrum to its heliocentric condition. Unless I have missed something, this is a slightly annoying feature in ISIS which could be improved.

There are also tools to manually calculate heliocentric correction and apply it to a spectrum under the Misc tab

Seasons greetings!
Robin