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jobovy / galpy

Galactic Dynamics in python
https://www.galpy.org
BSD 3-Clause "New" or "Revised" License
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Vlos nomenclature #233

Closed andycasey closed 9 years ago

andycasey commented 9 years ago

Hi Jo,

I've opened this issue to resolve a discussion regarding Vlos when initialising a galpy.orbit.Orbit. It's a simple question, but I'd like to be sure. It's arisen because the terms V_los, V_rad, V_hel are frequently (and incorrectly) used interchangeably by some people, myself included.

When you refer to Vlos as the line-of-sight velocity in the documentation, do you mean the velocity observed from the telescope on some night of observation (what I usually call the line-of-sight velocity), or do you mean the observed velocity after accounting for the motion of the Earth (e.g., the heliocentric velocity)?

I would assume Vlos is actually the latter because galpy has no knowledge of where the object was observed from or when. Otherwise the Vlos would vary by up to +/- 30 km/s and the inferred orbit would be slightly different depending on which nights' observations were used.

Is my assumption correct or have I missed something here?

Thanks, Andy

jobovy commented 9 years ago

Hi Andy,

The Vlos input is the heliocentric line-of-sight velocity. I dislike using 'radial velocity' or Vrad for this, as I always confuse that with the Galactocentric radial velocity, but you're right that Vhel would probably be a better name. I've clarified the documentation.

Thanks,

Jo

andycasey commented 9 years ago

Hi Jo,

I thought that might be the reason why you had named it Vlos. Thanks for clarifying that.

Cheers