The Origin of the Radial Velocity Zero Point Offset

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The Origin of the Radial Velocity Zero Point
Offset

• George Seabroke
• Paul Cass
• Arnaud Siebert

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ravedr-reduced twilight spectra (timeline light
to dark colours)
1st arc
Radial velocity (km/s)
Low order polynomial fit
Low order polynomial fit
Plots courtesy of Arnaud Siebert
2nd arc
Fibre number
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Blue
?
Red

• 1st raw arc divided by 2nd raw arc

Fibre number
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Hermann Carl Vogel (1834-1902)
George Mitchell Seabroke (1848-1918) British
Astronomical Association (BAA) Presidential
Portrait 1900
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Blue
?
Red

• 1st raw arc divided by 2nd raw arc

Fibre number
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Grating equation m?/ni ?g(sin?i sin?i)

• ? projected separation between fringes in plane
  of grating ?groove spacing on ruled grating

• ? grating period

Fig.1 from Baldry, Bland-Hawthorn, Robertson
(2004)
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Grating equation m?/ni ?g(sin?i sin?i)

• ? projected separation between fringes in plane
  of grating ?groove spacing on ruled grating

• ? grating period

Same pixel receives redder light i.e. arc lines
blueshifted
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Grating equation m?/ni ?g(sin?i sin?i)

• ? projected separation between fringes in plane
  of grating ?groove spacing on ruled grating

• ? grating period

? has to decrease to still get constructive
interference
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Conclusions

• Temperature is responsible for RV zero point
  offsets
• Negative pixel shift with temperature increase
  suggests VPH grating is not expanding with
  temperature.
• Thermal expansion coefficient of steel 1.44
  times that of glass
• Metal components in spectrograph responsible for
  radial velocity zero point offset?
• Put a heater in the spectrograph room?

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