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andycasey / orchestra

Twangle? Orchestra? Melody?
http://harps.guru
MIT License
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Consider revising the barycentric velocities in the HARPS data products #1

Open andycasey opened 7 years ago

andycasey commented 7 years ago

Amaury Triaud (Cambridge) informed me that when people have extracted all RV information from HARPS data for one star (over many programs) in the past, they sometimes will report the existence of multiple planets. But when one goes back and recalculates the barycentric velocity corrections using the same stellar position and proper motion, those planetary signals disappear.

It could be that although the HARPS pipeline will calculate RVs and barycentric velocities the same way, the precision in stellar position and proper motions may be enough to produce stellar RV signals at the level of ~cm/s.

We should investigate if this is the case, and decide whether we should recalculate barycentric velocities for all sources.

andycasey commented 7 years ago

Section 5.1 of the HARPS user manual (http://www.eso.org/sci/facilities/lasilla/instruments/harps/doc/manual/userman2_1.pdf) notes:

to achieve an accurate solar system barycentric Radial velocity, correction of 1 m/s, the target coordinates must be known to within 6" including proper motion

megbedell commented 7 years ago

I've also heard that this is a known (but unpublished) issue for the Tau Ceti RVs in particular. Different observing groups have apparently specified slightly different proper motions for the target in the observing template, which induces a false signal in the RVs after barycentric correction. This might be a problem at the cm/s level only for the nearest/highest PM targets though.

shbhuk commented 6 years ago

Hi, Has there been any update on this?

megbedell commented 5 years ago

Bumping this up again because I'm thinking about it w.r.t. wobble!

@benmontet also brings up the point that variable cloud coverage during the exposure can change the flux-weighted net BERV for the exposure. I'm not sure whether HARPS has a system in place to correct for this and I'm having trouble finding any info on how exactly the BERV is calculated in the DRS... anybody know more?

andycasey commented 5 years ago

I know nothing! (on this, and other things)

Sent from my Commodore 64

On 30 Nov 2018, at 03:59, Megan Bedell notifications@github.com wrote:

Bumping this up again because I'm thinking about it w.r.t. wobble!

@benmontet also brings up the point that variable cloud coverage during the exposure can change the flux-weighted net BERV for the exposure. I'm not sure whether HARPS has a system in place to correct for this and I'm having trouble finding any info on how exactly the BERV is calculated in the DRS... anybody know more?

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